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Network Working Group W. Koch
Internet-Draft
Updates: 4880 (if approved) Jun 2017
Intended status: Standards Track
Expires: December 3, 2017
OpenPGP Message Format
draft-ietf-openpgp-rfc4880bis-02
Abstract
{ Work in progress to update the OpenPGP specification from RFC4880 }
This document is maintained in order to publish all necessary
information needed to develop interoperable applications based on the
OpenPGP format. It is not a step-by-step cookbook for writing an
application. It describes only the format and methods needed to
read, check, generate, and write conforming packets crossing any
network. It does not deal with storage and implementation questions.
It does, however, discuss implementation issues necessary to avoid
security flaws.
OpenPGP software uses a combination of strong public-key and
symmetric cryptography to provide security services for electronic
communications and data storage. These services include
confidentiality, key management, authentication, and digital
signatures. This document specifies the message formats used in
OpenPGP.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 3, 2017.
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Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. {1} Introduction . . . . . . . . . . . . . . . . . . . . . . 5
1.1. {1.1} Terms . . . . . . . . . . . . . . . . . . . . . . . 5
2. {2} General functions . . . . . . . . . . . . . . . . . . . . 6
2.1. {2.1} Confidentiality via Encryption . . . . . . . . . . 6
2.2. {2.2} Authentication via Digital Signature . . . . . . . 7
2.3. {2.3} Compression . . . . . . . . . . . . . . . . . . . . 8
2.4. {2.4} Conversion to Radix-64 . . . . . . . . . . . . . . 8
2.5. {2.5} Signature-Only Applications . . . . . . . . . . . . 8
3. {3} Data Element Formats . . . . . . . . . . . . . . . . . . 8
3.1. {3.1} Scalar Numbers . . . . . . . . . . . . . . . . . . 9
3.2. {3.2} Multiprecision Integers . . . . . . . . . . . . . . 9
3.3. {3.3} Key IDs . . . . . . . . . . . . . . . . . . . . . . 9
3.4. {3.4} Text . . . . . . . . . . . . . . . . . . . . . . . 10
3.5. {3.5} Time Fields . . . . . . . . . . . . . . . . . . . . 10
3.6. {3.6} Keyrings . . . . . . . . . . . . . . . . . . . . . 10
3.7. {3.7} String-to-Key (S2K) Specifiers . . . . . . . . . . 10
3.7.1. {3.7.1} String-to-Key (S2K) Specifier Types . . . . . 10
3.7.2. {3.7.2} String-to-Key Usage . . . . . . . . . . . . . 12
4. {4} Packet Syntax . . . . . . . . . . . . . . . . . . . . . . 13
4.1. {4.1} Overview . . . . . . . . . . . . . . . . . . . . . 13
4.2. {4.2} Packet Headers . . . . . . . . . . . . . . . . . . 14
4.2.1. {4.2.1} Old Format Packet Lengths . . . . . . . . . . 14
4.2.2. {4.2.2} New Format Packet Lengths . . . . . . . . . . 15
4.2.3. {4.2.3} Packet Length Examples . . . . . . . . . . . 16
4.3. {4.3} Packet Tags . . . . . . . . . . . . . . . . . . . . 17
5. {5} Packet Types . . . . . . . . . . . . . . . . . . . . . . 17
5.1. {5.1} Public-Key Encrypted Session Key Packets (Tag 1) . 18
5.2. {5.2} Signature Packet (Tag 2) . . . . . . . . . . . . . 19
5.2.1. {5.2.1} Signature Types . . . . . . . . . . . . . . . 19
5.2.2. {5.2.2} Version 3 Signature Packet Format . . . . . . 22
5.2.3. {5.2.3} Version 4 Signature Packet Format . . . . . . 24
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5.2.4. {5.2.4} Computing Signatures . . . . . . . . . . . . 40
5.3. {5.3} Symmetric-Key Encrypted Session Key Packets (Tag 3) 42
5.4. {5.4} One-Pass Signature Packets (Tag 4) . . . . . . . . 43
5.5. {5.5} Key Material Packet . . . . . . . . . . . . . . . . 44
5.5.1. {5.5.1} Key Packet Variants . . . . . . . . . . . . . 44
5.5.2. {5.5.2} Public-Key Packet Formats . . . . . . . . . . 45
5.5.3. {5.5.3} Secret-Key Packet Formats . . . . . . . . . . 46
5.6. Algorithm-specific Parts of Keys . . . . . . . . . . . . 48
5.6.1. Algorithm-Specific Part for RSA Keys . . . . . . . . 48
5.6.2. Algorithm-Specific Part for DSA Keys . . . . . . . . 49
5.6.3. Algorithm-Specific Part for Elgamal Keys . . . . . . 49
5.6.4. Algorithm-Specific Part for ECDSA Keys . . . . . . . 49
5.6.5. Algorithm-Specific Part for EdDSA Keys . . . . . . . 50
5.6.6. Algorithm-Specific Part for ECDH Keys . . . . . . . . 50
5.7. {5.6} Compressed Data Packet (Tag 8) . . . . . . . . . . 51
5.8. {5.7} Symmetrically Encrypted Data Packet (Tag 9) . . . . 51
5.9. {5.8} Marker Packet (Obsolete Literal Packet) (Tag 10) . 52
5.10. {5.9} Literal Data Packet (Tag 11) . . . . . . . . . . . 53
5.11. {5.10} Trust Packet (Tag 12) . . . . . . . . . . . . . . 54
5.12. {5.11} User ID Packet (Tag 13) . . . . . . . . . . . . . 54
5.13. {5.12} User Attribute Packet (Tag 17) . . . . . . . . . . 54
5.13.1. {5.12.1} The Image Attribute Subpacket . . . . . . . 55
5.13.2. User ID Attribute Subpacket . . . . . . . . . . . . 55
5.14. {5.13} Sym. Encrypted Integrity Protected Data Packet
(Tag 18) . . . . . . . . . . . . . . . . . . . . . . . . 56
5.15. {5.14} Modification Detection Code Packet (Tag 19) . . . 59
5.16. AEAD Encrypted Data Packet (Tag 20) . . . . . . . . . . . 60
5.16.1. EAX Mode . . . . . . . . . . . . . . . . . . . . . . 61
6. {6} Radix-64 Conversions . . . . . . . . . . . . . . . . . . 61
6.1. {6.1} An Implementation of the CRC-24 in "C" . . . . . . 62
6.2. {6.2} Forming ASCII Armor . . . . . . . . . . . . . . . . 62
6.3. {6.3} Encoding Binary in Radix-64 . . . . . . . . . . . . 65
6.4. {6.4} Decoding Radix-64 . . . . . . . . . . . . . . . . . 66
6.5. {6.5} Examples of Radix-64 . . . . . . . . . . . . . . . 67
6.6. {6.6} Example of an ASCII Armored Message . . . . . . . . 67
7. {7} Cleartext Signature Framework . . . . . . . . . . . . . . 68
7.1. {7.1} Dash-Escaped Text . . . . . . . . . . . . . . . . . 68
8. {8} Regular Expressions . . . . . . . . . . . . . . . . . . . 69
9. {9} Constants . . . . . . . . . . . . . . . . . . . . . . . . 70
9.1. {9.1} Public-Key Algorithms . . . . . . . . . . . . . . . 70
9.2. ECC Curve OID . . . . . . . . . . . . . . . . . . . . . . 71
9.3. {9.2} Symmetric-Key Algorithms . . . . . . . . . . . . . 71
9.4. {9.3} Compression Algorithms . . . . . . . . . . . . . . 72
9.5. {9.4} Hash Algorithms . . . . . . . . . . . . . . . . . . 72
9.6. AEAD Algorithms . . . . . . . . . . . . . . . . . . . . . 73
10. {10} IANA Considerations . . . . . . . . . . . . . . . . . . 73
10.1. {10.1} New String-to-Key Specifier Types . . . . . . . . 74
10.2. {10.2} New Packets . . . . . . . . . . . . . . . . . . . 74
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10.2.1. {10.2.1} User Attribute Types . . . . . . . . . . . 74
10.2.2. {10.2.1.1} Image Format Subpacket Types . . . . . . 74
10.2.3. {10.2.2} New Signature Subpackets . . . . . . . . . 75
10.2.4. {10.2.3} New Packet Versions . . . . . . . . . . . . 77
10.3. {10.3} New Algorithms . . . . . . . . . . . . . . . . . 77
10.3.1. {10.3.1} Public-Key Algorithms . . . . . . . . . . . 78
10.3.2. {10.3.2} Symmetric-Key Algorithms . . . . . . . . . 78
10.3.3. {10.3.3} Hash Algorithms . . . . . . . . . . . . . . 78
10.3.4. {10.3.4} Compression Algorithms . . . . . . . . . . 79
11. {11} Packet Composition . . . . . . . . . . . . . . . . . . . 79
11.1. {11.1} Transferable Public Keys . . . . . . . . . . . . 79
11.2. {11.2} Transferable Secret Keys . . . . . . . . . . . . 81
11.3. {11.3} OpenPGP Messages . . . . . . . . . . . . . . . . 81
11.4. {11.4} Detached Signatures . . . . . . . . . . . . . . . 82
12. {12} Enhanced Key Formats . . . . . . . . . . . . . . . . . . 82
12.1. {12.1} Key Structures . . . . . . . . . . . . . . . . . 82
12.2. {12.2} Key IDs and Fingerprints . . . . . . . . . . . . 84
13. Elliptic Curve Cryptography . . . . . . . . . . . . . . . . . 85
13.1. Supported ECC Curves . . . . . . . . . . . . . . . . . . 86
13.2. ECDSA and ECDH Conversion Primitives . . . . . . . . . . 86
13.3. EdDSA Point Format . . . . . . . . . . . . . . . . . . . 86
13.4. Key Derivation Function . . . . . . . . . . . . . . . . 87
13.5. EC DH Algorithm (ECDH) . . . . . . . . . . . . . . . . . 87
14. {13} Notes on Algorithms . . . . . . . . . . . . . . . . . . 90
14.1. {13.1} PKCS#1 Encoding in OpenPGP . . . . . . . . . . . 90
14.1.1. {13.1.1} EME-PKCS1-v1_5-ENCODE . . . . . . . . . . . 90
14.1.2. {13.1.2} EME-PKCS1-v1_5-DECODE . . . . . . . . . . . 91
14.1.3. {13.1.3} EMSA-PKCS1-v1_5 . . . . . . . . . . . . . . 92
14.2. {13.2} Symmetric Algorithm Preferences . . . . . . . . . 93
14.3. {13.3} Other Algorithm Preferences . . . . . . . . . . . 94
14.3.1. {13.3.1} Compression Preferences . . . . . . . . . . 94
14.3.2. {13.3.2} Hash Algorithm Preferences . . . . . . . . 95
14.4. {13.4} Plaintext . . . . . . . . . . . . . . . . . . . . 95
14.5. {13.5} RSA . . . . . . . . . . . . . . . . . . . . . . . 95
14.6. {13.6} DSA . . . . . . . . . . . . . . . . . . . . . . . 95
14.7. {13.7} Elgamal . . . . . . . . . . . . . . . . . . . . . 96
14.8. EdDSA . . . . . . . . . . . . . . . . . . . . . . . . . 96
14.9. {13.8} Reserved Algorithm Numbers . . . . . . . . . . . 96
14.10. {13.9} OpenPGP CFB Mode . . . . . . . . . . . . . . . . 97
14.11. {13.10} Private or Experimental Parameters . . . . . . . 98
14.12. {13.11} Extension of the MDC System . . . . . . . . . . 98
14.13. {13.12} Meta-Considerations for Expansion . . . . . . . 99
15. {14} Security Considerations . . . . . . . . . . . . . . . . 99
16. Compatibility Profiles . . . . . . . . . . . . . . . . . . . 106
16.1. OpenPGP ECC Profile . . . . . . . . . . . . . . . . . . 106
16.2. Suite-B Profile . . . . . . . . . . . . . . . . . . . . 107
16.3. Security Strength at 192 Bits . . . . . . . . . . . . . 107
16.4. Security Strength at 128 Bits . . . . . . . . . . . . . 107
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17. {15} Implementation Nits . . . . . . . . . . . . . . . . . . 107
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 109
18.1. Normative References . . . . . . . . . . . . . . . . . . 109
18.2. Informative References . . . . . . . . . . . . . . . . . 112
Appendix A. Test vectors . . . . . . . . . . . . . . . . . . . . 112
A.1. Sample EdDSA key . . . . . . . . . . . . . . . . . . . . 112
A.2. Sample EdDSA signature . . . . . . . . . . . . . . . . . 113
Appendix B. ECC Point compression flag bytes . . . . . . . . . . 114
Appendix C. Changes since RFC-4880 . . . . . . . . . . . . . . . 114
Appendix D. The principal authors of RFC-4880 are as follows: . 114
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 115
1. {1} Introduction
{ This is work in progress to update OpenPGP. Editorial notes are
enclosed in curly braces. The section numbers from RFC4880 are also
indicated in curly braces. }
This document provides information on the message-exchange packet
formats used by OpenPGP to provide encryption, decryption, signing,
and key management functions. It is a revision of RFC 2440, "OpenPGP
Message Format", which itself replaces RFC 1991, "PGP Message
Exchange Formats" [RFC1991] [RFC2440].
This document obsoletes: RFC 5581 (Camellia cipher) and RFC 6637 (ECC
for OpenPGP)
1.1. {1.1} Terms
o OpenPGP - This is a term for security software that uses PGP 5.x
as a basis, formalized in RFC 2440 and this document.
o PGP - Pretty Good Privacy. PGP is a family of software systems
developed by Philip R. Zimmermann from which OpenPGP is based.
o PGP 2.6.x - This version of PGP has many variants, hence the term
PGP 2.6.x. It used only RSA, MD5, and IDEA for its cryptographic
transforms. An informational RFC, RFC 1991, was written
describing this version of PGP.
o PGP 5.x - This version of PGP is formerly known as "PGP 3" in the
community and also in the predecessor of this document, RFC 1991.
It has new formats and corrects a number of problems in the PGP
2.6.x design. It is referred to here as PGP 5.x because that
software was the first release of the "PGP 3" code base.
o GnuPG - GNU Privacy Guard, also called GPG. GnuPG is an OpenPGP
implementation that avoids all encumbered algorithms.
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Consequently, early versions of GnuPG did not include RSA public
keys. GnuPG may or may not have (depending on version) support
for IDEA or other encumbered algorithms.
"PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of PGP
Corporation and are used with permission. The term "OpenPGP" refers
to the protocol described in this and related documents.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The key words "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME
FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG
APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in
this document when used to describe namespace allocation are to be
interpreted as described in [RFC2434].
2. {2} General functions
OpenPGP provides data integrity services for messages and data files
by using these core technologies:
o digital signatures
o encryption
o compression
o Radix-64 conversion
In addition, OpenPGP provides key management and certificate
services, but many of these are beyond the scope of this document.
2.1. {2.1} Confidentiality via Encryption
OpenPGP combines symmetric-key encryption and public-key encryption
to provide confidentiality. When made confidential, first the object
is encrypted using a symmetric encryption algorithm. Each symmetric
key is used only once, for a single object. A new "session key" is
generated as a random number for each object (sometimes referred to
as a session). Since it is used only once, the session key is bound
to the message and transmitted with it. To protect the key, it is
encrypted with the receiver's public key. The sequence is as
follows:
1. The sender creates a message.
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2. The sending OpenPGP generates a random number to be used as a
session key for this message only.
3. The session key is encrypted using each recipient's public key.
These "encrypted session keys" start the message.
4. The sending OpenPGP encrypts the message using the session key,
which forms the remainder of the message. Note that the message
is also usually compressed.
5. The receiving OpenPGP decrypts the session key using the
recipient's private key.
6. The receiving OpenPGP decrypts the message using the session key.
If the message was compressed, it will be decompressed.
With symmetric-key encryption, an object may be encrypted with a
symmetric key derived from a passphrase (or other shared secret), or
a two-stage mechanism similar to the public-key method described
above in which a session key is itself encrypted with a symmetric
algorithm keyed from a shared secret.
Both digital signature and confidentiality services may be applied to
the same message. First, a signature is generated for the message
and attached to the message. Then the message plus signature is
encrypted using a symmetric session key. Finally, the session key is
encrypted using public-key encryption and prefixed to the encrypted
block.
2.2. {2.2} Authentication via Digital Signature
The digital signature uses a hash code or message digest algorithm,
and a public-key signature algorithm. The sequence is as follows:
1. The sender creates a message.
2. The sending software generates a hash code of the message.
3. The sending software generates a signature from the hash code
using the sender's private key.
4. The binary signature is attached to the message.
5. The receiving software keeps a copy of the message signature.
6. The receiving software generates a new hash code for the received
message and verifies it using the message's signature. If the
verification is successful, the message is accepted as authentic.
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2.3. {2.3} Compression
OpenPGP implementations SHOULD compress the message after applying
the signature but before encryption.
If an implementation does not implement compression, its authors
should be aware that most OpenPGP messages in the world are
compressed. Thus, it may even be wise for a space-constrained
implementation to implement decompression, but not compression.
Furthermore, compression has the added side effect that some types of
attacks can be thwarted by the fact that slightly altered, compressed
data rarely uncompresses without severe errors. This is hardly
rigorous, but it is operationally useful. These attacks can be
rigorously prevented by implementing and using Modification Detection
Codes as described in sections following.
2.4. {2.4} Conversion to Radix-64
OpenPGP's underlying native representation for encrypted messages,
signature certificates, and keys is a stream of arbitrary octets.
Some systems only permit the use of blocks consisting of seven-bit,
printable text. For transporting OpenPGP's native raw binary octets
through channels that are not safe to raw binary data, a printable
encoding of these binary octets is needed. OpenPGP provides the
service of converting the raw 8-bit binary octet stream to a stream
of printable ASCII characters, called Radix-64 encoding or ASCII
Armor.
Implementations SHOULD provide Radix-64 conversions.
2.5. {2.5} Signature-Only Applications
OpenPGP is designed for applications that use both encryption and
signatures, but there are a number of problems that are solved by a
signature-only implementation. Although this specification requires
both encryption and signatures, it is reasonable for there to be
subset implementations that are non-conformant only in that they omit
encryption.
3. {3} Data Element Formats
This section describes the data elements used by OpenPGP.
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3.1. {3.1} Scalar Numbers
Scalar numbers are unsigned and are always stored in big-endian
format. Using n[k] to refer to the kth octet being interpreted, the
value of a two-octet scalar is ((n[0] << 8) + n[1]). The value of a
four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) +
n[3]).
3.2. {3.2} Multiprecision Integers
Multiprecision integers (also called MPIs) are unsigned integers used
to hold large integers such as the ones used in cryptographic
calculations.
An MPI consists of two pieces: a two-octet scalar that is the length
of the MPI in bits followed by a string of octets that contain the
actual integer.
These octets form a big-endian number; a big-endian number can be
made into an MPI by prefixing it with the appropriate length.
Examples:
(all numbers are in hexadecimal)
The string of octets [00 01 01] forms an MPI with the value 1. The
string [00 09 01 FF] forms an MPI with the value of 511.
Additional rules:
The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.
The length field of an MPI describes the length starting from its
most significant non-zero bit. Thus, the MPI [00 02 01] is not
formed correctly. It should be [00 01 01].
Unused bits of an MPI MUST be zero.
Also note that when an MPI is encrypted, the length refers to the
plaintext MPI. It may be ill-formed in its ciphertext.
3.3. {3.3} Key IDs
A Key ID is an eight-octet scalar that identifies a key.
Implementations SHOULD NOT assume that Key IDs are unique. The
section "Enhanced Key Formats" below describes how Key IDs are
formed.
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3.4. {3.4} Text
Unless otherwise specified, the character set for text is the UTF-8
[RFC3629] encoding of Unicode [ISO10646].
3.5. {3.5} Time Fields
A time field is an unsigned four-octet number containing the number
of seconds elapsed since midnight, 1 January 1970 UTC.
3.6. {3.6} Keyrings
A keyring is a collection of one or more keys in a file or database.
Traditionally, a keyring is simply a sequential list of keys, but may
be any suitable database. It is beyond the scope of this standard to
discuss the details of keyrings or other databases.
3.7. {3.7} String-to-Key (S2K) Specifiers
String-to-key (S2K) specifiers are used to convert passphrase strings
into symmetric-key encryption/decryption keys. They are used in two
places, currently: to encrypt the secret part of private keys in the
private keyring, and to convert passphrases to encryption keys for
symmetrically encrypted messages.
3.7.1. {3.7.1} String-to-Key (S2K) Specifier Types
There are three types of S2K specifiers currently supported, and some
reserved values:
+-------------+---------------------------+
| ID | S2K Type |
+-------------+---------------------------+
| 0 | Simple S2K |
| 1 | Salted S2K |
| 2 | Reserved value |
| 3 | Iterated and Salted S2K |
| 100 to 110 | Private/Experimental S2K |
+-------------+---------------------------+
These are described in the following Sections.
3.7.1.1. {3.7.1.1} Simple S2K
This directly hashes the string to produce the key data. See below
for how this hashing is done.
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Octet 0: 0x00
Octet 1: hash algorithm
Simple S2K hashes the passphrase to produce the session key. The
manner in which this is done depends on the size of the session key
(which will depend on the cipher used) and the size of the hash
algorithm's output. If the hash size is greater than the session key
size, the high-order (leftmost) octets of the hash are used as the
key.
If the hash size is less than the key size, multiple instances of the
hash context are created -- enough to produce the required key data.
These instances are preloaded with 0, 1, 2, ... octets of zeros (that
is to say, the first instance has no preloading, the second gets
preloaded with 1 octet of zero, the third is preloaded with two
octets of zeros, and so forth).
As the data is hashed, it is given independently to each hash
context. Since the contexts have been initialized differently, they
will each produce different hash output. Once the passphrase is
hashed, the output data from the multiple hashes is concatenated,
first hash leftmost, to produce the key data, with any excess octets
on the right discarded.
3.7.1.2. {3.7.1.2} Salted S2K
This includes a "salt" value in the S2K specifier -- some arbitrary
data -- that gets hashed along with the passphrase string, to help
prevent dictionary attacks.
Octet 0: 0x01
Octet 1: hash algorithm
Octets 2-9: 8-octet salt value
Salted S2K is exactly like Simple S2K, except that the input to the
hash function(s) consists of the 8 octets of salt from the S2K
specifier, followed by the passphrase.
3.7.1.3. {3.7.1.3} Iterated and Salted S2K
This includes both a salt and an octet count. The salt is combined
with the passphrase and the resulting value is hashed repeatedly.
This further increases the amount of work an attacker must do to try
dictionary attacks.
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Octet 0: 0x03
Octet 1: hash algorithm
Octets 2-9: 8-octet salt value
Octet 10: count, a one-octet, coded value
The count is coded into a one-octet number using the following
formula:
#define EXPBIAS 6
count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);
The above formula is in C, where "Int32" is a type for a 32-bit
integer, and the variable "c" is the coded count, Octet 10.
Iterated-Salted S2K hashes the passphrase and salt data multiple
times. The total number of octets to be hashed is specified in the
encoded count in the S2K specifier. Note that the resulting count
value is an octet count of how many octets will be hashed, not an
iteration count.
Initially, one or more hash contexts are set up as with the other S2K
algorithms, depending on how many octets of key data are needed.
Then the salt, followed by the passphrase data, is repeatedly hashed
until the number of octets specified by the octet count has been
hashed. The one exception is that if the octet count is less than
the size of the salt plus passphrase, the full salt plus passphrase
will be hashed even though that is greater than the octet count.
After the hashing is done, the data is unloaded from the hash
context(s) as with the other S2K algorithms.
3.7.2. {3.7.2} String-to-Key Usage
Implementations SHOULD use salted or iterated-and-salted S2K
specifiers, as simple S2K specifiers are more vulnerable to
dictionary attacks.
3.7.2.1. {3.7.2.1} Secret-Key Encryption
An S2K specifier can be stored in the secret keyring to specify how
to convert the passphrase to a key that unlocks the secret data.
Older versions of PGP just stored a cipher algorithm octet preceding
the secret data or a zero to indicate that the secret data was
unencrypted. The MD5 hash function was always used to convert the
passphrase to a key for the specified cipher algorithm.
For compatibility, when an S2K specifier is used, the special value
253, 254, or 255 is stored in the position where the hash algorithm
octet would have been in the old data structure. This is then
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followed immediately by a one-octet algorithm identifier, and then by
the S2K specifier as encoded above.
Therefore, preceding the secret data there will be one of these
possibilities:
0: secret data is unencrypted (no passphrase)
255, 254, or 253: followed by algorithm octet and S2K specifier
Cipher alg: use Simple S2K algorithm using MD5 hash
This last possibility, the cipher algorithm number with an implicit
use of MD5 and IDEA, is provided for backward compatibility; it MAY
be understood, but SHOULD NOT be generated, and is deprecated.
These are followed by an Initial Vector of the same length as the
block size of the cipher for the decryption of the secret values, if
they are encrypted, and then the secret-key values themselves.
3.7.2.2. {3.7.2.2} Symmetric-Key Message Encryption
OpenPGP can create a Symmetric-key Encrypted Session Key (ESK) packet
at the front of a message. This is used to allow S2K specifiers to
be used for the passphrase conversion or to create messages with a
mix of symmetric-key ESKs and public-key ESKs. This allows a message
to be decrypted either with a passphrase or a public-key pair.
PGP 2.X always used IDEA with Simple string-to-key conversion when
encrypting a message with a symmetric algorithm. This is deprecated,
but MAY be used for backward-compatibility.
4. {4} Packet Syntax
This section describes the packets used by OpenPGP.
4.1. {4.1} Overview
An OpenPGP message is constructed from a number of records that are
traditionally called packets. A packet is a chunk of data that has a
tag specifying its meaning. An OpenPGP message, keyring,
certificate, and so forth consists of a number of packets. Some of
those packets may contain other OpenPGP packets (for example, a
compressed data packet, when uncompressed, contains OpenPGP packets).
Each packet consists of a packet header, followed by the packet body.
The packet header is of variable length.
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4.2. {4.2} Packet Headers
The first octet of the packet header is called the "Packet Tag". It
determines the format of the header and denotes the packet contents.
The remainder of the packet header is the length of the packet.
Note that the most significant bit is the leftmost bit, called bit 7.
A mask for this bit is 0x80 in hexadecimal.
+---------------+
PTag |7 6 5 4 3 2 1 0|
+---------------+
Bit 7 -- Always one
Bit 6 -- New packet format if set
PGP 2.6.x only uses old format packets. Thus, software that
interoperates with those versions of PGP must only use old format
packets. If interoperability is not an issue, the new packet format
is RECOMMENDED. Note that old format packets have four bits of
packet tags, and new format packets have six; some features cannot be
used and still be backward-compatible.
Also note that packets with a tag greater than or equal to 16 MUST
use new format packets. The old format packets can only express tags
less than or equal to 15.
Old format packets contain:
Bits 5-2 -- packet tag
Bits 1-0 -- length-type
New format packets contain:
Bits 5-0 -- packet tag
4.2.1. {4.2.1} Old Format Packet Lengths
The meaning of the length-type in old format packets is:
0 The packet has a one-octet length. The header is 2 octets long.
1 The packet has a two-octet length. The header is 3 octets long.
2 The packet has a four-octet length. The header is 5 octets long.
3 The packet is of indeterminate length. The header is 1 octet
long, and the implementation must determine how long the packet
is. If the packet is in a file, this means that the packet
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extends until the end of the file. In general, an implementation
SHOULD NOT use indeterminate-length packets except where the end
of the data will be clear from the context, and even then it is
better to use a definite length, or a new format header. The new
format headers described below have a mechanism for precisely
encoding data of indeterminate length.
4.2.2. {4.2.2} New Format Packet Lengths
New format packets have four possible ways of encoding length:
1. A one-octet Body Length header encodes packet lengths of up to
191 octets.
2. A two-octet Body Length header encodes packet lengths of 192 to
8383 octets.
3. A five-octet Body Length header encodes packet lengths of up to
4,294,967,295 (0xFFFFFFFF) octets in length. (This actually
encodes a four-octet scalar number.)
4. When the length of the packet body is not known in advance by the
issuer, Partial Body Length headers encode a packet of
indeterminate length, effectively making it a stream.
4.2.2.1. {4.2.2.1} One-Octet Lengths
A one-octet Body Length header encodes a length of 0 to 191 octets.
This type of length header is recognized because the one octet value
is less than 192. The body length is equal to:
bodyLen = 1st_octet;
4.2.2.2. {4.2.2.2} Two-Octet Lengths
A two-octet Body Length header encodes a length of 192 to 8383
octets. It is recognized because its first octet is in the range 192
to 223. The body length is equal to:
bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
4.2.2.3. {4.2.2.3} Five-Octet Lengths
A five-octet Body Length header consists of a single octet holding
the value 255, followed by a four-octet scalar. The body length is
equal to:
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bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |
(4th_octet << 8) | 5th_octet
This basic set of one, two, and five-octet lengths is also used
internally to some packets.
4.2.2.4. {4.2.2.4} Partial Body Lengths
A Partial Body Length header is one octet long and encodes the length
of only part of the data packet. This length is a power of 2, from 1
to 1,073,741,824 (2 to the 30th power). It is recognized by its one
octet value that is greater than or equal to 224, and less than 255.
The Partial Body Length is equal to:
partialBodyLen = 1 << (1st_octet & 0x1F);
Each Partial Body Length header is followed by a portion of the
packet body data. The Partial Body Length header specifies this
portion's length. Another length header (one octet, two-octet, five-
octet, or partial) follows that portion. The last length header in
the packet MUST NOT be a Partial Body Length header. Partial Body
Length headers may only be used for the non-final parts of the
packet.
Note also that the last Body Length header can be a zero-length
header.
An implementation MAY use Partial Body Lengths for data packets, be
they literal, compressed, or encrypted. The first partial length
MUST be at least 512 octets long. Partial Body Lengths MUST NOT be
used for any other packet types.
4.2.3. {4.2.3} Packet Length Examples
These examples show ways that new format packets might encode the
packet lengths.
A packet with length 100 may have its length encoded in one octet:
0x64. This is followed by 100 octets of data.
A packet with length 1723 may have its length encoded in two octets:
0xC5, 0xFB. This header is followed by the 1723 octets of data.
A packet with length 100000 may have its length encoded in five
octets: 0xFF, 0x00, 0x01, 0x86, 0xA0.
It might also be encoded in the following octet stream: 0xEF, first
32768 octets of data; 0xE1, next two octets of data; 0xE0, next one
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octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last 1693
octets of data. This is just one possible encoding, and many
variations are possible on the size of the Partial Body Length
headers, as long as a regular Body Length header encodes the last
portion of the data.
Please note that in all of these explanations, the total length of
the packet is the length of the header(s) plus the length of the
body.
4.3. {4.3} Packet Tags
The packet tag denotes what type of packet the body holds. Note that
old format headers can only have tags less than 16, whereas new
format headers can have tags as great as 63. The defined tags (in
decimal) are as follows:
+-----------+-----------------------------------------------------+
| Tag | Packet Type |
+-----------+-----------------------------------------------------+
| 0 | Reserved - a packet tag MUST NOT have this value |
| 1 | Public-Key Encrypted Session Key Packet |
| 2 | Signature Packet |
| 3 | Symmetric-Key Encrypted Session Key Packet |
| 4 | One-Pass Signature Packet |
| 5 | Secret-Key Packet |
| 6 | Public-Key Packet |
| 7 | Secret-Subkey Packet |
| 8 | Compressed Data Packet |
| 9 | Symmetrically Encrypted Data Packet |
| 10 | Marker Packet |
| 11 | Literal Data Packet |
| 12 | Trust Packet |
| 13 | User ID Packet |
| 14 | Public-Subkey Packet |
| 17 | User Attribute Packet |
| 18 | Sym. Encrypted and Integrity Protected Data Packet |
| 19 | Modification Detection Code Packet |
| 20 | AEAD Encrypted Data Packet |
| 60 to 63 | Private or Experimental Values |
+-----------+-----------------------------------------------------+
5. {5} Packet Types
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5.1. {5.1} Public-Key Encrypted Session Key Packets (Tag 1)
A Public-Key Encrypted Session Key packet holds the session key used
to encrypt a message. Zero or more Public-Key Encrypted Session Key
packets and/or Symmetric-Key Encrypted Session Key packets may
precede a Symmetrically Encrypted Data Packet, which holds an
encrypted message. The message is encrypted with the session key,
and the session key is itself encrypted and stored in the Encrypted
Session Key packet(s). The Symmetrically Encrypted Data Packet is
preceded by one Public-Key Encrypted Session Key packet for each
OpenPGP key to which the message is encrypted. The recipient of the
message finds a session key that is encrypted to their public key,
decrypts the session key, and then uses the session key to decrypt
the message.
The body of this packet consists of:
o A one-octet number giving the version number of the packet type.
The currently defined value for packet version is 3.
o An eight-octet number that gives the Key ID of the public key to
which the session key is encrypted. If the session key is
encrypted to a subkey, then the Key ID of this subkey is used here
instead of the Key ID of the primary key.
o A one-octet number giving the public-key algorithm used.
o A string of octets that is the encrypted session key. This string
takes up the remainder of the packet, and its contents are
dependent on the public-key algorithm used.
Algorithm Specific Fields for RSA encryption:
* Multiprecision integer (MPI) of RSA encrypted value m**e mod n.
Algorithm Specific Fields for Elgamal encryption:
* MPI of Elgamal (Diffie-Hellman) value g**k mod p.
* MPI of Elgamal (Diffie-Hellman) value m * y**k mod p.
Algorithm-Specific Fields for ECDH encryption:
* MPI of an EC point representing an ephemeral public key.
* a one-octet size, followed by a symmetric key encoded using the
method described in Section 13.5.
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The value "m" in the above formulas is derived from the session key
as follows. First, the session key is prefixed with a one-octet
algorithm identifier that specifies the symmetric encryption
algorithm used to encrypt the following Symmetrically Encrypted Data
Packet. Then a two-octet checksum is appended, which is equal to the
sum of the preceding session key octets, not including the algorithm
identifier, modulo 65536. This value is then encoded as described in
PKCS#1 block encoding EME-PKCS1-v1_5 in Section 7.2.1 of [RFC3447] to
form the "m" value used in the formulas above. See Section 13.1 of
this document for notes on OpenPGP's use of PKCS#1.
Note that when an implementation forms several PKESKs with one
session key, forming a message that can be decrypted by several keys,
the implementation MUST make a new PKCS#1 encoding for each key.
An implementation MAY accept or use a Key ID of zero as a "wild card"
or "speculative" Key ID. In this case, the receiving implementation
would try all available private keys, checking for a valid decrypted
session key. This format helps reduce traffic analysis of messages.
5.2. {5.2} Signature Packet (Tag 2)
A Signature packet describes a binding between some public key and
some data. The most common signatures are a signature of a file or a
block of text, and a signature that is a certification of a User ID.
Two versions of Signature packets are defined. Version 3 provides
basic signature information, while version 4 provides an expandable
format with subpackets that can specify more information about the
signature. PGP 2.6.x only accepts version 3 signatures.
Implementations SHOULD accept V3 signatures. Implementations SHOULD
generate V4 signatures.
Note that if an implementation is creating an encrypted and signed
message that is encrypted to a V3 key, it is reasonable to create a
V3 signature.
5.2.1. {5.2.1} Signature Types
There are a number of possible meanings for a signature, which are
indicated in a signature type octet in any given signature. Please
note that the vagueness of these meanings is not a flaw, but a
feature of the system. Because OpenPGP places final authority for
validity upon the receiver of a signature, it may be that one
signer's casual act might be more rigorous than some other
authority's positive act. See Section 5.2.4, "Computing Signatures",
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for detailed information on how to compute and verify signatures of
each type.
These meanings are as follows:
0x00 Signature of a binary document. This means the signer owns it,
created it, or certifies that it has not been modified.
0x01 Signature of a canonical text document. This means the signer
owns it, created it, or certifies that it has not been modified.
The signature is calculated over the text data with its line
endings converted to <CR><LF>.
0x02 Standalone signature. This signature is a signature of only
its own subpacket contents. It is calculated identically to a
signature over a zero-length binary document. Note that it
doesn't make sense to have a V3 standalone signature.
0x10 Generic certification of a User ID and Public-Key packet. The
issuer of this certification does not make any particular
assertion as to how well the certifier has checked that the owner
of the key is in fact the person described by the User ID.
0x11 Persona certification of a User ID and Public-Key packet. The
issuer of this certification has not done any verification of the
claim that the owner of this key is the User ID specified.
0x12 Casual certification of a User ID and Public-Key packet. The
issuer of this certification has done some casual verification of
the claim of identity.
0x13 Positive certification of a User ID and Public-Key packet. The
issuer of this certification has done substantial verification of
the claim of identity.
Most OpenPGP implementations make their "key signatures" as 0x10
certifications. Some implementations can issue 0x11-0x13
certifications, but few differentiate between the types.
0x18 Subkey Binding Signature This signature is a statement by the
top-level signing key that indicates that it owns the subkey.
This signature is calculated directly on the primary key and
subkey, and not on any User ID or other packets. A signature that
binds a signing subkey MUST have an Embedded Signature subpacket
in this binding signature that contains a 0x19 signature made by
the signing subkey on the primary key and subkey.
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0x19 Primary Key Binding Signature This signature is a statement by
a signing subkey, indicating that it is owned by the primary key
and subkey. This signature is calculated the same way as a 0x18
signature: directly on the primary key and subkey, and not on any
User ID or other packets.
0x1F Signature directly on a key This signature is calculated
directly on a key. It binds the information in the Signature
subpackets to the key, and is appropriate to be used for
subpackets that provide information about the key, such as the
Revocation Key subpacket. It is also appropriate for statements
that non-self certifiers want to make about the key itself, rather
than the binding between a key and a name.
0x20 Key revocation signature The signature is calculated directly
on the key being revoked. A revoked key is not to be used. Only
revocation signatures by the key being revoked, or by an
authorized revocation key, should be considered valid revocation
signatures.
0x28 Subkey revocation signature The signature is calculated
directly on the subkey being revoked. A revoked subkey is not to
be used. Only revocation signatures by the top-level signature
key that is bound to this subkey, or by an authorized revocation
key, should be considered valid revocation signatures.
0x30 Certification revocation signature This signature revokes an
earlier User ID certification signature (signature class 0x10
through 0x13) or direct-key signature (0x1F). It should be issued
by the same key that issued the revoked signature or an authorized
revocation key. The signature is computed over the same data as
the certificate that it revokes, and should have a later creation
date than that certificate.
0x40 Timestamp signature. This signature is only meaningful for the
timestamp contained in it.
0x50 Third-Party Confirmation signature. This signature is a
signature over some other OpenPGP Signature packet(s). It is
analogous to a notary seal on the signed data. A third-party
signature SHOULD include Signature Target subpacket(s) to give
easy identification. Note that we really do mean SHOULD. There
are plausible uses for this (such as a blind party that only sees
the signature, not the key or source document) that cannot include
a target subpacket.
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5.2.2. {5.2.2} Version 3 Signature Packet Format
The body of a version 3 Signature Packet contains:
o One-octet version number (3).
o One-octet length of following hashed material. MUST be 5.
o One-octet signature type.
o Four-octet creation time.
o Eight-octet Key ID of signer.
o One-octet public-key algorithm.
o One-octet hash algorithm.
o Two-octet field holding left 16 bits of signed hash value.
o One or more multiprecision integers comprising the signature.
This portion is algorithm specific, as described below.
The concatenation of the data to be signed, the signature type,
and creation time from the Signature packet (5 additional octets)
is hashed. The resulting hash value is used in the signature
algorithm. The high 16 bits (first two octets) of the hash are
included in the Signature packet to provide a quick test to reject
some invalid signatures.
Algorithm-Specific Fields for RSA signatures:
* Multiprecision integer (MPI) of RSA signature value m**d mod n.
Algorithm-Specific Fields for DSA and ECDSA signatures:
* MPI of DSA or ECDSA value r.
* MPI of DSA or ECDSA value s.
The signature calculation is based on a hash of the signed data, as
described above. The details of the calculation are different for
DSA signatures than for RSA signatures.
With RSA signatures, the hash value is encoded using PKCS#1 encoding
type EMSA-PKCS1-v1_5 as described in Section 9.2 of RFC 3447. This
requires inserting the hash value as an octet string into an ASN.1
structure. The object identifier for the type of hash being used is
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included in the structure. The hexadecimal representations for the
currently defined hash algorithms are as follows:
- MD5: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05
- RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01
- SHA-1: 0x2B, 0x0E, 0x03, 0x02, 0x1A
- SHA2-224: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04
- SHA2-256: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01
- SHA2-384: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02
- SHA2-512: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03
The ASN.1 Object Identifiers (OIDs) are as follows:
- MD5: 1.2.840.113549.2.5
- RIPEMD-160: 1.3.36.3.2.1
- SHA-1: 1.3.14.3.2.26
- SHA2-224: 2.16.840.1.101.3.4.2.4
- SHA2-256: 2.16.840.1.101.3.4.2.1
- SHA2-384: 2.16.840.1.101.3.4.2.2
- SHA2-512: 2.16.840.1.101.3.4.2.3
The full hash prefixes for these are as follows:
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- MD5: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00,
0x04, 0x10
- RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24,
0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14
- SHA-1: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E,
0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14
- SHA2-224: 0x30, 0x2D, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04, 0x05,
0x00, 0x04, 0x1C
- SHA2-256: 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,
0x00, 0x04, 0x20
- SHA2-384: 0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05,
0x00, 0x04, 0x30
- SHA2-512: 0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05,
0x00, 0x04, 0x40
DSA signatures MUST use hashes that are equal in size to the number
of bits of q, the group generated by the DSA key's generator value.
If the output size of the chosen hash is larger than the number of
bits of q, the hash result is truncated to fit by taking the number
of leftmost bits equal to the number of bits of q. This (possibly
truncated) hash function result is treated as a number and used
directly in the DSA signature algorithm.
5.2.3. {5.2.3} Version 4 Signature Packet Format
The body of a version 4 Signature packet contains:
o One-octet version number (4).
o One-octet signature type.
o One-octet public-key algorithm.
o One-octet hash algorithm.
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o Two-octet scalar octet count for following hashed subpacket data.
Note that this is the length in octets of all of the hashed
subpackets; a pointer incremented by this number will skip over
the hashed subpackets.
o Hashed subpacket data set (zero or more subpackets).
o Two-octet scalar octet count for the following unhashed subpacket
data. Note that this is the length in octets of all of the
unhashed subpackets; a pointer incremented by this number will
skip over the unhashed subpackets.
o Unhashed subpacket data set (zero or more subpackets).
o Two-octet field holding the left 16 bits of the signed hash value.
o One or more multiprecision integers comprising the signature.
This portion is algorithm specific:
Algorithm-Specific Fields for RSA signatures:
* Multiprecision integer (MPI) of RSA signature value m**d mod n.
Algorithm-Specific Fields for DSA or ECDSA signatures:
* MPI of DSA or ECDSA value r.
* MPI of DSA or ECDSA value s.
Algorithm-Specific Fields for EdDSA signatures:
* MPI of EdDSA compressed value r.
* MPI of EdDSA compressed value s.
The compressed version of R and S for use with EdDSA is described in
[I-D.irtf-cfrg-eddsa]. The version 3 signature format MUST NOT be
used with EdDSA.
The concatenation of the data being signed and the signature data
from the version number through the hashed subpacket data (inclusive)
is hashed. The resulting hash value is what is signed. The left 16
bits of the hash are included in the Signature packet to provide a
quick test to reject some invalid signatures.
There are two fields consisting of Signature subpackets. The first
field is hashed with the rest of the signature data, while the second
is unhashed. The second set of subpackets is not cryptographically
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protected by the signature and should include only advisory
information.
The algorithms for converting the hash function result to a signature
are described in a section below.
5.2.3.1. {5.2.3.1} Signature Subpacket Specification
A subpacket data set consists of zero or more Signature subpackets.
In Signature packets, the subpacket data set is preceded by a two-
octet scalar count of the length in octets of all the subpackets. A
pointer incremented by this number will skip over the subpacket data
set.
Each subpacket consists of a subpacket header and a body. The header
consists of:
o the subpacket length (1, 2, or 5 octets),
o the subpacket type (1 octet),
and is followed by the subpacket-specific data.
The length includes the type octet but not this length. Its format
is similar to the "new" format packet header lengths, but cannot have
Partial Body Lengths. That is:
if the 1st octet < 192, then
lengthOfLength = 1
subpacketLen = 1st_octet
if the 1st octet >= 192 and < 255, then
lengthOfLength = 2
subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
if the 1st octet = 255, then
lengthOfLength = 5
subpacket length = [four-octet scalar starting at 2nd_octet]
The value of the subpacket type octet may be:
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+-------------+-----------------------------------------+
| Type | Description |
+-------------+-----------------------------------------+
| 0 | Reserved |
| 1 | Reserved |
| 2 | Signature Creation Time |
| 3 | Signature Expiration Time |
| 4 | Exportable Certification |
| 5 | Trust Signature |
| 6 | Regular Expression |
| 7 | Revocable |
| 8 | Reserved |
| 9 | Key Expiration Time |
| 10 | Placeholder for backward compatibility |
| 11 | Preferred Symmetric Algorithms |
| 12 | Revocation Key |
| 13 to 15 | Reserved |
| 16 | Issuer |
| 17 to 19 | Reserved |
| 20 | Notation Data |
| 21 | Preferred Hash Algorithms |
| 22 | Preferred Compression Algorithms |
| 23 | Key Server Preferences |
| 24 | Preferred Key Server |
| 25 | Primary User ID |
| 26 | Policy URI |
| 27 | Key Flags |
| 28 | Signer's User ID |
| 29 | Reason for Revocation |
| 30 | Features |
| 31 | Signature Target |
| 32 | Embedded Signature |
| 33 | Issuer Fingerprint |
| 100 to 110 | Private or experimental |
+-------------+-----------------------------------------+
An implementation SHOULD ignore any subpacket of a type that it does
not recognize.
Bit 7 of the subpacket type is the "critical" bit. If set, it
denotes that the subpacket is one that is critical for the evaluator
of the signature to recognize. If a subpacket is encountered that is
marked critical but is unknown to the evaluating software, the
evaluator SHOULD consider the signature to be in error.
An evaluator may "recognize" a subpacket, but not implement it. The
purpose of the critical bit is to allow the signer to tell an
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evaluator that it would prefer a new, unknown feature to generate an
error than be ignored.
Implementations SHOULD implement the three preferred algorithm
subpackets (11, 21, and 22), as well as the "Reason for Revocation"
subpacket. Note, however, that if an implementation chooses not to
implement some of the preferences, it is required to behave in a
polite manner to respect the wishes of those users who do implement
these preferences.
5.2.3.2. {5.2.3.2} Signature Subpacket Types
A number of subpackets are currently defined. Some subpackets apply
to the signature itself and some are attributes of the key.
Subpackets that are found on a self-signature are placed on a
certification made by the key itself. Note that a key may have more
than one User ID, and thus may have more than one self-signature, and
differing subpackets.
A subpacket may be found either in the hashed or unhashed subpacket
sections of a signature. If a subpacket is not hashed, then the
information in it cannot be considered definitive because it is not
part of the signature proper.
5.2.3.3. {5.2.3.3} Notes on Self-Signatures
A self-signature is a binding signature made by the key to which the
signature refers. There are three types of self-signatures, the
certification signatures (types 0x10-0x13), the direct-key signature
(type 0x1F), and the subkey binding signature (type 0x18). For
certification self-signatures, each User ID may have a self-
signature, and thus different subpackets in those self-signatures.
For subkey binding signatures, each subkey in fact has a self-
signature. Subpackets that appear in a certification self-signature
apply to the user name, and subpackets that appear in the subkey
self-signature apply to the subkey. Lastly, subpackets on the
direct-key signature apply to the entire key.
Implementing software should interpret a self-signature's preference
subpackets as narrowly as possible. For example, suppose a key has
two user names, Alice and Bob. Suppose that Alice prefers the
symmetric algorithm CAST5, and Bob prefers IDEA or TripleDES. If the
software locates this key via Alice's name, then the preferred
algorithm is CAST5; if software locates the key via Bob's name, then
the preferred algorithm is IDEA. If the key is located by Key ID,
the algorithm of the primary User ID of the key provides the
preferred symmetric algorithm.
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Revoking a self-signature or allowing it to expire has a semantic
meaning that varies with the signature type. Revoking the self-
signature on a User ID effectively retires that user name. The self-
signature is a statement, "My name X is tied to my signing key K" and
is corroborated by other users' certifications. If another user
revokes their certification, they are effectively saying that they no
longer believe that name and that key are tied together. Similarly,
if the users themselves revoke their self-signature, then the users
no longer go by that name, no longer have that email address, etc.
Revoking a binding signature effectively retires that subkey.
Revoking a direct-key signature cancels that signature. Please see
the "Reason for Revocation" subpacket (Section 5.2.3.23) for more
relevant detail.
Since a self-signature contains important information about the key's
use, an implementation SHOULD allow the user to rewrite the self-
signature, and important information in it, such as preferences and
key expiration.
It is good practice to verify that a self-signature imported into an
implementation doesn't advertise features that the implementation
doesn't support, rewriting the signature as appropriate.
An implementation that encounters multiple self-signatures on the
same object may resolve the ambiguity in any way it sees fit, but it
is RECOMMENDED that priority be given to the most recent self-
signature.
5.2.3.4. {5.2.3.4} Signature Creation Time
(4-octet time field)
The time the signature was made.
MUST be present in the hashed area.
5.2.3.5. {5.2.3.5} Issuer
(8-octet Key ID)
The OpenPGP Key ID of the key issuing the signature. If the version
of that key is greater than 4, this subpacket MUST NOT be included in
the signature.
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5.2.3.6. {5.2.3.6} Key Expiration Time
(4-octet time field)
The validity period of the key. This is the number of seconds after
the key creation time that the key expires. If this is not present
or has a value of zero, the key never expires. This is found only on
a self-signature.
5.2.3.7. {5.2.3.7} Preferred Symmetric Algorithms
(array of one-octet values)
Symmetric algorithm numbers that indicate which algorithms the key
holder prefers to use. The subpacket body is an ordered list of
octets with the most preferred listed first. It is assumed that only
algorithms listed are supported by the recipient's software.
Algorithm numbers are in Section 9. This is only found on a self-
signature.
5.2.3.8. {5.2.3.8} Preferred Hash Algorithms
(array of one-octet values)
Message digest algorithm numbers that indicate which algorithms the
key holder prefers to receive. Like the preferred symmetric
algorithms, the list is ordered. Algorithm numbers are in Section 9.
This is only found on a self-signature.
5.2.3.9. {5.2.3.9} Preferred Compression Algorithms
(array of one-octet values)
Compression algorithm numbers that indicate which algorithms the key
holder prefers to use. Like the preferred symmetric algorithms, the
list is ordered. Algorithm numbers are in Section 9. If this
subpacket is not included, ZIP is preferred. A zero denotes that
uncompressed data is preferred; the key holder's software might have
no compression software in that implementation. This is only found
on a self-signature.
5.2.3.10. {5.2.3.10} Signature Expiration Time
(4-octet time field)
The validity period of the signature. This is the number of seconds
after the signature creation time that the signature expires. If
this is not present or has a value of zero, it never expires.
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5.2.3.11. {5.2.3.11} Exportable Certification
(1 octet of exportability, 0 for not, 1 for exportable)
This subpacket denotes whether a certification signature is
"exportable", to be used by other users than the signature's issuer.
The packet body contains a Boolean flag indicating whether the
signature is exportable. If this packet is not present, the
certification is exportable; it is equivalent to a flag containing a
1.
Non-exportable, or "local", certifications are signatures made by a
user to mark a key as valid within that user's implementation only.
Thus, when an implementation prepares a user's copy of a key for
transport to another user (this is the process of "exporting" the
key), any local certification signatures are deleted from the key.
The receiver of a transported key "imports" it, and likewise trims
any local certifications. In normal operation, there won't be any,
assuming the import is performed on an exported key. However, there
are instances where this can reasonably happen. For example, if an
implementation allows keys to be imported from a key database in
addition to an exported key, then this situation can arise.
Some implementations do not represent the interest of a single user
(for example, a key server). Such implementations always trim local
certifications from any key they handle.
5.2.3.12. {5.2.3.12} Revocable
(1 octet of revocability, 0 for not, 1 for revocable)
Signature's revocability status. The packet body contains a Boolean
flag indicating whether the signature is revocable. Signatures that
are not revocable have any later revocation signatures ignored. They
represent a commitment by the signer that he cannot revoke his
signature for the life of his key. If this packet is not present,
the signature is revocable.
5.2.3.13. {5.2.3.13} Trust Signature
(1 octet "level" (depth), 1 octet of trust amount)
Signer asserts that the key is not only valid but also trustworthy at
the specified level. Level 0 has the same meaning as an ordinary
validity signature. Level 1 means that the signed key is asserted to
be a valid trusted introducer, with the 2nd octet of the body
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specifying the degree of trust. Level 2 means that the signed key is
asserted to be trusted to issue level 1 trust signatures, i.e., that
it is a "meta introducer". Generally, a level n trust signature
asserts that a key is trusted to issue level n-1 trust signatures.
The trust amount is in a range from 0-255, interpreted such that
values less than 120 indicate partial trust and values of 120 or
greater indicate complete trust. Implementations SHOULD emit values
of 60 for partial trust and 120 for complete trust.
5.2.3.14. {5.2.3.14} Regular Expression
(null-terminated regular expression)
Used in conjunction with trust Signature packets (of level > 0) to
limit the scope of trust that is extended. Only signatures by the
target key on User IDs that match the regular expression in the body
of this packet have trust extended by the trust Signature subpacket.
The regular expression uses the same syntax as the Henry Spencer's
"almost public domain" regular expression [REGEX] package. A
description of the syntax is found in Section 8 below.
5.2.3.15. {5.2.3.15} Revocation Key
(1 octet of class, 1 octet of public-key algorithm ID, 20 or 25
octets of fingerprint)
V4 keys use the full 20 octet fingerprint; V5 keys use the leftmost
25 octets of the fingerprint
Authorizes the specified key to issue revocation signatures for this
key. Class octet must have bit 0x80 set. If the bit 0x40 is set,
then this means that the revocation information is sensitive. Other
bits are for future expansion to other kinds of authorizations. This
is found on a self-signature.
If the "sensitive" flag is set, the keyholder feels this subpacket
contains private trust information that describes a real-world
sensitive relationship. If this flag is set, implementations SHOULD
NOT export this signature to other users except in cases where the
data needs to be available: when the signature is being sent to the
designated revoker, or when it is accompanied by a revocation
signature from that revoker. Note that it may be appropriate to
isolate this subpacket within a separate signature so that it is not
combined with other subpackets that need to be exported.
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5.2.3.16. {5.2.3.16} Notation Data
(4 octets of flags, 2 octets of name length (M),
2 octets of value length (N),
M octets of name data,
N octets of value data)
This subpacket describes a "notation" on the signature that the
issuer wishes to make. The notation has a name and a value, each of
which are strings of octets. There may be more than one notation in
a signature. Notations can be used for any extension the issuer of
the signature cares to make. The "flags" field holds four octets of
flags.
All undefined flags MUST be zero. Defined flags are as follows:
First octet: 0x80 = human-readable. This note value is text.
Other octets: none.
Notation names are arbitrary strings encoded in UTF-8. They reside
in two namespaces: The IETF namespace and the user namespace.
The IETF namespace is registered with IANA. These names MUST NOT
contain the "@" character (0x40). This is a tag for the user
namespace.
Names in the user namespace consist of a UTF-8 string tag followed by
"@" followed by a DNS domain name. Note that the tag MUST NOT
contain an "@" character. For example, the "sample" tag used by
Example Corporation could be "sample@example.com".
Names in a user space are owned and controlled by the owners of that
domain. Obviously, it's bad form to create a new name in a DNS space
that you don't own.
Since the user namespace is in the form of an email address,
implementers MAY wish to arrange for that address to reach a person
who can be consulted about the use of the named tag. Note that due
to UTF-8 encoding, not all valid user space name tags are valid email
addresses.
If there is a critical notation, the criticality applies to that
specific notation and not to notations in general.
The following subsections define a set of standard notations.
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5.2.3.16.1. The 'manu' Notation
The "manu" notation is a string that declares the device
manufacturer's name. The certifier key is asserting this string
(which may or may not be related to the User ID of the certifier's
key).
5.2.3.16.2. The 'make' Notation
This notation defines the product make. It is a free form string.
5.2.3.16.3. The 'model' Notation
This notation defines the product model name/number. It is a free
form string.
5.2.3.16.4. The 'prodid' Notation
This notation contains the product identifier. It is a free form
string.
5.2.3.16.5. The 'pvers' Notation
This notation defines the product version number (which could be a
release number, year, or some other identifier to differentiate
different versions of the same make/model). It is a free form
string.
5.2.3.16.6. The 'lot' Notation
This notation defines the product lot number (which is an indicator
of the batch of product). It is a free form string.
5.2.3.16.7. The 'qty' Notation
This notation defines the quantity of items in this package. It is a
decimal integer representation with no punctuation, e.g. "10",
"1000", "10000", etc.
5.2.3.16.8. The 'loc' and 'dest' Notations
The "loc" and 'dest' notations declare a GeoLocation as defined by
RFC 5870 [RFC5870] but without the leading "geo:" header. For
example, if you had a GeoLocation URI of "geo:13.4125,103.8667" you
would encode that in these notations as "13.4125,103.8667".
The 'loc' notation is meant to encode the geo location where the
signature was made. The 'dest' notation is meant to encode the geo
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location where the device is "destined" (i.e., a "destination" for
the device).
5.2.3.16.9. The 'hash' Notation
A 'hash' notation is a means to include external data in the contents
of a signature without including the data itself. This is done by
hashing the external data separately and then including the data's
name and hash in the signature via this notation. This is useful,
for example, to have an external "manifest," "image," or other data
that might not be vital to the signature itself but still needs to be
protected and authenticated without requiring a second signature.
The 'hash' notation has the following structure: * A single byte
specifying the length of the name of the hashed data * A UTF-8 string
of the name of the hashed data * A single byte specifying the hash
algorithm (see section 9.4) * The binary hash output of the hashed
data using the specified algorithm. (The length of this data is
implicit based on the algorithm specified).
Due to its nature a 'hash' notation is not human readable and MUST
NOT be marked as such when used.
5.2.3.17. {5.2.3.17} Key Server Preferences
(N octets of flags)
This is a list of one-bit flags that indicate preferences that the
key holder has about how the key is handled on a key server. All
undefined flags MUST be zero.
First octet: 0x80 = No-modify the key holder requests that this key
only be modified or updated by the key holder or an administrator of
the key server.
This is found only on a self-signature.
5.2.3.18. {5.2.3.18} Preferred Key Server
(String)
This is a URI of a key server that the key holder prefers be used for
updates. Note that keys with multiple User IDs can have a preferred
key server for each User ID. Note also that since this is a URI, the
key server can actually be a copy of the key retrieved by ftp, http,
finger, etc.
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5.2.3.19. {5.2.3.19} Primary User ID
(1 octet, Boolean)
This is a flag in a User ID's self-signature that states whether this
User ID is the main User ID for this key. It is reasonable for an
implementation to resolve ambiguities in preferences, etc. by
referring to the primary User ID. If this flag is absent, its value
is zero. If more than one User ID in a key is marked as primary, the
implementation may resolve the ambiguity in any way it sees fit, but
it is RECOMMENDED that priority be given to the User ID with the most
recent self-signature.
When appearing on a self-signature on a User ID packet, this
subpacket applies only to User ID packets. When appearing on a self-
signature on a User Attribute packet, this subpacket applies only to
User Attribute packets. That is to say, there are two different and
independent "primaries" -- one for User IDs, and one for User
Attributes.
5.2.3.20. {5.2.3.20} Policy URI
(String)
This subpacket contains a URI of a document that describes the policy
under which the signature was issued.
5.2.3.21. {5.2.3.21} Key Flags
(N octets of flags)
This subpacket contains a list of binary flags that hold information
about a key. It is a string of octets, and an implementation MUST
NOT assume a fixed size. This is so it can grow over time. If a
list is shorter than an implementation expects, the unstated flags
are considered to be zero. The defined flags are as follows:
0x01 This key may be used to certify other keys.
0x02 This key may be used to sign data.
0x04 This key may be used to encrypt communications.
0x08 This key may be used to encrypt storage.
0x10 The private component of this key may have been split by a
secret-sharing mechanism.
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0x20 This key may be used for authentication.
0x80 The private component of this key may be in the possession of
more than one person.
Usage notes:
The flags in this packet may appear in self-signatures or in
certification signatures. They mean different things depending on
who is making the statement --- for example, a certification
signature that has the "sign data" flag is stating that the
certification is for that use. On the other hand, the
"communications encryption" flag in a self-signature is stating a
preference that a given key be used for communications. Note
however, that it is a thorny issue to determine what is
"communications" and what is "storage". This decision is left wholly
up to the implementation; the authors of this document do not claim
any special wisdom on the issue and realize that accepted opinion may
change.
The "split key" (0x10) and "group key" (0x80) flags are placed on a
self-signature only; they are meaningless on a certification
signature. They SHOULD be placed only on a direct-key signature
(type 0x1F) or a subkey signature (type 0x18), one that refers to the
key the flag applies to.
5.2.3.22. {5.2.3.22} Signer's User ID
(String)
This subpacket allows a keyholder to state which User ID is
responsible for the signing. Many keyholders use a single key for
different purposes, such as business communications as well as
personal communications. This subpacket allows such a keyholder to
state which of their roles is making a signature.
This subpacket is not appropriate to use to refer to a User Attribute
packet.
5.2.3.23. {5.2.3.23} Reason for Revocation
(1 octet of revocation code, N octets of reason string)
This subpacket is used only in key revocation and certification
revocation signatures. It describes the reason why the key or
certificate was revoked.
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The first octet contains a machine-readable code that denotes the
reason for the revocation:
+----------+--------------------------------------------------------+
| Code | Reason |
+----------+--------------------------------------------------------+
| 0 | No reason specified (key revocations or cert |
| | revocations) |
| 1 | Key is superseded (key revocations) |
| 2 | Key material has been compromised (key revocations) |
| 3 | Key is retired and no longer used (key revocations) |
| 32 | User ID information is no longer valid (cert |
| | revocations) |
| 100-110 | Private Use |
+----------+--------------------------------------------------------+
Following the revocation code is a string of octets that gives
information about the Reason for Revocation in human-readable form
(UTF-8). The string may be null, that is, of zero length. The
length of the subpacket is the length of the reason string plus one.
An implementation SHOULD implement this subpacket, include it in all
revocation signatures, and interpret revocations appropriately.
There are important semantic differences between the reasons, and
there are thus important reasons for revoking signatures.
If a key has been revoked because of a compromise, all signatures
created by that key are suspect. However, if it was merely
superseded or retired, old signatures are still valid. If the
revoked signature is the self-signature for certifying a User ID, a
revocation denotes that that user name is no longer in use. Such a
revocation SHOULD include a 0x20 code.
Note that any signature may be revoked, including a certification on
some other person's key. There are many good reasons for revoking a
certification signature, such as the case where the keyholder leaves
the employ of a business with an email address. A revoked
certification is no longer a part of validity calculations.
5.2.3.24. {5.2.3.24} Features
(N octets of flags)
The Features subpacket denotes which advanced OpenPGP features a
user's implementation supports. This is so that as features are
added to OpenPGP that cannot be backwards-compatible, a user can
state that they can use that feature. The flags are single bits that
indicate that a given feature is supported.
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This subpacket is similar to a preferences subpacket, and only
appears in a self-signature.
An implementation SHOULD NOT use a feature listed when sending to a
user who does not state that they can use it.
Defined features are as follows:
First octet:
0x01 - Modification Detection (packets 18 and 19)
0x02 - AEAD Encrypted Data Packet (packet 20) and version 5
Symmetric-Key Encrypted Session Key Packets (packet 3)
If an implementation implements any of the defined features, it
SHOULD implement the Features subpacket, too.
An implementation may freely infer features from other suitable
implementation-dependent mechanisms.
5.2.3.25. {5.2.3.25} Signature Target
(1 octet public-key algorithm, 1 octet hash algorithm, N octets hash)
This subpacket identifies a specific target signature to which a
signature refers. For revocation signatures, this subpacket provides
explicit designation of which signature is being revoked. For a
third-party or timestamp signature, this designates what signature is
signed. All arguments are an identifier of that target signature.
The N octets of hash data MUST be the size of the hash of the
signature. For example, a target signature with a SHA-1 hash MUST
have 20 octets of hash data.
5.2.3.26. {5.2.3.26} Embedded Signature
(1 signature packet body)
This subpacket contains a complete Signature packet body as specified
in Section 5.2 above. It is useful when one signature needs to refer
to, or be incorporated in, another signature.
5.2.3.27. Issuer Fingerprint
(1 octet key version number, N octets of fingerprint)
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The OpenPGP Key fingerprint of the key issuing the signature. This
subpacket SHOULD be included in all signatures. If the version of
the issuing key is 4 and an Issuer subpacket is also included in the
signature, the key ID of the Issuer subpacket MUST match the low 64
bits of the fingerprint.
Note that the length N of the fingerprint for a version 4 key is 20
octets. For a version 5 key the leftmost 25 octets of the
fingerprint are used (N=25).
5.2.4. {5.2.4} Computing Signatures
All signatures are formed by producing a hash over the signature
data, and then using the resulting hash in the signature algorithm.
For binary document signatures (type 0x00), the document data is
hashed directly. For text document signatures (type 0x01), the
document is canonicalized by converting line endings to <CR><LF>, and
the resulting data is hashed.
When a signature is made over a key, the hash data starts with the
octet 0x99, followed by a two-octet length of the key, and then body
of the key packet. (Note that this is an old-style packet header for
a key packet with two-octet length.) A subkey binding signature
(type 0x18) or primary key binding signature (type 0x19) then hashes
the subkey using the same format as the main key (also using 0x99 as
the first octet). Primary key revocation signatures (type 0x20) hash
only the key being revoked. Subkey revocation signature (type 0x28)
hash first the primary key and then the subkey being revoked.
A certification signature (type 0x10 through 0x13) hashes the User ID
being bound to the key into the hash context after the above data. A
V3 certification hashes the contents of the User ID or attribute
packet packet, without any header. A V4 or V5 certification hashes
the constant 0xB4 for User ID certifications or the constant 0xD1 for
User Attribute certifications, followed by a four-octet number giving
the length of the User ID or User Attribute data, and then the User
ID or User Attribute data.
When a signature is made over a Signature packet (type 0x50), the
hash data starts with the octet 0x88, followed by the four-octet
length of the signature, and then the body of the Signature packet.
(Note that this is an old-style packet header for a Signature packet
with the length-of-length set to zero.) The unhashed subpacket data
of the Signature packet being hashed is not included in the hash, and
the unhashed subpacket data length value is set to zero.
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Once the data body is hashed, then a trailer is hashed. A V3
signature hashes five octets of the packet body, starting from the
signature type field. This data is the signature type, followed by
the four-octet signature time. A V4 or V5 signature hashes the
packet body starting from its first field, the version number,
through the end of the hashed subpacket data. Thus, the fields
hashed are the signature version, the signature type, the public-key
algorithm, the hash algorithm, the hashed subpacket length, and the
hashed subpacket body.
V4 signatures also hash in a final trailer of six octets: the version
of the Signature packet, i.e., 0x04; 0xFF; and a four-octet, big-
endian number that is the length of the hashed data from the
Signature packet (note that this number does not include these final
six octets).
V5 signatures instead hash in a ten-octet trailer: the version of the
Signature packet, i.e., 0x05; 0xFF; and an eight-octet, big-endian
number that is the length of the hashed data from the Signature
packet (note that this number does not include these final ten
octets).
After all this has been hashed in a single hash context, the
resulting hash field is used in the signature algorithm and placed at
the end of the Signature packet.
5.2.4.1. {5.2.4.1} Subpacket Hints
It is certainly possible for a signature to contain conflicting
information in subpackets. For example, a signature may contain
multiple copies of a preference or multiple expiration times. In
most cases, an implementation SHOULD use the last subpacket in the
signature, but MAY use any conflict resolution scheme that makes more
sense. Please note that we are intentionally leaving conflict
resolution to the implementer; most conflicts are simply syntax
errors, and the wishy-washy language here allows a receiver to be
generous in what they accept, while putting pressure on a creator to
be stingy in what they generate.
Some apparent conflicts may actually make sense -- for example,
suppose a keyholder has a V3 key and a V4 key that share the same RSA
key material. Either of these keys can verify a signature created by
the other, and it may be reasonable for a signature to contain an
issuer subpacket for each key, as a way of explicitly tying those
keys to the signature.
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5.3. {5.3} Symmetric-Key Encrypted Session Key Packets (Tag 3)
The Symmetric-Key Encrypted Session Key packet holds the symmetric-
key encryption of a session key used to encrypt a message. Zero or
more Public-Key Encrypted Session Key packets and/or Symmetric-Key
Encrypted Session Key packets may precede a Symmetrically Encrypted
Data packet that holds an encrypted message. The message is
encrypted with a session key, and the session key is itself encrypted
and stored in the Encrypted Session Key packet or the Symmetric-Key
Encrypted Session Key packet.
If the Symmetrically Encrypted Data packet is preceded by one or more
Symmetric-Key Encrypted Session Key packets, each specifies a
passphrase that may be used to decrypt the message. This allows a
message to be encrypted to a number of public keys, and also to one
or more passphrases. This packet type is new and is not generated by
PGP 2.x or PGP 5.0.
A version 4 Symmetric-Key Encrypted Session Key packet consists of:
o A one-octet version number with value 4.
o A one-octet number describing the symmetric algorithm used.
o A string-to-key (S2K) specifier, length as defined above.
o Optionally, the encrypted session key itself, which is decrypted
with the string-to-key object.
If the encrypted session key is not present (which can be detected on
the basis of packet length and S2K specifier size), then the S2K
algorithm applied to the passphrase produces the session key for
decrypting the file, using the symmetric cipher algorithm from the
Symmetric-Key Encrypted Session Key packet.
If the encrypted session key is present, the result of applying the
S2K algorithm to the passphrase is used to decrypt just that
encrypted session key field, using CFB mode with an IV of all zeros.
The decryption result consists of a one-octet algorithm identifier
that specifies the symmetric-key encryption algorithm used to encrypt
the following Symmetrically Encrypted Data packet, followed by the
session key octets themselves.
Note: because an all-zero IV is used for this decryption, the S2K
specifier MUST use a salt value, either a Salted S2K or an Iterated-
Salted S2K. The salt value will ensure that the decryption key is
not repeated even if the passphrase is reused.
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A version 5 Symmetric-Key Encrypted Session Key packet consists of:
o A one-octet version number with value 5.
o A one-octet cipher algorithm.
o A one-octet AEAD algorithm.
o A string-to-key (S2K) specifier, length as defined above.
o A starting initialization vector of size specified by the AEAD
algorithm.
o The encrypted session key itself, which is decrypted with the
string-to-key object using the given cipher and AEAD mode.
o A final, summary authentication tag for the AEAD mode.
The encrypted session key is encrypted exactly as an AEAD Encrypted
Data packet with a chunk size octet of 10 would be. This implicit
chunk size octet is included in the normal calculations of additional
data.
5.4. {5.4} One-Pass Signature Packets (Tag 4)
The One-Pass Signature packet precedes the signed data and contains
enough information to allow the receiver to begin calculating any
hashes needed to verify the signature. It allows the Signature
packet to be placed at the end of the message, so that the signer can
compute the entire signed message in one pass.
A One-Pass Signature does not interoperate with PGP 2.6.x or earlier.
The body of this packet consists of:
o A one-octet version number. The current version is 3.
o A one-octet signature type. Signature types are described in
Section 5.2.1.
o A one-octet number describing the hash algorithm used.
o A one-octet number describing the public-key algorithm used.
o An eight-octet number holding the Key ID of the signing key.
o A one-octet number holding a flag showing whether the signature is
nested. A zero value indicates that the next packet is another
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One-Pass Signature packet that describes another signature to be
applied to the same message data.
Note that if a message contains more than one one-pass signature,
then the Signature packets bracket the message; that is, the first
Signature packet after the message corresponds to the last one-pass
packet and the final Signature packet corresponds to the first one-
pass packet.
5.5. {5.5} Key Material Packet
A key material packet contains all the information about a public or
private key. There are four variants of this packet type, and two
major versions. Consequently, this section is complex.
5.5.1. {5.5.1} Key Packet Variants
5.5.1.1. {5.5.1.1} Public-Key Packet (Tag 6)
A Public-Key packet starts a series of packets that forms an OpenPGP
key (sometimes called an OpenPGP certificate).
5.5.1.2. {5.5.1.2} Public-Subkey Packet (Tag 14)
A Public-Subkey packet (tag 14) has exactly the same format as a
Public-Key packet, but denotes a subkey. One or more subkeys may be
associated with a top-level key. By convention, the top-level key
provides signature services, and the subkeys provide encryption
services.
Note: in PGP 2.6.x, tag 14 was intended to indicate a comment packet.
This tag was selected for reuse because no previous version of PGP
ever emitted comment packets but they did properly ignore them.
Public-Subkey packets are ignored by PGP 2.6.x and do not cause it to
fail, providing a limited degree of backward compatibility.
5.5.1.3. {5.5.1.3} Secret-Key Packet (Tag 5)
A Secret-Key packet contains all the information that is found in a
Public-Key packet, including the public-key material, but also
includes the secret-key material after all the public-key fields.
5.5.1.4. {5.5.1.4} Secret-Subkey Packet (Tag 7)
A Secret-Subkey packet (tag 7) is the subkey analog of the Secret Key
packet and has exactly the same format.
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5.5.2. {5.5.2} Public-Key Packet Formats
There are two versions of key-material packets. Version 3 packets
were first generated by PGP 2.6. Version 4 keys first appeared in
PGP 5.0 and are the preferred key version for OpenPGP.
OpenPGP implementations MUST create keys with version 4 format. V3
keys are deprecated; an implementation MUST NOT generate a V3 key,
but MAY accept it.
A version 3 public key or public-subkey packet contains:
o A one-octet version number (3).
o A four-octet number denoting the time that the key was created.
o A two-octet number denoting the time in days that this key is
valid. If this number is zero, then it does not expire.
o A one-octet number denoting the public-key algorithm of this key.
o A series of multiprecision integers comprising the key material:
* a multiprecision integer (MPI) of RSA public modulus n;
* an MPI of RSA public encryption exponent e.
V3 keys are deprecated. They contain three weaknesses. First, it is
relatively easy to construct a V3 key that has the same Key ID as any
other key because the Key ID is simply the low 64 bits of the public
modulus. Secondly, because the fingerprint of a V3 key hashes the
key material, but not its length, there is an increased opportunity
for fingerprint collisions. Third, there are weaknesses in the MD5
hash algorithm that make developers prefer other algorithms. See
below for a fuller discussion of Key IDs and fingerprints.
V2 keys are identical to the deprecated V3 keys except for the
version number. An implementation MUST NOT generate them and MAY
accept or reject them as it sees fit.
The version 4 format is similar to the version 3 format except for
the absence of a validity period. This has been moved to the
Signature packet. In addition, fingerprints of version 4 keys are
calculated differently from version 3 keys, as described in the
section "Enhanced Key Formats".
A version 4 packet contains:
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o A one-octet version number (4).
o A four-octet number denoting the time that the key was created.
o A one-octet number denoting the public-key algorithm of this key.
o A series of values comprising the key material. This is
algorithm-specific and described in section XXXX.
The version 5 format is similar to the version 4 format except for
the addition of a count for the key material. This count helps
parsing secret key packets (which are an extension of the public key
packet format) in the case of an unknown algoritm. In addition,
fingerprints of version 5 keys are calculated differently from
version 4 keys, as described in the section "Enhanced Key Formats".
A version 5 packet contains:
o A one-octet version number (5).
o A four-octet number denoting the time that the key was created.
o A one-octet number denoting the public-key algorithm of this key.
o A four-octet scalar octet count for the following key material.
o A series of values comprising the key material. This is
algorithm-specific and described in section XXXX.
5.5.3. {5.5.3} Secret-Key Packet Formats
The Secret-Key and Secret-Subkey packets contain all the data of the
Public-Key and Public-Subkey packets, with additional algorithm-
specific secret-key data appended, usually in encrypted form.
The packet contains:
o A Public-Key or Public-Subkey packet, as described above.
o One octet indicating string-to-key usage conventions. Zero
indicates that the secret-key data is not encrypted. 255 or 254
indicates that a string-to-key specifier is being given. Any
other value is a symmetric-key encryption algorithm identifier. A
version 5 packet MUST NOT use the value 255.
o Only for a version 5 packet, a one-octet scalar octet count of the
next 3 optional fields.
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o [Optional] If string-to-key usage octet was 255, 254, or 253, a
one- octet symmetric encryption algorithm.
o [Optional] If string-to-key usage octet was 253, a one-octet AEAD
algorithm.
o [Optional] If string-to-key usage octet was 255, 254, or 253, a
string-to-key specifier. The length of the string-to-key
specifier is implied by its type, as described above.
o [Optional] If secret data is encrypted (string-to-key usage octet
not zero), an Initial Vector (IV) of the same length as the
cipher's block size.
o Only for a version 5 packet, a four-octet scalar octet count for
the following key material.
o Plain or encrypted series of values comprising the secret key
material. This is algorithm-specific and described in section
XXXX.
o If the string-to-key usage octet is zero or 255, then a two-octet
checksum of the plaintext of the algorithm-specific portion (sum
of all octets, mod 65536). If the string-to-key usage octet was
254, then a 20-octet SHA-1 hash of the plaintext of the algorithm-
specific portion. This checksum or hash is encrypted together
with the algorithm-specific fields (if string-to-key usage octet
is not zero). If the string-to-key usage octet was 253, then an
AEAD authentication tag is included here. Note that for all other
values, a two-octet checksum is required.
Note that the version 5 packet format adds two count values to help
parsing packets with unknown S2K or public key algorithms.
Secret MPI values can be encrypted using a passphrase. If a string-
to-key specifier is given, that describes the algorithm for
converting the passphrase to a key, else a simple MD5 hash of the
passphrase is used. Implementations MUST use a string-to-key
specifier; the simple hash is for backward compatibility and is
deprecated, though implementations MAY continue to use existing
private keys in the old format. The cipher for encrypting the MPIs
is specified in the Secret-Key packet.
Encryption/decryption of the secret data is done in CFB mode using
the key created from the passphrase and the Initial Vector from the
packet. A different mode is used with V3 keys (which are only RSA)
than with other key formats. With V3 keys, the MPI bit count prefix
(i.e., the first two octets) is not encrypted. Only the MPI non-
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prefix data is encrypted. Furthermore, the CFB state is
resynchronized at the beginning of each new MPI value, so that the
CFB block boundary is aligned with the start of the MPI data.
With V4 and V5 keys, a simpler method is used. All secret MPI values
are encrypted, including the MPI bitcount prefix.
If the string-to-key usage octet is 253, the encrypted MPI values are
encrypted as one combined plaintext exactly as an AEAD Encrypted Data
packet with a chunk size octet of 10 would be. This implicit chunk
size octet is included in the normal calculations of additional data.
The two-octet checksum that follows the algorithm-specific portion is
the algebraic sum, mod 65536, of the plaintext of all the algorithm-
specific octets (including MPI prefix and data). With V3 keys, the
checksum is stored in the clear. With V4 keys, the checksum is
encrypted like the algorithm-specific data. This value is used to
check that the passphrase was correct. However, this checksum is
deprecated; an implementation SHOULD NOT use it, but should rather
use the SHA-1 hash denoted with a usage octet of 254. The reason for
this is that there are some attacks that involve undetectably
modifying the secret key.
5.6. Algorithm-specific Parts of Keys
The public and secret key format specifies algorithm-specific parts
of a key. The following sections describe them in detail.
5.6.1. Algorithm-Specific Part for RSA Keys
The public key is this series of multiprecision integers:
o MPI of RSA public modulus n;
o MPI of RSA public encryption exponent e.
The secret key is this series of multiprecision integers:
o MPI of RSA secret exponent d;
o MPI of RSA secret prime value p;
o MPI of RSA secret prime value q (p < q);
o MPI of u, the multiplicative inverse of p, mod q.
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5.6.2. Algorithm-Specific Part for DSA Keys
The public key is this series of multiprecision integers:
o MPI of DSA prime p;
o MPI of DSA group order q (q is a prime divisor of p-1);
o MPI of DSA group generator g;
o MPI of DSA public-key value y (= g**x mod p where x is secret).
The secret key is this single multiprecision integer:
o MPI of DSA secret exponent x.
5.6.3. Algorithm-Specific Part for Elgamal Keys
The public key is this series of multiprecision integers:
o MPI of Elgamal prime p;
o MPI of Elgamal group generator g;
o MPI of Elgamal public key value y (= g**x mod p where x is
secret).
The secret key is this single multiprecision integer:
o MPI of Elgamal secret exponent x.
5.6.4. Algorithm-Specific Part for ECDSA Keys
The public key is this series of values:
o a variable-length field containing a curve OID, formatted as
follows:
* a one-octet size of the following field; values 0 and 0xFF are
reserved for future extensions,
* the octets representing a curve OID, defined in section
11{FIXME};
o a MPI of an EC point representing a public key.
The secret key is this single multiprecision integer:
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o MPI of an integer representing the secret key, which is a scalar
of the public EC point.
5.6.5. Algorithm-Specific Part for EdDSA Keys
The public key is this series of values:
o a variable-length field containing a curve OID, formatted as
follows:
* a one-octet size of the following field; values 0 and 0xFF are
reserved for future extensions,
* the octets representing a curve OID, defined in section
NN{FIXME};
o a MPI of an EC point representing a public key Q as described
under EdDSA Point Format below.
The secret key is this single multiprecision integer:
o MPI of an integer representing the secret key, which is a scalar
of the public EC point.
5.6.6. Algorithm-Specific Part for ECDH Keys
The public key is this series of values:
o a variable-length field containing a curve OID, formatted as
follows:
* a one-octet size of the following field; values 0 and 0xFF are
reserved for future extensions,
* the octets representing a curve OID, defined in
Section 11{FIXME};
o a MPI of an EC point representing a public key;
o a variable-length field containing KDF parameters, formatted as
follows:
* a one-octet size of the following fields; values 0 and 0xff are
reserved for future extensions;
* a one-octet value 1, reserved for future extensions;
* a one-octet hash function ID used with a KDF;
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* a one-octet algorithm ID for the symmetric algorithm used to
wrap the symmetric key used for the message encryption; see
Section 8 for details.
Observe that an ECDH public key is composed of the same sequence of
fields that define an ECDSA key, plus the KDF parameters field.
The secret key is this single multiprecision integer:
o MPI of an integer representing the secret key, which is a scalar
of the public EC point.
5.7. {5.6} Compressed Data Packet (Tag 8)
The Compressed Data packet contains compressed data. Typically, this
packet is found as the contents of an encrypted packet, or following
a Signature or One-Pass Signature packet, and contains a literal data
packet.
The body of this packet consists of:
o One octet that gives the algorithm used to compress the packet.
o Compressed data, which makes up the remainder of the packet.
A Compressed Data Packet's body contains an block that compresses
some set of packets. See section "Packet Composition" for details on
how messages are formed.
ZIP-compressed packets are compressed with raw RFC 1951 [RFC1951]
DEFLATE blocks. Note that PGP V2.6 uses 13 bits of compression. If
an implementation uses more bits of compression, PGP V2.6 cannot
decompress it.
ZLIB-compressed packets are compressed with RFC 1950 [RFC1950] ZLIB-
style blocks.
BZip2-compressed packets are compressed using the BZip2 [BZ2]
algorithm.
5.8. {5.7} Symmetrically Encrypted Data Packet (Tag 9)
The Symmetrically Encrypted Data packet contains data encrypted with
a symmetric-key algorithm. When it has been decrypted, it contains
other packets (usually a literal data packet or compressed data
packet, but in theory other Symmetrically Encrypted Data packets or
sequences of packets that form whole OpenPGP messages).
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The body of this packet consists of:
o Encrypted data, the output of the selected symmetric-key cipher
operating in OpenPGP's variant of Cipher Feedback (CFB) mode.
The symmetric cipher used may be specified in a Public-Key or
Symmetric-Key Encrypted Session Key packet that precedes the
Symmetrically Encrypted Data packet. In that case, the cipher
algorithm octet is prefixed to the session key before it is
encrypted. If no packets of these types precede the encrypted data,
the IDEA algorithm is used with the session key calculated as the MD5
hash of the passphrase, though this use is deprecated.
The data is encrypted in CFB mode, with a CFB shift size equal to the
cipher's block size. The Initial Vector (IV) is specified as all
zeros. Instead of using an IV, OpenPGP prefixes a string of length
equal to the block size of the cipher plus two to the data before it
is encrypted. The first block-size octets (for example, 8 octets for
a 64-bit block length) are random, and the following two octets are
copies of the last two octets of the IV. For example, in an 8-octet
block, octet 9 is a repeat of octet 7, and octet 10 is a repeat of
octet 8. In a cipher of length 16, octet 17 is a repeat of octet 15
and octet 18 is a repeat of octet 16. As a pedantic clarification,
in both these examples, we consider the first octet to be numbered 1.
After encrypting the first block-size-plus-two octets, the CFB state
is resynchronized. The last block-size octets of ciphertext are
passed through the cipher and the block boundary is reset.
The repetition of 16 bits in the random data prefixed to the message
allows the receiver to immediately check whether the session key is
incorrect. See the "Security Considerations" section for hints on
the proper use of this "quick check".
5.9. {5.8} Marker Packet (Obsolete Literal Packet) (Tag 10)
An experimental version of PGP used this packet as the Literal
packet, but no released version of PGP generated Literal packets with
this tag. With PGP 5.x, this packet has been reassigned and is
reserved for use as the Marker packet.
The body of this packet consists of:
o The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8).
Such a packet MUST be ignored when received. It may be placed at the
beginning of a message that uses features not available in PGP 2.6.x
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in order to cause that version to report that newer software is
necessary to process the message.
5.10. {5.9} Literal Data Packet (Tag 11)
A Literal Data packet contains the body of a message; data that is
not to be further interpreted.
The body of this packet consists of:
o A one-octet field that describes how the data is formatted.
If it is a 'b' (0x62), then the Literal packet contains binary
data. If it is a 't' (0x74), then it contains text data, and thus
may need line ends converted to local form, or other text-mode
changes. The tag 'u' (0x75) means the same as 't', but also
indicates that implementation believes that the literal data
contains UTF-8 text.
Early versions of PGP also defined a value of 'l' as a 'local'
mode for machine-local conversions. RFC 1991 [RFC1991]
incorrectly stated this local mode flag as '1' (ASCII numeral
one). Both of these local modes are deprecated.
o File name as a string (one-octet length, followed by a file name).
This may be a zero-length string. Commonly, if the source of the
encrypted data is a file, this will be the name of the encrypted
file. An implementation MAY consider the file name in the Literal
packet to be a more authoritative name than the actual file name.
If the special name "_CONSOLE" is used, the message is considered
to be "for your eyes only". This advises that the message data is
unusually sensitive, and the receiving program should process it
more carefully, perhaps avoiding storing the received data to
disk, for example.
o A four-octet number that indicates a date associated with the
literal data. Commonly, the date might be the modification date
of a file, or the time the packet was created, or a zero that
indicates no specific time.
o The remainder of the packet is literal data.
Text data is stored with <CR><LF> text endings (i.e., network-
normal line endings). These should be converted to native line
endings by the receiving software.
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5.11. {5.10} Trust Packet (Tag 12)
The Trust packet is used only within keyrings and is not normally
exported. Trust packets contain data that record the user's
specifications of which key holders are trustworthy introducers,
along with other information that implementing software uses for
trust information. The format of Trust packets is defined by a given
implementation.
Trust packets SHOULD NOT be emitted to output streams that are
transferred to other users, and they SHOULD be ignored on any input
other than local keyring files.
5.12. {5.11} User ID Packet (Tag 13)
A User ID packet consists of UTF-8 text that is intended to represent
the name and email address of the key holder. By convention, it
includes an RFC 2822 [RFC2822] mail name-addr, but there are no
restrictions on its content. The packet length in the header
specifies the length of the User ID.
5.13. {5.12} User Attribute Packet (Tag 17)
The User Attribute packet is a variation of the User ID packet. It
is capable of storing more types of data than the User ID packet,
which is limited to text. Like the User ID packet, a User Attribute
packet may be certified by the key owner ("self-signed") or any other
key owner who cares to certify it. Except as noted, a User Attribute
packet may be used anywhere that a User ID packet may be used.
While User Attribute packets are not a required part of the OpenPGP
standard, implementations SHOULD provide at least enough
compatibility to properly handle a certification signature on the
User Attribute packet. A simple way to do this is by treating the
User Attribute packet as a User ID packet with opaque contents, but
an implementation may use any method desired.
The User Attribute packet is made up of one or more attribute
subpackets. Each subpacket consists of a subpacket header and a
body. The header consists of:
o the subpacket length (1, 2, or 5 octets)
o the subpacket type (1 octet)
and is followed by the subpacket specific data.
The following table lists the currently known subpackets:
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+----------+------------------------------+
| Type | Attribute Subpacket |
+----------+------------------------------+
| 1 | Image Attribute Subpacket |
| [TBD1] | User ID Attribute Subpacket |
| 100-110 | Private/Experimental Use |
+----------+------------------------------+
An implementation SHOULD ignore any subpacket of a type that it does
not recognize.
5.13.1. {5.12.1} The Image Attribute Subpacket
The Image Attribute subpacket is used to encode an image, presumably
(but not required to be) that of the key owner.
The Image Attribute subpacket begins with an image header. The first
two octets of the image header contain the length of the image
header. Note that unlike other multi-octet numerical values in this
document, due to a historical accident this value is encoded as a
little-endian number. The image header length is followed by a
single octet for the image header version. The only currently
defined version of the image header is 1, which is a 16-octet image
header. The first three octets of a version 1 image header are thus
0x10, 0x00, 0x01.
The fourth octet of a version 1 image header designates the encoding
format of the image. The only currently defined encoding format is
the value 1 to indicate JPEG. Image format types 100 through 110 are
reserved for private or experimental use. The rest of the version 1
image header is made up of 12 reserved octets, all of which MUST be
set to 0.
The rest of the image subpacket contains the image itself. As the
only currently defined image type is JPEG, the image is encoded in
the JPEG File Interchange Format (JFIF), a standard file format for
JPEG images [JFIF].
An implementation MAY try to determine the type of an image by
examination of the image data if it is unable to handle a particular
version of the image header or if a specified encoding format value
is not recognized.
5.13.2. User ID Attribute Subpacket
A User ID Attribute subpacket has type #[IANA -- assignment TBD1].
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A User ID Attribute subpacket, just like a User ID packet, consists
of UTF-8 text that is intended to represent the name and email
address of the key holder. By convention, it includes an RFC 2822
[RFC2822] mail name-addr, but there are no restrictions on its
content. For devices using OpenPGP for device certificates, it may
just be the device identifier. The packet length in the header
specifies the length of the User ID.
Because User Attribute subpackets can be used anywhere a User ID
packet can be used, implementations MAY choose to trust a signed User
Attribute subpacket that includes a User ID Attribute subpacket.
5.14. {5.13} Sym. Encrypted Integrity Protected Data Packet (Tag 18)
The Symmetrically Encrypted Integrity Protected Data packet is a
variant of the Symmetrically Encrypted Data packet. It is a new
feature created for OpenPGP that addresses the problem of detecting a
modification to encrypted data. It is used in combination with a
Modification Detection Code packet.
There is a corresponding feature in the features Signature subpacket
that denotes that an implementation can properly use this packet
type. An implementation MUST support decrypting these packets and
SHOULD prefer generating them to the older Symmetrically Encrypted
Data packet when possible. Since this data packet protects against
modification attacks, this standard encourages its proliferation.
While blanket adoption of this data packet would create
interoperability problems, rapid adoption is nevertheless important.
An implementation SHOULD specifically denote support for this packet,
but it MAY infer it from other mechanisms.
For example, an implementation might infer from the use of a cipher
such as Advanced Encryption Standard (AES) or Twofish that a user
supports this feature. It might place in the unhashed portion of
another user's key signature a Features subpacket. It might also
present a user with an opportunity to regenerate their own self-
signature with a Features subpacket.
This packet contains data encrypted with a symmetric-key algorithm
and protected against modification by the SHA-1 hash algorithm. When
it has been decrypted, it will typically contain other packets (often
a Literal Data packet or Compressed Data packet). The last decrypted
packet in this packet's payload MUST be a Modification Detection Code
packet.
The body of this packet consists of:
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o A one-octet version number. The only currently defined value is
1.
o Encrypted data, the output of the selected symmetric-key cipher
operating in Cipher Feedback mode with shift amount equal to the
block size of the cipher (CFB-n where n is the block size).
The symmetric cipher used MUST be specified in a Public-Key or
Symmetric-Key Encrypted Session Key packet that precedes the
Symmetrically Encrypted Data packet. In either case, the cipher
algorithm octet is prefixed to the session key before it is
encrypted.
The data is encrypted in CFB mode, with a CFB shift size equal to the
cipher's block size. The Initial Vector (IV) is specified as all
zeros. Instead of using an IV, OpenPGP prefixes an octet string to
the data before it is encrypted. The length of the octet string
equals the block size of the cipher in octets, plus two. The first
octets in the group, of length equal to the block size of the cipher,
are random; the last two octets are each copies of their 2nd
preceding octet. For example, with a cipher whose block size is 128
bits or 16 octets, the prefix data will contain 16 random octets,
then two more octets, which are copies of the 15th and 16th octets,
respectively. Unlike the Symmetrically Encrypted Data Packet, no
special CFB resynchronization is done after encrypting this prefix
data. See "OpenPGP CFB Mode" below for more details.
The repetition of 16 bits in the random data prefixed to the message
allows the receiver to immediately check whether the session key is
incorrect.
The plaintext of the data to be encrypted is passed through the SHA-1
hash function, and the result of the hash is appended to the
plaintext in a Modification Detection Code packet. The input to the
hash function includes the prefix data described above; it includes
all of the plaintext, and then also includes two octets of values
0xD3, 0x14. These represent the encoding of a Modification Detection
Code packet tag and length field of 20 octets.
The resulting hash value is stored in a Modification Detection Code
(MDC) packet, which MUST use the two octet encoding just given to
represent its tag and length field. The body of the MDC packet is
the 20-octet output of the SHA-1 hash.
The Modification Detection Code packet is appended to the plaintext
and encrypted along with the plaintext using the same CFB context.
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During decryption, the plaintext data should be hashed with SHA-1,
including the prefix data as well as the packet tag and length field
of the Modification Detection Code packet. The body of the MDC
packet, upon decryption, is compared with the result of the SHA-1
hash.
Any failure of the MDC indicates that the message has been modified
and MUST be treated as a security problem. Failures include a
difference in the hash values, but also the absence of an MDC packet,
or an MDC packet in any position other than the end of the plaintext.
Any failure SHOULD be reported to the user.
Note: future designs of new versions of this packet should consider
rollback attacks since it will be possible for an attacker to change
the version back to 1.
NON-NORMATIVE EXPLANATION
The MDC system, as packets 18 and 19 are called, were created to
provide an integrity mechanism that is less strong than a
signature, yet stronger than bare CFB encryption.
It is a limitation of CFB encryption that damage to the
ciphertext will corrupt the affected cipher blocks and the block
following. Additionally, if data is removed from the end of a
CFB-encrypted block, that removal is undetectable. (Note also
that CBC mode has a similar limitation, but data removed from
the front of the block is undetectable.)
The obvious way to protect or authenticate an encrypted block is
to digitally sign it. However, many people do not wish to
habitually sign data, for a large number of reasons beyond the
scope of this document. Suffice it to say that many people
consider properties such as deniability to be as valuable as
integrity.
OpenPGP addresses this desire to have more security than raw
encryption and yet preserve deniability with the MDC system. An
MDC is intentionally not a MAC. Its name was not selected by
accident. It is analogous to a checksum.
Despite the fact that it is a relatively modest system, it has
proved itself in the real world. It is an effective defense to
several attacks that have surfaced since it has been created.
It has met its modest goals admirably.
Consequently, because it is a modest security system, it has
modest requirements on the hash function(s) it employs. It does
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not rely on a hash function being collision-free, it relies on a
hash function being one-way. If a forger, Frank, wishes to send
Alice a (digitally) unsigned message that says, "I've always
secretly loved you, signed Bob", it is far easier for him to
construct a new message than it is to modify anything
intercepted from Bob. (Note also that if Bob wishes to
communicate secretly with Alice, but without authentication or
identification and with a threat model that includes forgers, he
has a problem that transcends mere cryptography.)
Note also that unlike nearly every other OpenPGP subsystem,
there are no parameters in the MDC system. It hard-defines
SHA-1 as its hash function. This is not an accident. It is an
intentional choice to avoid downgrade and cross-grade attacks
while making a simple, fast system. (A downgrade attack would
be an attack that replaced SHA2-256 with SHA-1, for example. A
cross-grade attack would replace SHA-1 with another 160-bit
hash, such as RIPE-MD/160, for example.)
However, given the present state of hash function cryptanalysis
and cryptography, it may be desirable to upgrade the MDC system
to a new hash function. See Section 13.11 in the "IANA
Considerations" for guidance.
5.15. {5.14} Modification Detection Code Packet (Tag 19)
The Modification Detection Code packet contains a SHA-1 hash of
plaintext data, which is used to detect message modification. It is
only used with a Symmetrically Encrypted Integrity Protected Data
packet. The Modification Detection Code packet MUST be the last
packet in the plaintext data that is encrypted in the Symmetrically
Encrypted Integrity Protected Data packet, and MUST appear in no
other place.
A Modification Detection Code packet MUST have a length of 20 octets.
The body of this packet consists of:
o A 20-octet SHA-1 hash of the preceding plaintext data of the
Symmetrically Encrypted Integrity Protected Data packet, including
prefix data, the tag octet, and length octet of the Modification
Detection Code packet.
Note that the Modification Detection Code packet MUST always use a
new format encoding of the packet tag, and a one-octet encoding of
the packet length. The reason for this is that the hashing rules for
modification detection include a one-octet tag and one-octet length
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in the data hash. While this is a bit restrictive, it reduces
complexity.
5.16. AEAD Encrypted Data Packet (Tag 20)
This packet contains data encrypted with an authenticated encryption
and additional data (AEAD) construction. When it has been decrypted,
it will typically contain other packets (often a Literal Data packet
or Compressed Data packet).
The body of this packet consists of:
o A one-octet version number. The only currently defined value is
1.
o A one-octet cipher algorithm.
o A one-octet AEAD algorithm.
o A one-octet chunk size.
o A starting initialization vector of size specified by the AEAD
algorithm.
o Encrypted data, the output of the selected symmetric-key cipher
operating in the given AEAD mode.
o A final, summary authentication tag for the AEAD mode.
An AEAD encrypted data packet consists of one or more chunks of data.
The plaintext of each chunk is of a size specified using the chunk
size octet using the method specified below.
The encrypted data consists of the encryption of each chunk of
plaintext, followed immediately by the relevant authentication tag.
If the last chunk of plaintext is smaller than the chunk size, the
ciphertext for that data may be shorter; it is nevertheless followed
by a full authentication tag.
For each chunk, the AEAD construction is given the packet header,
version number, cipher algorithm octet, AEAD algorithm octet, chunk
size octet, and an eight-octet, big-endian chunk index as additional
data. The index of the first chunk is zero.
After the final chunk, the AEAD algorithm is used to produce a final
authentication tag encrypting the empty string. This AEAD instance
is given the additional data specified above, plus an eight-octet,
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big-endian value specifying the total number of plaintext octets
encrypted. This allows detection of a truncated ciphertext.
The chunk size octet specifies the size of chunks using the following
formula (in C), where c is the chunk size octet:
chunk_size = ((uint64_t)1 << (c + 6))
An implementation MUST support chunk size octets with values from 0
to 56. Chunk size octets with other values are reserved for future
extensions.
A new random initialization vector MUST be used for each message.
5.16.1. EAX Mode
The only currently defined AEAD algorithm is EAX Mode [EAX]. This
algorithm can only use block ciphers with 16-octet blocks. The
starting initialization vector and authentication tag are both 16
octets long.
The starting initialization vector for this mode MUST be unique and
unpredictable.
The nonce for EAX mode is computed by treating the starting
initialization vector as a 16-octet, big-endian value and exclusive-
oring the low eight octets of it with the chunk index.
The security of EAX requires that the nonce is never reused, hence
the requirement that the starting initialization vector be unique.
6. {6} Radix-64 Conversions
As stated in the introduction, OpenPGP's underlying native
representation for objects is a stream of arbitrary octets, and some
systems desire these objects to be immune to damage caused by
character set translation, data conversions, etc.
In principle, any printable encoding scheme that met the requirements
of the unsafe channel would suffice, since it would not change the
underlying binary bit streams of the native OpenPGP data structures.
The OpenPGP standard specifies one such printable encoding scheme to
ensure interoperability.
OpenPGP's Radix-64 encoding is composed of two parts: a base64
encoding of the binary data and a checksum. The base64 encoding is
identical to the MIME base64 content-transfer-encoding [RFC2045].
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The checksum is a 24-bit Cyclic Redundancy Check (CRC) converted to
four characters of radix-64 encoding by the same MIME base64
transformation, preceded by an equal sign (=). The CRC is computed
by using the generator 0x864CFB and an initialization of 0xB704CE.
The accumulation is done on the data before it is converted to radix-
64, rather than on the converted data. A sample implementation of
this algorithm is in the next section.
The checksum with its leading equal sign MAY appear on the first line
after the base64 encoded data.
Rationale for CRC-24: The size of 24 bits fits evenly into printable
base64. The nonzero initialization can detect more errors than a
zero initialization.
6.1. {6.1} An Implementation of the CRC-24 in "C"
<CODE BEGINS>
#define CRC24_INIT 0xB704CEL
#define CRC24_POLY 0x1864CFBL
typedef long crc24;
crc24 crc_octets(unsigned char *octets, size_t len)
{
crc24 crc = CRC24_INIT;
int i;
while (len--) {
crc ^= (*octets++) << 16;
for (i = 0; i < 8; i++) {
crc <<= 1;
if (crc & 0x1000000)
crc ^= CRC24_POLY;
}
}
return crc & 0xFFFFFFL;
}
<CODE ENDS>
6.2. {6.2} Forming ASCII Armor
When OpenPGP encodes data into ASCII Armor, it puts specific headers
around the Radix-64 encoded data, so OpenPGP can reconstruct the data
later. An OpenPGP implementation MAY use ASCII armor to protect raw
binary data. OpenPGP informs the user what kind of data is encoded
in the ASCII armor through the use of the headers.
Concatenating the following data creates ASCII Armor:
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o An Armor Header Line, appropriate for the type of data
o Armor Headers
o A blank (zero-length, or containing only whitespace) line
o The ASCII-Armored data
o An Armor Checksum
o The Armor Tail, which depends on the Armor Header Line
An Armor Header Line consists of the appropriate header line text
surrounded by five (5) dashes ('-', 0x2D) on either side of the
header line text. The header line text is chosen based upon the type
of data that is being encoded in Armor, and how it is being encoded.
Header line texts include the following strings:
BEGIN PGP MESSAGE Used for signed, encrypted, or compressed files.
BEGIN PGP PUBLIC KEY BLOCK Used for armoring public keys.
BEGIN PGP PRIVATE KEY BLOCK Used for armoring private keys.
BEGIN PGP MESSAGE, PART X/Y Used for multi-part messages, where the
armor is split amongst Y parts, and this is the Xth part out of Y.
BEGIN PGP MESSAGE, PART X Used for multi-part messages, where this
is the Xth part of an unspecified number of parts. Requires the
MESSAGE-ID Armor Header to be used.
BEGIN PGP SIGNATURE Used for detached signatures, OpenPGP/MIME
signatures, and cleartext signatures. Note that PGP 2.x uses
BEGIN PGP MESSAGE for detached signatures.
Note that all these Armor Header Lines are to consist of a complete
line. That is to say, there is always a line ending preceding the
starting five dashes, and following the ending five dashes. The
header lines, therefore, MUST start at the beginning of a line, and
MUST NOT have text other than whitespace following them on the same
line. These line endings are considered a part of the Armor Header
Line for the purposes of determining the content they delimit. This
is particularly important when computing a cleartext signature (see
below).
The Armor Headers are pairs of strings that can give the user or the
receiving OpenPGP implementation some information about how to decode
or use the message. The Armor Headers are a part of the armor, not a
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part of the message, and hence are not protected by any signatures
applied to the message.
The format of an Armor Header is that of a key-value pair. A colon
(':' 0x38) and a single space (0x20) separate the key and value.
OpenPGP should consider improperly formatted Armor Headers to be
corruption of the ASCII Armor. Unknown keys should be reported to
the user, but OpenPGP should continue to process the message.
Note that some transport methods are sensitive to line length. While
there is a limit of 76 characters for the Radix-64 data
(Section 6.3), there is no limit to the length of Armor Headers.
Care should be taken that the Armor Headers are short enough to
survive transport. One way to do this is to repeat an Armor Header
key multiple times with different values for each so that no one line
is overly long.
Currently defined Armor Header Keys are as follows:
o "Version", which states the OpenPGP implementation and version
used to encode the message.
o "Comment", a user-defined comment. OpenPGP defines all text to be
in UTF-8. A comment may be any UTF-8 string. However, the whole
point of armoring is to provide seven-bit-clean data.
Consequently, if a comment has characters that are outside the US-
ASCII range of UTF, they may very well not survive transport.
o "Hash", a comma-separated list of hash algorithms used in this
message. This is used only in cleartext signed messages.
o "MessageID", a 32-character string of printable characters. The
string must be the same for all parts of a multi-part message that
uses the "PART X" Armor Header. MessageID strings should be
unique enough that the recipient of the mail can associate all the
parts of a message with each other. A good checksum or
cryptographic hash function is sufficient.
The MessageID SHOULD NOT appear unless it is in a multi-part
message. If it appears at all, it MUST be computed from the
finished (encrypted, signed, etc.) message in a deterministic
fashion, rather than contain a purely random value. This is to
allow the legitimate recipient to determine that the MessageID
cannot serve as a covert means of leaking cryptographic key
information.
o "Charset", a description of the character set that the plaintext
is in. Please note that OpenPGP defines text to be in UTF-8. An
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implementation will get best results by translating into and out
of UTF-8. However, there are many instances where this is easier
said than done. Also, there are communities of users who have no
need for UTF-8 because they are all happy with a character set
like ISO Latin-5 or a Japanese character set. In such instances,
an implementation MAY override the UTF-8 default by using this
header key. An implementation MAY implement this key and any
translations it cares to; an implementation MAY ignore it and
assume all text is UTF-8.
The Armor Tail Line is composed in the same manner as the Armor
Header Line, except the string "BEGIN" is replaced by the string
"END".
6.3. {6.3} Encoding Binary in Radix-64
The encoding process represents 24-bit groups of input bits as output
strings of 4 encoded characters. Proceeding from left to right, a
24-bit input group is formed by concatenating three 8-bit input
groups. These 24 bits are then treated as four concatenated 6-bit
groups, each of which is translated into a single digit in the
Radix-64 alphabet. When encoding a bit stream with the Radix-64
encoding, the bit stream must be presumed to be ordered with the most
significant bit first. That is, the first bit in the stream will be
the high-order bit in the first 8-bit octet, and the eighth bit will
be the low-order bit in the first 8-bit octet, and so on.
+--first octet--+-second octet--+--third octet--+
|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
+-----------+---+-------+-------+---+-----------+
|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|
+--1.index--+--2.index--+--3.index--+--4.index--+
Each 6-bit group is used as an index into an array of 64 printable
characters from the table below. The character referenced by the
index is placed in the output string.
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Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad) =
15 P 32 g 49 x
16 Q 33 h 50 y
The encoded output stream must be represented in lines of no more
than 76 characters each.
Special processing is performed if fewer than 24 bits are available
at the end of the data being encoded. There are three possibilities:
1. The last data group has 24 bits (3 octets). No special
processing is needed.
2. The last data group has 16 bits (2 octets). The first two
6-bit groups are processed as above. The third (incomplete)
data group has two zero-value bits added to it, and is
processed as above. A pad character (=) is added to the
output.
3. The last data group has 8 bits (1 octet). The first 6-bit
group is processed as above. The second (incomplete) data
group has four zero-value bits added to it, and is processed
as above. Two pad characters (=) are added to the output.
6.4. {6.4} Decoding Radix-64
In Radix-64 data, characters other than those in the table, line
breaks, and other white space probably indicate a transmission error,
about which a warning message or even a message rejection might be
appropriate under some circumstances. Decoding software must ignore
all white space.
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Because it is used only for padding at the end of the data, the
occurrence of any "=" characters may be taken as evidence that the
end of the data has been reached (without truncation in transit). No
such assurance is possible, however, when the number of octets
transmitted was a multiple of three and no "=" characters are
present.
6.5. {6.5} Examples of Radix-64
Input data: 0x14FB9C03D97E
Hex: 1 4 F B 9 C | 0 3 D 9 7 E
8-bit: 00010100 11111011 10011100 | 00000011 11011001 01111110
6-bit: 000101 001111 101110 011100 | 000000 111101 100101 111110
Decimal: 5 15 46 28 0 61 37 62
Output: F P u c A 9 l +
Input data: 0x14FB9C03D9
Hex: 1 4 F B 9 C | 0 3 D 9
8-bit: 00010100 11111011 10011100 | 00000011 11011001
pad with 00
6-bit: 000101 001111 101110 011100 | 000000 111101 100100
Decimal: 5 15 46 28 0 61 36
pad with =
Output: F P u c A 9 k =
Input data: 0x14FB9C03
Hex: 1 4 F B 9 C | 0 3
8-bit: 00010100 11111011 10011100 | 00000011
pad with 0000
6-bit: 000101 001111 101110 011100 | 000000 110000
Decimal: 5 15 46 28 0 48
pad with = =
Output: F P u c A w = =
6.6. {6.6} Example of an ASCII Armored Message
-----BEGIN PGP MESSAGE-----
Version: OpenPrivacy 0.99
yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS
vBSFjNSiVHsuAA==
=njUN
-----END PGP MESSAGE-----
Note that this example has extra indenting; an actual armored message
would have no leading whitespace.
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7. {7} Cleartext Signature Framework
It is desirable to be able to sign a textual octet stream without
ASCII armoring the stream itself, so the signed text is still
readable without special software. In order to bind a signature to
such a cleartext, this framework is used. (Note that this framework
is not intended to be reversible. RFC 3156 [RFC3156] defines another
way to sign cleartext messages for environments that support MIME.)
The cleartext signed message consists of:
o The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a
single line,
o One or more "Hash" Armor Headers,
o Exactly one empty line not included into the message digest,
o The dash-escaped cleartext that is included into the message
digest,
o The ASCII armored signature(s) including the '-----BEGIN PGP
SIGNATURE-----' Armor Header and Armor Tail Lines.
If the "Hash" Armor Header is given, the specified message digest
algorithm(s) are used for the signature. If there are no such
headers, MD5 is used. If MD5 is the only hash used, then an
implementation MAY omit this header for improved V2.x compatibility.
If more than one message digest is used in the signature, the "Hash"
armor header contains a comma-delimited list of used message digests.
Current message digest names are described below with the algorithm
IDs.
An implementation SHOULD add a line break after the cleartext, but
MAY omit it if the cleartext ends with a line break. This is for
visual clarity.
7.1. {7.1} Dash-Escaped Text
The cleartext content of the message must also be dash-escaped.
Dash-escaped cleartext is the ordinary cleartext where every line
starting with a dash '-' (0x2D) is prefixed by the sequence dash '-'
(0x2D) and space ' ' (0x20). This prevents the parser from
recognizing armor headers of the cleartext itself. An implementation
MAY dash-escape any line, SHOULD dash-escape lines commencing "From"
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followed by a space, and MUST dash-escape any line commencing in a
dash. The message digest is computed using the cleartext itself, not
the dash-escaped form.
As with binary signatures on text documents, a cleartext signature is
calculated on the text using canonical <CR><LF> line endings. The
line ending (i.e., the <CR><LF>) before the '-----BEGIN PGP
SIGNATURE-----' line that terminates the signed text is not
considered part of the signed text.
When reversing dash-escaping, an implementation MUST strip the string
"- " if it occurs at the beginning of a line, and SHOULD warn on "-"
and any character other than a space at the beginning of a line.
Also, any trailing whitespace -- spaces (0x20) and tabs (0x09) -- at
the end of any line is removed when the cleartext signature is
generated.
8. {8} Regular Expressions
A regular expression is zero or more branches, separated by '|'. It
matches anything that matches one of the branches.
A branch is zero or more pieces, concatenated. It matches a match
for the first, followed by a match for the second, etc.
A piece is an atom possibly followed by '_', '+', or '?'. An atom
followed by '_' matches a sequence of 0 or more matches of the atom.
An atom followed by '+' matches a sequence of 1 or more matches of
the atom. An atom followed by '?' matches a match of the atom, or
the null string.
An atom is a regular expression in parentheses (matching a match for
the regular expression), a range (see below), '.' (matching any
single character), '^' (matching the null string at the beginning of
the input string), '$' (matching the null string at the end of the
input string), a '\' followed by a single character (matching that
character), or a single character with no other significance
(matching that character).
A range is a sequence of characters enclosed in '[]'. It normally
matches any single character from the sequence. If the sequence
begins with '^', it matches any single character not from the rest of
the sequence. If two characters in the sequence are separated by
'-', this is shorthand for the full list of ASCII characters between
them (e.g., '[0-9]' matches any decimal digit). To include a literal
']' in the sequence, make it the first character (following a
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possible '^'). To include a literal '-', make it the first or last
character.
9. {9} Constants
This section describes the constants used in OpenPGP.
Note that these tables are not exhaustive lists; an implementation
MAY implement an algorithm not on these lists, so long as the
algorithm numbers are chosen from the private or experimental
algorithm range.
See the section "Notes on Algorithms" below for more discussion of
the algorithms.
9.1. {9.1} Public-Key Algorithms
+-----------+----------------------------------------------------+
| ID | Algorithm |
+-----------+----------------------------------------------------+
| 1 | RSA (Encrypt or Sign) [HAC] |
| 2 | RSA Encrypt-Only [HAC] |
| 3 | RSA Sign-Only [HAC] |
| 16 | Elgamal (Encrypt-Only) [ELGAMAL] [HAC] |
| 17 | DSA (Digital Signature Algorithm) [FIPS186] [HAC] |
| 18 | ECDH public key algorithm |
| 19 | ECDSA public key algorithm [FIPS186] |
| 20 | Reserved (formerly Elgamal Encrypt or Sign) |
| 21 | Reserved for Diffie-Hellman |
| | (X9.42, as defined for IETF-S/MIME) |
| 22 | EdDSA [I-D.irtf-cfrg-eddsa] |
| 100--110 | Private/Experimental algorithm |
+-----------+----------------------------------------------------+
Implementations MUST implement DSA and ECDSA for signatures, and
Elgamal and ECDH for encryption. Implementations SHOULD implement
RSA keys (1). RSA Encrypt-Only (2) and RSA Sign-Only are deprecated
and SHOULD NOT be generated, but may be interpreted. See
Section 13.5. See Section 13.8 for notes on Elgamal Encrypt or Sign
(20), and X9.42 (21). Implementations MAY implement any other
algorithm.
A compatible specification of ECDSA is given in [RFC6090] as "KT-I
Signatures" and in [SEC1]; ECDH is defined in Section 13.5 this
document.
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9.2. ECC Curve OID
The parameter curve OID is an array of octets that define a named
curve. The table below specifies the exact sequence of bytes for
each named curve referenced in this document:
+------------------------+-----+------------------+-----------------+
| ASN.1 Object | OID | Curve OID bytes | Curve name |
| Identifier | len | in hexadecimal | |
| | | representation | |
+------------------------+-----+------------------+-----------------+
| 1.2.840.10045.3.1.7 | 8 | 2A 86 48 CE 3D | NIST P-256 |
| | | 03 01 07 | |
| 1.3.132.0.34 | 5 | 2B 81 04 00 22 | NIST P-384 |
| 1.3.132.0.35 | 5 | 2B 81 04 00 23 | NIST P-521 |
| 1.3.36.3.3.2.8.1.1.7 | 9 | 2B 24 03 03 02 | brainpoolP256r1 |
| | | 08 01 01 07 | |
| 1.3.36.3.3.2.8.1.1.13 | 9 | 2B 24 03 03 02 | brainpoolP512r1 |
| | | 08 01 01 0D | |
| 1.3.6.1.4.1.11591.15.1 | 9 | 2B 06 01 04 01 | Ed25519 |
| | | DA 47 0F 01 | |
| 1.3.6.1.4.1.3029.1.5.1 | 10 | 2B 06 01 04 01 | Curve25519 |
| | | 97 55 01 05 01 | |
+------------------------+-----+------------------+-----------------+
The sequence of octets in the third column is the result of applying
the Distinguished Encoding Rules (DER) to the ASN.1 Object Identifier
with subsequent truncation. The truncation removes the two fields of
encoded Object Identifier. The first omitted field is one octet
representing the Object Identifier tag, and the second omitted field
is the length of the Object Identifier body. For example, the
complete ASN.1 DER encoding for the NIST P-256 curve OID is "06 08 2A
86 48 CE 3D 03 01 07", from which the first entry in the table above
is constructed by omitting the first two octets. Only the truncated
sequence of octets is the valid representation of a curve OID.
9.3. {9.2} Symmetric-Key Algorithms
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+-----------+-----------------------------------------------+
| ID | Algorithm |
+-----------+-----------------------------------------------+
| 0 | Plaintext or unencrypted data |
| 1 | IDEA [IDEA] |
| 2 | TripleDES (DES-EDE, [SCHNEIER] [HAC] |
| | - 168 bit key derived from 192) |
| 3 | CAST5 (128 bit key, as per [RFC2144]) |
| 4 | Blowfish (128 bit key, 16 rounds) [BLOWFISH] |
| 5 | Reserved |
| 6 | Reserved |
| 7 | AES with 128-bit key [AES] |
| 8 | AES with 192-bit key |
| 9 | AES with 256-bit key |
| 10 | Twofish with 256-bit key [TWOFISH] |
| 11 | Camellia with 128-bit key [RFC3713] |
| 12 | Camellia with 192-bit key |
| 13 | Camellia with 256-bit key |
| 100--110 | Private/Experimental algorithm |
+-----------+-----------------------------------------------+
Implementations MUST implement TripleDES. Implementations SHOULD
implement AES-128 and CAST5. Implementations that interoperate with
PGP 2.6 or earlier need to support IDEA, as that is the only
symmetric cipher those versions use. Implementations MAY implement
any other algorithm.
9.4. {9.3} Compression Algorithms
+-----------+---------------------------------+
| ID | Algorithm |
+-----------+---------------------------------+
| 0 | Uncompressed |
| 1 | ZIP [RFC1951] |
| 2 | ZLIB [RFC1950] |
| 3 | BZip2 [BZ2] |
| 100--110 | Private/Experimental algorithm |
+-----------+---------------------------------+
Implementations MUST implement uncompressed data. Implementations
SHOULD implement ZIP. Implementations MAY implement any other
algorithm.
9.5. {9.4} Hash Algorithms
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+-----------+---------------------------------+--------------+
| ID | Algorithm | Text Name |
+-----------+---------------------------------+--------------+
| 1 | MD5 [HAC] | "MD5" |
| 2 | SHA-1 [FIPS180] | "SHA1" |
| 3 | RIPE-MD/160 [HAC] | "RIPEMD160" |
| 4 | Reserved | |
| 5 | Reserved | |
| 6 | Reserved | |
| 7 | Reserved | |
| 8 | SHA2-256 [FIPS180] | "SHA256" |
| 9 | SHA2-384 [FIPS180] | "SHA384" |
| 10 | SHA2-512 [FIPS180] | "SHA512" |
| 11 | SHA2-224 [FIPS180] | "SHA224" |
| 12 | SHA3-256 [FIPS202] | "SHA3-256" |
| 13 | Reserved | |
| 14 | SHA3-512 [FIPS202] | "SHA3-512" |
| 100--110 | Private/Experimental algorithm | |
+-----------+---------------------------------+--------------+
Implementations MUST implement SHA2-256. Implementations MAY
implement other algorithms. Implementations SHOULD NOT create
messages which require the use of SHA-1 with the exception of
computing version 4 key fingerprints and for purposes of the MDC
packet. Implementations SHOULD NOT use MD5 or RIPE-MD/160.
9.6. AEAD Algorithms
+-----------+---------------------------------+
| ID | Algorithm |
+-----------+---------------------------------+
| 1 | EAX [EAX] |
| 100--110 | Private/Experimental algorithm |
+-----------+---------------------------------+
Implementations MUST implement EAX. Implementations MAY implement
other algorithms.
10. {10} IANA Considerations
OpenPGP is highly parameterized, and consequently there are a number
of considerations for allocating parameters for extensions. This
section describes how IANA should look at extensions to the protocol
as described in this document.
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10.1. {10.1} New String-to-Key Specifier Types
OpenPGP S2K specifiers contain a mechanism for new algorithms to turn
a string into a key. This specification creates a registry of S2K
specifier types. The registry includes the S2K type, the name of the
S2K, and a reference to the defining specification. The initial
values for this registry can be found in Section 3.7.1. Adding a new
S2K specifier MUST be done through the IETF CONSENSUS method, as
described in [RFC2434].
10.2. {10.2} New Packets
Major new features of OpenPGP are defined through new packet types.
This specification creates a registry of packet types. The registry
includes the packet type, the name of the packet, and a reference to
the defining specification. The initial values for this registry can
be found in Section 4.3. Adding a new packet type MUST be done
through the IETF CONSENSUS method, as described in [RFC2434].
10.2.1. {10.2.1} User Attribute Types
The User Attribute packet permits an extensible mechanism for other
types of certificate identification. This specification creates a
registry of User Attribute types. The registry includes the User
Attribute type, the name of the User Attribute, and a reference to
the defining specification. The initial values for this registry can
be found in Section 5.12. Adding a new User Attribute type MUST be
done through the IETF CONSENSUS method, as described in [RFC2434].
This document requests that IANA register the User ID Attribute Type
found in Section 5.12.2:
+--------+------------+-----------------------------+
| Value | Attribute | Reference |
+--------+------------+-----------------------------+
| TBD1 | User ID | This Document Section 5.12 |
+--------+------------+-----------------------------+
10.2.2. {10.2.1.1} Image Format Subpacket Types
Within User Attribute packets, there is an extensible mechanism for
other types of image-based user attributes. This specification
creates a registry of Image Attribute subpacket types. The registry
includes the Image Attribute subpacket type, the name of the Image
Attribute subpacket, and a reference to the defining specification.
The initial values for this registry can be found in Section 5.12.1.
Adding a new Image Attribute subpacket type MUST be done through the
IETF CONSENSUS method, as described in [RFC2434].
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10.2.3. {10.2.2} New Signature Subpackets
OpenPGP signatures contain a mechanism for signed (or unsigned) data
to be added to them for a variety of purposes in the Signature
subpackets as discussed in Section 5.2.3.1. This specification
creates a registry of Signature subpacket types. The registry
includes the Signature subpacket type, the name of the subpacket, and
a reference to the defining specification. The initial values for
this registry can be found in Section 5.2.3.1. Adding a new
Signature subpacket MUST be done through the IETF CONSENSUS method,
as described in [RFC2434].
10.2.3.1. {10.2.2.1} Signature Notation Data Subpackets
OpenPGP signatures further contain a mechanism for extensions in
signatures. These are the Notation Data subpackets, which contain a
key/value pair. Notations contain a user space that is completely
unmanaged and an IETF space.
This specification creates a registry of Signature Notation Data
types. The registry includes the Signature Notation Data type, the
name of the Signature Notation Data, its allowed values, and a
reference to the defining specification. The initial values for this
registry can be found in Section 5.2.3.16. Adding a new Signature
Notation Data subpacket MUST be done through the EXPERT REVIEW
method, as described in [RFC2434].
This document requests IANA register the following Signature Notation
Data types:
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+---------------+--------+--------------------+---------------------+
| Allowed | Name | Type | Reference |
| Values | | | |
+---------------+--------+--------------------+---------------------+
| Any String | manu | Manufacturer Name | This Doc Section |
| | | | 5.2.3.16.1 |
| Any String | make | Product Make | This Doc Section |
| | | | 5.2.3.16.2 |
| Any String | model | Product Model | This Doc Section |
| | | | 5.2.3.16.3 |
| Any String | prodid | Product ID | This Doc Section |
| | | | 5.2.3.16.4 |
| Any String | pvers | Product Version | This Doc Section |
| | | | 5.2.3.16.5 |
| Any String | lot | Product Lot Number | This Doc Section |
| | | | 5.2.3.16.6 |
| Decimal | qty | Package Quantity | This Doc Section |
| Integer | | | 5.2.3.16.7 |
| String | | | |
| A geo: URI | loc | Current Geo- | This Doc Section |
| | | | 5.2.3.16.8 |
| without the | | location | |
| "geo:" | | Latitude/Longitude | |
| A geo: URI | dest | Destination Geo- | This Doc Section |
| | | | 5.2.3.16.8 |
| without the | | location | |
| "geo:" | | Latitude/Longitude | |
| Hash Notation | hash | The Hash of | This Doc Section |
| | | | 5.2.3.16.9 |
| data | | external data | |
+---------------+--------+--------------------+---------------------+
10.2.3.2. {10.2.2.2} Key Server Preference Extensions
OpenPGP signatures contain a mechanism for preferences to be
specified about key servers. This specification creates a registry
of key server preferences. The registry includes the key server
preference, the name of the preference, and a reference to the
defining specification. The initial values for this registry can be
found in Section 5.2.3.17. Adding a new key server preference MUST
be done through the IETF CONSENSUS method, as described in [RFC2434].
10.2.3.3. {10.2.2.3} Key Flags Extensions
OpenPGP signatures contain a mechanism for flags to be specified
about key usage. This specification creates a registry of key usage
flags. The registry includes the key flags value, the name of the
flag, and a reference to the defining specification. The initial
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values for this registry can be found in Section 5.2.3.21. Adding a
new key usage flag MUST be done through the IETF CONSENSUS method, as
described in [RFC2434].
10.2.3.4. {10.2.2.4} Reason for Revocation Extensions
OpenPGP signatures contain a mechanism for flags to be specified
about why a key was revoked. This specification creates a registry
of "Reason for Revocation" flags. The registry includes the "Reason
for Revocation" flags value, the name of the flag, and a reference to
the defining specification. The initial values for this registry can
be found in Section 5.2.3.23. Adding a new feature flag MUST be done
through the IETF CONSENSUS method, as described in [RFC2434].
10.2.3.5. {10.2.2.5} Implementation Features
OpenPGP signatures contain a mechanism for flags to be specified
stating which optional features an implementation supports. This
specification creates a registry of feature-implementation flags.
The registry includes the feature-implementation flags value, the
name of the flag, and a reference to the defining specification. The
initial values for this registry can be found in Section 5.2.3.24.
Adding a new feature-implementation flag MUST be done through the
IETF CONSENSUS method, as described in [RFC2434].
Also see Section 13.12 for more information about when feature flags
are needed.
10.2.4. {10.2.3} New Packet Versions
The core OpenPGP packets all have version numbers, and can be revised
by introducing a new version of an existing packet. This
specification creates a registry of packet types. The registry
includes the packet type, the number of the version, and a reference
to the defining specification. The initial values for this registry
can be found in Section 5. Adding a new packet version MUST be done
through the IETF CONSENSUS method, as described in [RFC2434].
10.3. {10.3} New Algorithms
Section 9 lists the core algorithms that OpenPGP uses. Adding in a
new algorithm is usually simple. For example, adding in a new
symmetric cipher usually would not need anything more than allocating
a constant for that cipher. If that cipher had other than a 64-bit
or 128-bit block size, there might need to be additional
documentation describing how OpenPGP-CFB mode would be adjusted.
Similarly, when DSA was expanded from a maximum of 1024-bit public
keys to 3072-bit public keys, the revision of FIPS 186 contained
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enough information itself to allow implementation. Changes to this
document were made mainly for emphasis.
10.3.1. {10.3.1} Public-Key Algorithms
OpenPGP specifies a number of public-key algorithms. This
specification creates a registry of public-key algorithm identifiers.
The registry includes the algorithm name, its key sizes and
parameters, and a reference to the defining specification. The
initial values for this registry can be found in Section 9. Adding a
new public-key algorithm MUST be done through the IETF CONSENSUS
method, as described in [RFC2434].
This document requests IANA register the following public-key
algorithm:
+-----+-----------------------------+------------+
| ID | Algorithm | Reference |
+-----+-----------------------------+------------+
| 22 | EdDSA public key algorithm | This doc |
+-----+-----------------------------+------------+
[Notes to RFC-Editor: Please remove the table above on publication.
It is desirable not to reuse old or reserved algorithms because some
existing tools might print a wrong description. A higher number is
also an indication for a newer algorithm. As of now 22 is the next
free number.]
10.3.2. {10.3.2} Symmetric-Key Algorithms
OpenPGP specifies a number of symmetric-key algorithms. This
specification creates a registry of symmetric-key algorithm
identifiers. The registry includes the algorithm name, its key sizes
and block size, and a reference to the defining specification. The
initial values for this registry can be found in Section 9. Adding a
new symmetric-key algorithm MUST be done through the IETF CONSENSUS
method, as described in [RFC2434].
10.3.3. {10.3.3} Hash Algorithms
OpenPGP specifies a number of hash algorithms. This specification
creates a registry of hash algorithm identifiers. The registry
includes the algorithm name, a text representation of that name, its
block size, an OID hash prefix, and a reference to the defining
specification. The initial values for this registry can be found in
Section 9 for the algorithm identifiers and text names, and
Section 5.2.2 for the OIDs and expanded signature prefixes. Adding a
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new hash algorithm MUST be done through the IETF CONSENSUS method, as
described in [RFC2434].
This document requests IANA register the following hash algorithms:
+-----+------------+------------+
| ID | Algorithm | Reference |
+-----+------------+------------+
| 12 | SHA3-256 | This doc |
| 13 | Reserved | |
| 14 | SHA3-512 | This doc |
+-----+------------+------------+
[Notes to RFC-Editor: Please remove the table above on publication.
It is desirable not to reuse old or reserved algorithms because some
existing tools might print a wrong description. The ID 13 has been
reserved so that the SHA3 algorithm IDs align nicely with their SHA2
counterparts.]
10.3.4. {10.3.4} Compression Algorithms
OpenPGP specifies a number of compression algorithms. This
specification creates a registry of compression algorithm
identifiers. The registry includes the algorithm name and a
reference to the defining specification. The initial values for this
registry can be found in Section 9.3. Adding a new compression key
algorithm MUST be done through the IETF CONSENSUS method, as
described in [RFC2434].
11. {11} Packet Composition
OpenPGP packets are assembled into sequences in order to create
messages and to transfer keys. Not all possible packet sequences are
meaningful and correct. This section describes the rules for how
packets should be placed into sequences.
11.1. {11.1} Transferable Public Keys
OpenPGP users may transfer public keys. The essential elements of a
transferable public key are as follows:
o One Public-Key packet
o Zero or more revocation signatures
o Zero or more User ID packets
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o After each User ID packet, zero or more Signature packets
(certifications)
o Zero or more User Attribute packets
o After each User Attribute packet, zero or more Signature packets
(certifications)
o Zero or more Subkey packets
o After each Subkey packet, one Signature packet, plus optionally a
revocation
The Public-Key packet occurs first. Each of the following User ID
packets provides the identity of the owner of this public key. If
there are multiple User ID packets, this corresponds to multiple
means of identifying the same unique individual user; for example, a
user may have more than one email address, and construct a User ID
for each one.
Immediately following each User ID packet, there are zero or more
Signature packets. Each Signature packet is calculated on the
immediately preceding User ID packet and the initial Public-Key
packet. The signature serves to certify the corresponding public key
and User ID. In effect, the signer is testifying to his or her
belief that this public key belongs to the user identified by this
User ID.
Within the same section as the User ID packets, there are zero or
more User Attribute packets. Like the User ID packets, a User
Attribute packet is followed by zero or more Signature packets
calculated on the immediately preceding User Attribute packet and the
initial Public-Key packet.
User Attribute packets and User ID packets may be freely intermixed
in this section, so long as the signatures that follow them are
maintained on the proper User Attribute or User ID packet.
After the User ID packet or Attribute packet, there may be zero or
more Subkey packets. In general, subkeys are provided in cases where
the top-level public key is a signature-only key. However, any V4
key may have subkeys, and the subkeys may be encryption-only keys,
signature-only keys, or general-purpose keys. V3 keys MUST NOT have
subkeys.
Each Subkey packet MUST be followed by one Signature packet, which
should be a subkey binding signature issued by the top-level key.
For subkeys that can issue signatures, the subkey binding signature
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MUST contain an Embedded Signature subpacket with a primary key
binding signature (0x19) issued by the subkey on the top-level key.
Subkey and Key packets may each be followed by a revocation Signature
packet to indicate that the key is revoked. Revocation signatures
are only accepted if they are issued by the key itself, or by a key
that is authorized to issue revocations via a Revocation Key
subpacket in a self-signature by the top-level key.
Transferable public-key packet sequences may be concatenated to allow
transferring multiple public keys in one operation.
11.2. {11.2} Transferable Secret Keys
OpenPGP users may transfer secret keys. The format of a transferable
secret key is the same as a transferable public key except that
secret-key and secret-subkey packets are used instead of the public
key and public-subkey packets. Implementations SHOULD include self-
signatures on any user IDs and subkeys, as this allows for a complete
public key to be automatically extracted from the transferable secret
key. Implementations MAY choose to omit the self-signatures,
especially if a transferable public key accompanies the transferable
secret key.
11.3. {11.3} OpenPGP Messages
An OpenPGP message is a packet or sequence of packets that
corresponds to the following grammatical rules (comma represents
sequential composition, and vertical bar separates alternatives):
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OpenPGP Message :- Encrypted Message | Signed Message |
Compressed Message | Literal Message.
Compressed Message :- Compressed Data Packet.
Literal Message :- Literal Data Packet.
ESK :- Public-Key Encrypted Session Key Packet |
Symmetric-Key Encrypted Session Key Packet.
ESK Sequence :- ESK | ESK Sequence, ESK.
Encrypted Data :- Symmetrically Encrypted Data Packet |
Symmetrically Encrypted Integrity Protected Data Packet
Encrypted Message :- Encrypted Data | ESK Sequence, Encrypted Data.
One-Pass Signed Message :- One-Pass Signature Packet,
OpenPGP Message, Corresponding Signature Packet.
Signed Message :- Signature Packet, OpenPGP Message |
One-Pass Signed Message.
In addition, decrypting a Symmetrically Encrypted Data packet or a
Symmetrically Encrypted Integrity Protected Data packet as well as
decompressing a Compressed Data packet must yield a valid OpenPGP
Message.
11.4. {11.4} Detached Signatures
Some OpenPGP applications use so-called "detached signatures". For
example, a program bundle may contain a file, and with it a second
file that is a detached signature of the first file. These detached
signatures are simply a Signature packet stored separately from the
data for which they are a signature.
12. {12} Enhanced Key Formats
12.1. {12.1} Key Structures
The format of an OpenPGP V3 key is as follows. Entries in square
brackets are optional and ellipses indicate repetition.
RSA Public Key
[Revocation Self Signature]
User ID [Signature ...]
[User ID [Signature ...] ...]
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Each signature certifies the RSA public key and the preceding User
ID. The RSA public key can have many User IDs and each User ID can
have many signatures. V3 keys are deprecated. Implementations MUST
NOT generate new V3 keys, but MAY continue to use existing ones.
The format of an OpenPGP V4 key that uses multiple public keys is
similar except that the other keys are added to the end as "subkeys"
of the primary key.
Primary-Key
[Revocation Self Signature]
[Direct Key Signature...]
[User ID [Signature ...] ...]
[User Attribute [Signature ...] ...]
[[Subkey [Binding-Signature-Revocation]
Primary-Key-Binding-Signature] ...]
A subkey always has a single signature after it that is issued using
the primary key to tie the two keys together. This binding signature
may be in either V3 or V4 format, but SHOULD be V4. Subkeys that can
issue signatures MUST have a V4 binding signature due to the REQUIRED
embedded primary key binding signature.
In the above diagram, if the binding signature of a subkey has been
revoked, the revoked key may be removed, leaving only one key.
In a V4 key, the primary key SHOULD be a key capable of
certification. There are cases, such as device certificates, where
the primary key may not be capable of certification. A primary key
capable of making signatures SHOULD be accompanied by either a
certification signature (on a User ID or User Attribute) or a
signature directly on the key.
Implementations SHOULD accept encryption-only primary keys without a
signature. It also SHOULD allow importing any key accompanied either
by a certification signature or a signature on itself. It MAY accept
signature-capable primary keys without an accompanying signature.
The subkeys may be keys of any other type. There may be other
constructions of V4 keys, too. For example, there may be a single-
key RSA key in V4 format, a DSA primary key with an RSA encryption
key, or RSA primary key with an Elgamal subkey, etc.
It is also possible to have a signature-only subkey. This permits a
primary key that collects certifications (key signatures), but is
used only for certifying subkeys that are used for encryption and
signatures.
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12.2. {12.2} Key IDs and Fingerprints
For a V3 key, the eight-octet Key ID consists of the low 64 bits of
the public modulus of the RSA key.
The fingerprint of a V3 key is formed by hashing the body (but not
the two-octet length) of the MPIs that form the key material (public
modulus n, followed by exponent e) with MD5. Note that both V3 keys
and MD5 are deprecated.
A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99,
followed by the two-octet packet length, followed by the entire
Public-Key packet starting with the version field. The Key ID is the
low-order 64 bits of the fingerprint. Here are the fields of the
hash material, with the example of a DSA key:
a.1) 0x99 (1 octet)
a.2) two-octet scalar octet count of (b)-(e)
b) version number = 4 (1 octet);
c) timestamp of key creation (4 octets);
d) algorithm (1 octet): 17 = DSA (example);
e) Algorithm-specific fields.
Algorithm-Specific Fields for DSA keys (example):
e.1) MPI of DSA prime p;
e.2) MPI of DSA group order q (q is a prime divisor of p-1);
e.3) MPI of DSA group generator g;
e.4) MPI of DSA public-key value y (= g\*\*x mod p where x is secret).
A V5 fingerprint is the 256-bit SHA2-256 hash of the octet 0x99,
followed by the four-octet packet length, followed by the entire
Public-Key packet starting with the version field. The Key ID is the
high-order 64 bits of the fingerprint. Here are the fields of the
hash material, with the example of a DSA key:
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a.1) 0x9A (1 octet)
a.2) four-octet scalar octet count of (b)-(f)
b) version number = 5 (1 octet);
c) timestamp of key creation (4 octets);
d) algorithm (1 octet): 17 = DSA (example);
e) four-octet scalar octet count for the following key material;
f) algorithm-specific fields.
Algorithm-Specific Fields for DSA keys (example):
f.1) MPI of DSA prime p;
f.2) MPI of DSA group order q (q is a prime divisor of p-1);
f.3) MPI of DSA group generator g;
f.4) MPI of DSA public-key value y (= g\*\*x mod p where x is secret).
Note that it is possible for there to be collisions of Key IDs -- two
different keys with the same Key ID. Note that there is a much
smaller, but still non-zero, probability that two different keys have
the same fingerprint.
Also note that if V3, V4, and V5 format keys share the same RSA key
material, they will have different Key IDs as well as different
fingerprints.
Finally, the Key ID and fingerprint of a subkey are calculated in the
same way as for a primary key, including the 0x99 (V3 and V4 key) or
0x9A (V5 key) as the first octet (even though this is not a valid
packet ID for a public subkey).
13. Elliptic Curve Cryptography
This section descripes algorithms and parameters used with Elliptic
Curve Cryptography (ECC) keys. A thorough introduction to ECC can be
found in [KOBLITZ].
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13.1. Supported ECC Curves
This document references five named prime field curves, defined in
[FIPS186] as "Curve P-256", "Curve P-384", and "Curve P-521"; and
defined in [RFC5639] as "brainpoolP256r1", and "brainpoolP512r1".
Further curve "Ed25519", defined in [I-D.irtf-cfrg-eddsa] is
referenced for use with the EdDSA algorithm.
The named curves are referenced as a sequence of bytes in this
document, called throughout, curve OID. Section 9.2 describes in
detail how this sequence of bytes is formed.
13.2. ECDSA and ECDH Conversion Primitives
This document only defines the uncompressed point format for ECDSA
and ECDH. The point is encoded in the Multiprecision Integer (MPI)
format. The content of the MPI is the following:
B = 04 || x || y
where x and y are coordinates of the point P = (x, y), each encoded
in the big-endian format and zero-padded to the adjusted underlying
field size. The adjusted underlying field size is the underlying
field size that is rounded up to the nearest 8-bit boundary.
Therefore, the exact size of the MPI payload is 515 bits for "Curve
P-256", 771 for "Curve P-384", and 1059 for "Curve P-521".
Even though the zero point, also called the point at infinity, may
occur as a result of arithmetic operations on points of an elliptic
curve, it SHALL NOT appear in data structures defined in this
document.
This encoding is compatible with the definition given in [SEC1].
If other conversion methods are defined in the future, a compliant
application MUST NOT use a new format when in doubt that any
recipient can support it. Consider, for example, that while both the
public key and the per-recipient ECDH data structure, respectively
defined in Sections 9{FIXME} and 10{FIXME}, contain an encoded point
field, the format changes to the field in Section 10{FIXME} only
affect a given recipient of a given message.
13.3. EdDSA Point Format
The EdDSA algorithm defines a specific point compression format. To
indicate the use of this compression format and to make sure that the
key can be represented in the Multiprecision Integer (MPI) format the
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octet string specifying the point is prefixed with the octet 0x40.
This encoding is an extension of the encoding given in [SEC1] which
uses 0x04 to indicate an uncompressed point.
For example, the length of a public key for the curve Ed25519 is 263
bit: 7 bit to represent the 0x40 prefix octet and 32 octets for the
native value of the public key.
13.4. Key Derivation Function
A key derivation function (KDF) is necessary to implement the EC
encryption. The Concatenation Key Derivation Function (Approved
Alternative 1) [SP800-56A] with the KDF hash function that is
SHA2-256 [FIPS180] or stronger is REQUIRED. See Section 12{FIXME}
for the details regarding the choice of the hash function.
For convenience, the synopsis of the encoding method is given below
with significant simplifications attributable to the restricted
choice of hash functions in this document. However, [SP800-56A] is
the normative source of the definition.
// Implements KDF( X, oBits, Param );
// Input: point X = (x,y)
// oBits - the desired size of output
// hBits - the size of output of hash function Hash
// Param - octets representing the parameters
// Assumes that oBits <= hBits
// Convert the point X to the octet string, see section 6{FIXME}:
// ZB' = 04 || x || y
// and extract the x portion from ZB'
ZB = x;
MB = Hash ( 00 || 00 || 00 || 01 || ZB || Param );
return oBits leftmost bits of MB.
Note that ZB in the KDF description above is the compact
representation of X, defined in Section 4.2 of [RFC6090].
13.5. EC DH Algorithm (ECDH)
The method is a combination of an ECC Diffie-Hellman method to
establish a shared secret, a key derivation method to process the
shared secret into a derived key, and a key wrapping method that uses
the derived key to protect a session key used to encrypt a message.
The One-Pass Diffie-Hellman method C(1, 1, ECC CDH) [SP800-56A] MUST
be implemented with the following restrictions: the ECC CDH primitive
employed by this method is modified to always assume the cofactor as
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1, the KDF specified in Section 7 is used, and the KDF parameters
specified below are used.
The KDF parameters are encoded as a concatenation of the following 5
variable-length and fixed-length fields, compatible with the
definition of the OtherInfo bitstring [SP800-56A]:
o a variable-length field containing a curve OID, formatted as
follows:
* a one-octet size of the following field
* the octets representing a curve OID, defined in Section 11
o a one-octet public key algorithm ID defined in Section 5
o a variable-length field containing KDF parameters, identical to
the corresponding field in the ECDH public key, formatted as
follows:
* a one-octet size of the following fields; values 0 and 0xff are
reserved for future extensions
* a one-octet value 01, reserved for future extensions
* a one-octet hash function ID used with the KDF
* a one-octet algorithm ID for the symmetric algorithm used to
wrap the symmetric key for message encryption; see Section 8
for details
o 20 octets representing the UTF-8 encoding of the string "Anonymous
Sender ", which is the octet sequence 41 6E 6F 6E 79 6D 6F 75 73
20 53 65 6E 64 65 72 20 20 20 20
o 20 octets representing a recipient encryption subkey or a master
key fingerprint, identifying the key material that is needed for
the decryption. For version 5 keys the 20 leftmost octets of the
fingerprint are used.
The size of the KDF parameters sequence, defined above, is either 54
for the NIST curve P-256 or 51 for the curves P-384 and P-521.
The key wrapping method is described in [RFC3394]. KDF produces a
symmetric key that is used as a key-encryption key (KEK) as specified
in [RFC3394]. Refer to Section 13{FIXME} for the details regarding
the choice of the KEK algorithm, which SHOULD be one of three AES
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algorithms. Key wrapping and unwrapping is performed with the
default initial value of [RFC3394].
The input to the key wrapping method is the value "m" derived from
the session key, as described in Section 5.1{FIXME}, "Public-Key
Encrypted Session Key Packets (Tag 1)", except that the PKCS #1.5
padding step is omitted. The result is padded using the method
described in [PKCS5] to the 8-byte granularity. For example, the
following AES-256 session key, in which 32 octets are denoted from k0
to k31, is composed to form the following 40 octet sequence:
09 k0 k1 ... k31 c0 c1 05 05 05 05 05
The octets c0 and c1 above denote the checksum. This encoding allows
the sender to obfuscate the size of the symmetric encryption key used
to encrypt the data. For example, assuming that an AES algorithm is
used for the session key, the sender MAY use 21, 13, and 5 bytes of
padding for AES-128, AES-192, and AES-256, respectively, to provide
the same number of octets, 40 total, as an input to the key wrapping
method.
The output of the method consists of two fields. The first field is
the MPI containing the ephemeral key used to establish the shared
secret. The second field is composed of the following two fields:
o a one-octet encoding the size in octets of the result of the key
wrapping method; the value 255 is reserved for future extensions;
o up to 254 octets representing the result of the key wrapping
method, applied to the 8-byte padded session key, as described
above.
Note that for session key sizes 128, 192, and 256 bits, the size of
the result of the key wrapping method is, respectively, 32, 40, and
48 octets, unless the size obfuscation is used.
For convenience, the synopsis of the encoding method is given below;
however, this section, [SP800-56A], and [RFC3394] are the normative
sources of the definition.
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Obtain the authenticated recipient public key R
Generate an ephemeral key pair {v, V=vG}
Compute the shared point S = vR;
m = symm_alg_ID || session key || checksum || pkcs5_padding;
curve_OID_len = (byte)len(curve_OID);
Param = curve_OID_len || curve_OID || public_key_alg_ID || 03
|| 01 || KDF_hash_ID || KEK_alg_ID for AESKeyWrap || "Anonymous
Sender " || recipient_fingerprint;
Z_len = the key size for the KEK_alg_ID used with AESKeyWrap
Compute Z = KDF( S, Z_len, Param );
Compute C = AESKeyWrap( Z, m ) as per [RFC3394]
VB = convert point V to the octet string
Output (MPI(VB) || len(C) || C).
The decryption is the inverse of the method given. Note that the
recipient obtains the shared secret by calculating
S = rV = rvG, where (r,R) is the recipient's key pair.
Consistent with Section 5.13{FIXME}, "Sym. Encrypted Integrity
Protected Data Packet (Tag 18)", a Modification Detection Code (MDC)
MUST be used anytime the symmetric key is protected by ECDH.
14. {13} Notes on Algorithms
14.1. {13.1} PKCS#1 Encoding in OpenPGP
This standard makes use of the PKCS#1 functions EME-PKCS1-v1_5 and
EMSA-PKCS1-v1_5. However, the calling conventions of these functions
has changed in the past. To avoid potential confusion and
interoperability problems, we are including local copies in this
document, adapted from those in PKCS#1 v2.1 [RFC3447]. RFC 3447
should be treated as the ultimate authority on PKCS#1 for OpenPGP.
Nonetheless, we believe that there is value in having a self-
contained document that avoids problems in the future with needed
changes in the conventions.
14.1.1. {13.1.1} EME-PKCS1-v1_5-ENCODE
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Input:
k = the length in octets of the key modulus
M = message to be encoded, an octet string of length mLen, where mLen
\<= k - 11
Output:
EM = encoded message, an octet string of length k
Error: "message too long"
1. Length checking: If mLen > k - 11, output "message too long"
and stop.
2. Generate an octet string PS of length k - mLen - 3 consisting
of pseudo-randomly generated nonzero octets. The length of PS
will be at least eight octets.
3. Concatenate PS, the message M, and other padding to form an
encoded message EM of length k octets as
EM = 0x00 || 0x02 || PS || 0x00 || M.
4. Output EM.
14.1.2. {13.1.2} EME-PKCS1-v1_5-DECODE
Input:
EM = encoded message, an octet string
Output:
M = message, an octet string
Error: "decryption error"
To decode an EME-PKCS1_v1_5 message, separate the encoded message EM
into an octet string PS consisting of nonzero octets and a message M
as follows
EM = 0x00 || 0x02 || PS || 0x00 || M.
If the first octet of EM does not have hexadecimal value 0x00, if the
second octet of EM does not have hexadecimal value 0x02, if there is
no octet with hexadecimal value 0x00 to separate PS from M, or if the
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length of PS is less than 8 octets, output "decryption error" and
stop. See also the security note in Section 14 regarding differences
in reporting between a decryption error and a padding error.
14.1.3. {13.1.3} EMSA-PKCS1-v1_5
This encoding method is deterministic and only has an encoding
operation.
Option:
Hash - a hash function in which hLen denotes the length in octets
of the hash function output
Input:
M = message to be encoded
emLen = intended length in octets of the encoded message, at least
tLen + 11, where tLen is the octet length of the DER encoding
T of a certain value computed during the encoding operation
Output:
EM = encoded message, an octet string of length emLen
Errors: "message too long";
"intended encoded message length too short"
Steps:
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1. Apply the hash function to the message M to produce a hash
value H:
H = Hash(M).
If the hash function outputs "message too long," output
"message too long" and stop.
2. Using the list in Section 5.2.2, produce an ASN.1 DER value
for the hash function used. Let T be the full hash prefix
from Section 5.2.2, and let tLen be the length in octets of T.
3. If emLen < tLen + 11, output "intended encoded message length
too short" and stop.
4. Generate an octet string PS consisting of emLen - tLen - 3
octets with hexadecimal value 0xFF. The length of PS will be
at least 8 octets.
5. Concatenate PS, the hash prefix T, and other padding to form
the encoded message EM as
EM = 0x00 || 0x01 || PS || 0x00 || T.
6. Output EM.
14.2. {13.2} Symmetric Algorithm Preferences
The symmetric algorithm preference is an ordered list of algorithms
that the keyholder accepts. Since it is found on a self-signature,
it is possible that a keyholder may have multiple, different
preferences. For example, Alice may have TripleDES only specified
for "alice@work.com" but CAST5, Blowfish, and TripleDES specified for
"alice@home.org". Note that it is also possible for preferences to
be in a subkey's binding signature.
Since TripleDES is the MUST-implement algorithm, if it is not
explicitly in the list, it is tacitly at the end. However, it is
good form to place it there explicitly. Note also that if an
implementation does not implement the preference, then it is
implicitly a TripleDES-only implementation.
An implementation MUST NOT use a symmetric algorithm that is not in
the recipient's preference list. When encrypting to more than one
recipient, the implementation finds a suitable algorithm by taking
the intersection of the preferences of the recipients. Note that the
MUST-implement algorithm, TripleDES, ensures that the intersection is
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not null. The implementation may use any mechanism to pick an
algorithm in the intersection.
If an implementation can decrypt a message that a keyholder doesn't
have in their preferences, the implementation SHOULD decrypt the
message anyway, but MUST warn the keyholder that the protocol has
been violated. For example, suppose that Alice, above, has software
that implements all algorithms in this specification. Nonetheless,
she prefers subsets for work or home. If she is sent a message
encrypted with IDEA, which is not in her preferences, the software
warns her that someone sent her an IDEA-encrypted message, but it
would ideally decrypt it anyway.
14.3. {13.3} Other Algorithm Preferences
Other algorithm preferences work similarly to the symmetric algorithm
preference, in that they specify which algorithms the keyholder
accepts. There are two interesting cases that other comments need to
be made about, though, the compression preferences and the hash
preferences.
14.3.1. {13.3.1} Compression Preferences
Compression has been an integral part of PGP since its first days.
OpenPGP and all previous versions of PGP have offered compression.
In this specification, the default is for messages to be compressed,
although an implementation is not required to do so. Consequently,
the compression preference gives a way for a keyholder to request
that messages not be compressed, presumably because they are using a
minimal implementation that does not include compression.
Additionally, this gives a keyholder a way to state that it can
support alternate algorithms.
Like the algorithm preferences, an implementation MUST NOT use an
algorithm that is not in the preference vector. If the preferences
are not present, then they are assumed to be [ZIP(1),
Uncompressed(0)].
Additionally, an implementation MUST implement this preference to the
degree of recognizing when to send an uncompressed message. A robust
implementation would satisfy this requirement by looking at the
recipient's preference and acting accordingly. A minimal
implementation can satisfy this requirement by never generating a
compressed message, since all implementations can handle messages
that have not been compressed.
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14.3.2. {13.3.2} Hash Algorithm Preferences
Typically, the choice of a hash algorithm is something the signer
does, rather than the verifier, because a signer rarely knows who is
going to be verifying the signature. This preference, though, allows
a protocol based upon digital signatures ease in negotiation.
Thus, if Alice is authenticating herself to Bob with a signature, it
makes sense for her to use a hash algorithm that Bob's software uses.
This preference allows Bob to state in his key which algorithms Alice
may use.
Since SHA2-256 is the MUST-implement hash algorithm, if it is not
explicitly in the list, it is tacitly at the end. However, it is
good form to place it there explicitly.
14.4. {13.4} Plaintext
Algorithm 0, "plaintext", may only be used to denote secret keys that
are stored in the clear. Implementations MUST NOT use plaintext in
Symmetrically Encrypted Data packets; they must use Literal Data
packets to encode unencrypted or literal data.
14.5. {13.5} RSA
There are algorithm types for RSA Sign-Only, and RSA Encrypt-Only
keys. These types are deprecated. The "key flags" subpacket in a
signature is a much better way to express the same idea, and
generalizes it to all algorithms. An implementation SHOULD NOT
create such a key, but MAY interpret it.
An implementation SHOULD NOT implement RSA keys of size less than
1024 bits.
14.6. {13.6} DSA
An implementation SHOULD NOT implement DSA keys of size less than
1024 bits. It MUST NOT implement a DSA key with a q size of less
than 160 bits. DSA keys MUST also be a multiple of 64 bits, and the
q size MUST be a multiple of 8 bits. The Digital Signature Standard
(DSS) [FIPS186] specifies that DSA be used in one of the following
ways:
o 1024-bit key, 160-bit q, SHA-1, SHA2--224, SHA2-256, SHA2-384, or
SHA2-512 hash
o 2048-bit key, 224-bit q, SHA2-224, SHA2-256, SHA2-384, or SHA2-512
hash
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o 2048-bit key, 256-bit q, SHA2-256, SHA2-384, or SHA2-512 hash
o 3072-bit key, 256-bit q, SHA2-256, SHA2-384, or SHA2-512 hash
The above key and q size pairs were chosen to best balance the
strength of the key with the strength of the hash. Implementations
SHOULD use one of the above key and q size pairs when generating DSA
keys. If DSS compliance is desired, one of the specified SHA hashes
must be used as well. [FIPS186] is the ultimate authority on DSS,
and should be consulted for all questions of DSS compliance.
Note that earlier versions of this standard only allowed a 160-bit q
with no truncation allowed, so earlier implementations may not be
able to handle signatures with a different q size or a truncated
hash.
14.7. {13.7} Elgamal
An implementation SHOULD NOT implement Elgamal keys of size less than
1024 bits.
14.8. EdDSA
Although the EdDSA algorithm allows arbitrary data as input, its use
with OpenPGP requires that a digest of the message is used as input
(pre-hashed). See section XXXXX, "Computing Signatures" for details.
Truncation of the resulting digest is never applied; the resulting
digest value is used verbatim as input to the EdDSA algorithm.
14.9. {13.8} Reserved Algorithm Numbers
A number of algorithm IDs have been reserved for algorithms that
would be useful to use in an OpenPGP implementation, yet there are
issues that prevent an implementer from actually implementing the
algorithm. These are marked in Section 9.1, "Public-Key Algorithms",
as "reserved for".
The reserved public-key algorithm X9.42 (21) does not have the
necessary parameters, parameter order, or semantics defined.
Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures
with a public-key identifier of 20. These are no longer permitted.
An implementation MUST NOT generate such keys. An implementation
MUST NOT generate Elgamal signatures. See [BLEICHENBACHER].
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14.10. {13.9} OpenPGP CFB Mode
OpenPGP does symmetric encryption using a variant of Cipher Feedback
mode (CFB mode). This section describes the procedure it uses in
detail. This mode is what is used for Symmetrically Encrypted Data
Packets; the mechanism used for encrypting secret-key material is
similar, and is described in the sections above.
In the description below, the value BS is the block size in octets of
the cipher. Most ciphers have a block size of 8 octets. The AES and
Twofish have a block size of 16 octets. Also note that the
description below assumes that the IV and CFB arrays start with an
index of 1 (unlike the C language, which assumes arrays start with a
zero index).
OpenPGP CFB mode uses an initialization vector (IV) of all zeros, and
prefixes the plaintext with BS+2 octets of random data, such that
octets BS+1 and BS+2 match octets BS-1 and BS. It does a CFB
resynchronization after encrypting those BS+2 octets.
Thus, for an algorithm that has a block size of 8 octets (64 bits),
the IV is 10 octets long and octets 7 and 8 of the IV are the same as
octets 9 and 10. For an algorithm with a block size of 16 octets
(128 bits), the IV is 18 octets long, and octets 17 and 18 replicate
octets 15 and 16. Those extra two octets are an easy check for a
correct key.
Step by step, here is the procedure:
1. The feedback register (FR) is set to the IV, which is all zeros.
2. FR is encrypted to produce FRE (FR Encrypted). This is the
encryption of an all-zero value.
3. FRE is xored with the first BS octets of random data prefixed to
the plaintext to produce C[1] through C[BS], the first BS octets
of ciphertext.
4. FR is loaded with C[1] through C[BS].
5. FR is encrypted to produce FRE, the encryption of the first BS
octets of ciphertext.
6. The left two octets of FRE get xored with the next two octets of
data that were prefixed to the plaintext. This produces C[BS+1]
and C[BS+2], the next two octets of ciphertext.
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7. (The resynchronization step) FR is loaded with C[3] through
C[BS+2].
8. FRE is xored with the first BS octets of the given plaintext,
now that we have finished encrypting the BS+2 octets of prefixed
data. This produces C[BS+3] through C[BS+(BS+2)], the next BS
octets of ciphertext.
9. FR is encrypted to produce FRE.
10. FR is loaded with C[BS+3] to C[BS + (BS+2)] (which is C11-C18
for an 8-octet block).
11. FR is encrypted to produce FRE.
12. FRE is xored with the next BS octets of plaintext, to produce
the next BS octets of ciphertext. These are loaded into FR, and
the process is repeated until the plaintext is used up.
14.11. {13.10} Private or Experimental Parameters
S2K specifiers, Signature subpacket types, user attribute types,
image format types, and algorithms described in Section 9 all reserve
the range 100 to 110 for private and experimental use. Packet types
reserve the range 60 to 63 for private and experimental use. These
are intentionally managed with the PRIVATE USE method, as described
in [RFC2434].
However, implementations need to be careful with these and promote
them to full IANA-managed parameters when they grow beyond the
original, limited system.
14.12. {13.11} Extension of the MDC System
As described in the non-normative explanation in Section 5.13, the
MDC system is uniquely unparameterized in OpenPGP. This was an
intentional decision to avoid cross-grade attacks. If the MDC system
is extended to a stronger hash function, care must be taken to avoid
downgrade and cross-grade attacks.
One simple way to do this is to create new packets for a new MDC.
For example, instead of the MDC system using packets 18 and 19, a new
MDC could use 20 and 21. This has obvious drawbacks (it uses two
packet numbers for each new hash function in a space that is limited
to a maximum of 60).
Another simple way to extend the MDC system is to create new versions
of packet 18, and reflect this in packet 19. For example, suppose
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that V2 of packet 18 implicitly used SHA2-256. This would require
packet 19 to have a length of 32 octets. The change in the version
in packet 18 and the size of packet 19 prevent a downgrade attack.
There are two drawbacks to this latter approach. The first is that
using the version number of a packet to carry algorithm information
is not tidy from a protocol-design standpoint. It is possible that
there might be several versions of the MDC system in common use, but
this untidiness would reflect untidiness in cryptographic consensus
about hash function security. The second is that different versions
of packet 19 would have to have unique sizes. If there were two
versions each with 256-bit hashes, they could not both have 32-octet
packet 19s without admitting the chance of a cross-grade attack.
Yet another, complex approach to extend the MDC system would be a
hybrid of the two above -- create a new pair of MDC packets that are
fully parameterized, and yet protected from downgrade and cross-
grade.
Any change to the MDC system MUST be done through the IETF CONSENSUS
method, as described in [RFC2434].
14.13. {13.12} Meta-Considerations for Expansion
If OpenPGP is extended in a way that is not backwards-compatible,
meaning that old implementations will not gracefully handle their
absence of a new feature, the extension proposal can be declared in
the key holder's self-signature as part of the Features signature
subpacket.
We cannot state definitively what extensions will not be upwards-
compatible, but typically new algorithms are upwards-compatible,
whereas new packets are not.
If an extension proposal does not update the Features system, it
SHOULD include an explanation of why this is unnecessary. If the
proposal contains neither an extension to the Features system nor an
explanation of why such an extension is unnecessary, the proposal
SHOULD be rejected.
15. {14} Security Considerations
o As with any technology involving cryptography, you should check
the current literature to determine if any algorithms used here
have been found to be vulnerable to attack.
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o This specification uses Public-Key Cryptography technologies. It
is assumed that the private key portion of a public-private key
pair is controlled and secured by the proper party or parties.
o Certain operations in this specification involve the use of random
numbers. An appropriate entropy source should be used to generate
these numbers (see [RFC4086]).
o The MD5 hash algorithm has been found to have weaknesses, with
collisions found in a number of cases. MD5 is deprecated for use
in OpenPGP. Implementations MUST NOT generate new signatures
using MD5 as a hash function. They MAY continue to consider old
signatures that used MD5 as valid.
o SHA2-224 and SHA2-384 require the same work as SHA2-256 and
SHA2-512, respectively. In general, there are few reasons to use
them outside of DSS compatibility. You need a situation where one
needs more security than smaller hashes, but does not want to have
the full 256-bit or 512-bit data length.
o Many security protocol designers think that it is a bad idea to
use a single key for both privacy (encryption) and integrity
(signatures). In fact, this was one of the motivating forces
behind the V4 key format with separate signature and encryption
keys. If you as an implementer promote dual-use keys, you should
at least be aware of this controversy.
o The DSA algorithm will work with any hash, but is sensitive to the
quality of the hash algorithm. Verifiers should be aware that
even if the signer used a strong hash, an attacker could have
modified the signature to use a weak one. Only signatures using
acceptably strong hash algorithms should be accepted as valid.
o As OpenPGP combines many different asymmetric, symmetric, and hash
algorithms, each with different measures of strength, care should
be taken that the weakest element of an OpenPGP message is still
sufficiently strong for the purpose at hand. While consensus
about the strength of a given algorithm may evolve, NIST Special
Publication 800-57 [SP800-57] recommends the following list of
equivalent strengths:
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Asymmetric | Hash | Symmetric
key size | size | key size
------------+--------+-----------
1024 160 80
2048 224 112
3072 256 128
7680 384 192
15360 512 256
o There is a somewhat-related potential security problem in
signatures. If an attacker can find a message that hashes to the
same hash with a different algorithm, a bogus signature structure
can be constructed that evaluates correctly.
For example, suppose Alice DSA signs message M using hash
algorithm H. Suppose that Mallet finds a message M' that has the
same hash value as M with H'. Mallet can then construct a
signature block that verifies as Alice's signature of M' with H'.
However, this would also constitute a weakness in either H or H'
or both. Should this ever occur, a revision will have to be made
to this document to revise the allowed hash algorithms.
o If you are building an authentication system, the recipient may
specify a preferred signing algorithm. However, the signer would
be foolish to use a weak algorithm simply because the recipient
requests it.
o Some of the encryption algorithms mentioned in this document have
been analyzed less than others. For example, although CAST5 is
presently considered strong, it has been analyzed less than
TripleDES. Other algorithms may have other controversies
surrounding them.
o In late summer 2002, Jallad, Katz, and Schneier published an
interesting attack on the OpenPGP protocol and some of its
implementations [JKS02]. In this attack, the attacker modifies a
message and sends it to a user who then returns the erroneously
decrypted message to the attacker. The attacker is thus using the
user as a random oracle, and can often decrypt the message.
Compressing data can ameliorate this attack. The incorrectly
decrypted data nearly always decompresses in ways that defeat the
attack. However, this is not a rigorous fix, and leaves open some
small vulnerabilities. For example, if an implementation does not
compress a message before encryption (perhaps because it knows it
was already compressed), then that message is vulnerable. Because
of this happenstance -- that modification attacks can be thwarted
by decompression errors -- an implementation SHOULD treat a
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decompression error as a security problem, not merely a data
problem.
This attack can be defeated by the use of Modification Detection,
provided that the implementation does not let the user naively
return the data to the attacker. An implementation MUST treat an
MDC failure as a security problem, not merely a data problem.
In either case, the implementation MAY allow the user access to
the erroneous data, but MUST warn the user as to potential
security problems should that data be returned to the sender.
While this attack is somewhat obscure, requiring a special set of
circumstances to create it, it is nonetheless quite serious as it
permits someone to trick a user to decrypt a message.
Consequently, it is important that:
1. Implementers treat MDC errors and decompression failures as
security problems.
2. Implementers implement Modification Detection with all due
speed and encourage its spread.
3. Users migrate to implementations that support Modification
Detection with all due speed.
o PKCS#1 has been found to be vulnerable to attacks in which a
system that reports errors in padding differently from errors in
decryption becomes a random oracle that can leak the private key
in mere millions of queries. Implementations must be aware of
this attack and prevent it from happening. The simplest solution
is to report a single error code for all variants of decryption
errors so as not to leak information to an attacker.
o Some technologies mentioned here may be subject to government
control in some countries.
o In winter 2005, Serge Mister and Robert Zuccherato from Entrust
released a paper describing a way that the "quick check" in
OpenPGP CFB mode can be used with a random oracle to decrypt two
octets of every cipher block [MZ05]. They recommend as prevention
not using the quick check at all.
Many implementers have taken this advice to heart for any data
that is symmetrically encrypted and for which the session key is
public-key encrypted. In this case, the quick check is not needed
as the public-key encryption of the session key should guarantee
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that it is the right session key. In other cases, the
implementation should use the quick check with care.
On the one hand, there is a danger to using it if there is a
random oracle that can leak information to an attacker. In
plainer language, there is a danger to using the quick check if
timing information about the check can be exposed to an attacker,
particularly via an automated service that allows rapidly repeated
queries.
On the other hand, it is inconvenient to the user to be informed
that they typed in the wrong passphrase only after a petabyte of
data is decrypted. There are many cases in cryptographic
engineering where the implementer must use care and wisdom, and
this is one.
o Refer to [FIPS186], B.4.1, for the method to generate a uniformly
distributed ECC private key.
o The curves proposed in this document correspond to the symmetric
key sizes 128 bits, 192 bits, and 256 bits, as described in the
table below. This allows a compliant application to offer
balanced public key security, which is compatible with the
symmetric key strength for each AES algorithm defined here.
The following table defines the hash and the symmetric encryption
algorithm that SHOULD be used with a given curve for ECDSA or
ECDH. A stronger hash algorithm or a symmetric key algorithm MAY
be used for a given ECC curve. However, note that the increase in
the strength of the hash algorithm or the symmetric key algorithm
may not increase the overall security offered by the given ECC
key.
Curve name | ECC | RSA | Hash size | Symmetric
| strength | strength, | | key size
| | informative | |
-----------+----------+-------------+-----------+-----------
NIST P-256 256 3072 256 128
NIST P-384 384 7680 384 192
NIST P-521 521 15360 512 256
Requirement levels indicated elsewhere in this document lead to
the following combinations of algorithms in the OpenPGP profile:
MUST implement NIST curve P-256 / SHA2-256 / AES-128, SHOULD
implement NIST curve P-521 / SHA2-512 / AES-256, MAY implement
NIST curve P-384 / SHA2-384 / AES-256, among other allowed
combinations.
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Consistent with the table above, the following table defines the
KDF hash algorithm and the AES KEK encryption algorithm that
SHOULD be used with a given curve for ECDH. A stronger KDF hash
algorithm or AES KEK algorithm MAY be used for a given ECC curve.
Curve name | Recommended KDF | Recommended KEK
| hash algorithm | encryption algorithm
-----------+-----------------+-----------------------
NIST P-256 SHA2-256 AES-128
NIST P-384 SHA2-384 AES-192
NIST P-521 SHA2-512 AES-256
This document explicitly discourages the use of algorithms other
than AES as a KEK algorithm because backward compatibility of the
ECDH format is not a concern. The KEK algorithm is only used
within the scope of a Public-Key Encrypted Session Key Packet,
which represents an ECDH key recipient of a message. Compare this
with the algorithm used for the session key of the message, which
MAY be different from a KEK algorithm.
Compliant applications SHOULD implement, advertise through key
preferences, and use the strongest algorithms specified in this
document.
Note that the symmetric algorithm preference list may make it
impossible to use the balanced strength of symmetric key
algorithms for a corresponding public key. For example, the
presence of the symmetric key algorithm IDs and their order in the
key preference list affects the algorithm choices available to the
encoding side, which in turn may make the adherence to the table
above infeasible. Therefore, compliance with this specification
is a concern throughout the life of the key, starting immediately
after the key generation when the key preferences are first added
to a key. It is generally advisable to position a symmetric
algorithm ID of strength matching the public key at the head of
the key preference list.
Encryption to multiple recipients often results in an unordered
intersection subset. For example, if the first recipient's set is
{A, B} and the second's is {B, A}, the intersection is an
unordered set of two algorithms, A and B. In this case, a
compliant application SHOULD choose the stronger encryption
algorithm.
Resource constraints, such as limited computational power, is a
likely reason why an application might prefer to use the weakest
algorithm. On the other side of the spectrum are applications
that can implement every algorithm defined in this document. Most
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applications are expected to fall into either of two categories.
A compliant application in the second, or strongest, category
SHOULD prefer AES-256 to AES-192.
SHA-1 MUST NOT be used with the ECDSA or the KDF in the ECDH
method.
MDC MUST be used when a symmetric encryption key is protected by
ECDH. None of the ECC methods described in this document are
allowed with deprecated V3 keys. A compliant application MUST
only use iterated and salted S2K to protect private keys, as
defined in Section 3.7.1.3{FIXME}, "Iterated and Salted S2K".
Side channel attacks are a concern when a compliant application's
use of the OpenPGP format can be modeled by a decryption or
signing oracle model, for example, when an application is a
network service performing decryption to unauthenticated remote
users. ECC scalar multiplication operations used in ECDSA and
ECDH are vulnerable to side channel attacks. Countermeasures can
often be taken at the higher protocol level, such as limiting the
number of allowed failures or time-blinding of the operations
associated with each network interface. Mitigations at the scalar
multiplication level seek to eliminate any measurable distinction
between the ECC point addition and doubling operations.
o Although technically possible, the EdDSA algorithm MUST NOT be
used with a digest algorithms weaker than SHA2-256.
OpenPGP was designed with security in mind, with many smart,
intelligent people spending a lot of time thinking about the
ramifications of their decisions. Removing the requirement for self-
certifying User ID (and User Attribute) packets on a key means that
someone could surreptitiously add an unwanted ID to a key and sign
it. If enough "trusted" people sign that surreptitious identity then
other people might believe it. The attack could wind up sending
encrypted mail destined for alice to some other target, bob, because
someone added "alice" to bob's key without bob's consent.
In the case of device certificates the device itself does not have
any consent. It is given an identity by the device manufacturer and
the manufacturer can insert that ID on the device certificate,
signing it with the manufacturer's key. If another people wants to
label the device by another name, they can do so. There is no harm
in multiple IDs, because the verification is all done based on who
has signed those IDs.
When a key can self-sign, it is still suggested to self-certify IDs,
even if it no longer required by this modification to OpenPGP. This
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at least signals to recipients of keys that yes, the owner of this
key asserts that this identity belongs to herself. Note, however,
that mallet could still assert that he is 'alice' and could even
self-certify that. So the attack is not truly different. Moreover,
in the case of device certificates, it's more the manufacturer than
the device that wants to assert an identity (even if the device could
self-certify).
There is no signaling whether a key is using this looser-requirement
key format. An attacker could therefore just remove the self-
signature off a published key. However one would hope that wide
publication would result in another copy still having that signature
and it being returned quickly. However, the lack of signaling also
means that a user with an application following RFC 4880 directly
would see a key following this specification as "broken" and may not
accept it.
On a different note, including the "geo" notation could leak
information about where a signer is located. However it is just an
assertion (albeit a signed assertion) so there is no verifiable truth
to the location information released. Similarly, all the rest of the
signature notations are pure assertions, so they should be taken with
the trustworthiness of the signer.
Combining the User ID with the User Attribute means that an ID and
image would not be separable. For a person this is probably not
good, but for a device it's unlikely the image will change so it
makes sense to combine the ID and image into a single signed packet
with a single signature.
16. Compatibility Profiles
16.1. OpenPGP ECC Profile
A compliant application MUST implement NIST curve P-256, SHOULD
implement NIST curve P-521, SHOULD implemend Ed25519, SHOULD
implement Curve25519, MAY implement NIST curve P-384, MAY implement
brainpoolP256r1, MAY implement brainpoolP512r1, and MAY implement
Curve25519, as defined in Section 11. A compliant application MUST
implement SHA2-256 and SHOULD implement SHA2-384 and SHA2-512. A
compliant application MUST implement AES-128 and SHOULD implement
AES-256.
A compliant application SHOULD follow Section 13{FIXME} regarding the
choice of the following algorithms for each curve:
o the KDF hash algorithm,
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o the KEK algorithm,
o the message digest algorithm and the hash algorithm used in the
key certifications,
o the symmetric algorithm used for message encryption.
It is recommended that the chosen symmetric algorithm for message
encryption be no less secure than the KEK algorithm.
16.2. Suite-B Profile
A subset of algorithms allowed by this document can be used to
achieve [SuiteB] compatibility. The references to [SuiteB] in this
document are informative. This document is primarily concerned with
format specification, leaving additional security restrictions
unspecified, such as matching the assigned security level of
information to authorized recipients or interoperability concerns
arising from fewer allowed algorithms in [SuiteB] than allowed by
this document.
16.3. Security Strength at 192 Bits
To achieve the security strength of 192 bits, [SuiteB] requires NIST
curve P-384, AES-256, and SHA2-384. The symmetric algorithm
restriction means that the algorithm of KEK used for key wrapping in
Section 8 and an OpenPGP session key used for message encryption must
be AES-256. The hash algorithm restriction means that the hash
algorithms of KDF and the OpenPGP message digest calculation must be
SHA2-384.
16.4. Security Strength at 128 Bits
The set of algorithms in Section 12.2.1{FIXME} is extended to allow
NIST curve P-256, AES-128, and SHA2-256.
17. {15} Implementation Nits
This section is a collection of comments to help an implementer,
particularly with an eye to backward compatibility. Previous
implementations of PGP are not OpenPGP compliant. Often the
differences are small, but small differences are frequently more
vexing than large differences. Thus, this is a non-comprehensive
list of potential problems and gotchas for a developer who is trying
to be backward-compatible.
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o The IDEA algorithm is patented, and yet it is required for PGP 2.x
interoperability. It is also the de-facto preferred algorithm for
a V3 key with a V3 self-signature (or no self- signature).
o When exporting a private key, PGP 2.x generates the header "BEGIN
PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY BLOCK".
All previous versions ignore the implied data type, and look
directly at the packet data type.
o PGP 2.0 through 2.5 generated V2 Public-Key packets. These are
identical to the deprecated V3 keys except for the version number.
An implementation MUST NOT generate them and may accept or reject
them as it sees fit. Some older PGP versions generated V2 PKESK
packets (Tag 1) as well. An implementation may accept or reject
V2 PKESK packets as it sees fit, and MUST NOT generate them.
o PGP 2.6.x will not accept key-material packets with versions
greater than 3.
o There are many ways possible for two keys to have the same key
material, but different fingerprints (and thus Key IDs). Perhaps
the most interesting is an RSA key that has been "upgraded" to V4
format, but since a V4 fingerprint is constructed by hashing the
key creation time along with other things, two V4 keys created at
different times, yet with the same key material will have
different fingerprints.
o If an implementation is using zlib to interoperate with PGP 2.x,
then the "windowBits" parameter should be set to -13.
o The 0x19 back signatures were not required for signing subkeys
until relatively recently. Consequently, there may be keys in the
wild that do not have these back signatures. Implementing
software may handle these keys as it sees fit.
o OpenPGP does not put limits on the size of public keys. However,
larger keys are not necessarily better keys. Larger keys take
more computation time to use, and this can quickly become
impractical. Different OpenPGP implementations may also use
different upper bounds for public key sizes, and so care should be
taken when choosing sizes to maintain interoperability. As of
2007 most implementations have an upper bound of 4096 bits.
o ASCII armor is an optional feature of OpenPGP. The OpenPGP
working group strives for a minimal set of mandatory-to-implement
features, and since there could be useful implementations that
only use binary object formats, this is not a "MUST" feature for
an implementation. For example, an implementation that is using
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OpenPGP as a mechanism for file signatures may find ASCII armor
unnecessary. OpenPGP permits an implementation to declare what
features it does and does not support, but ASCII armor is not one
of these. Since most implementations allow binary and armored
objects to be used indiscriminately, an implementation that does
not implement ASCII armor may find itself with compatibility
issues with general-purpose implementations. Moreover,
implementations of OpenPGP-MIME [RFC3156] already have a
requirement for ASCII armor so those implementations will
necessarily have support.
18. References
18.1. Normative References
[AES] NIST, "FIPS PUB 197, Advanced Encryption Standard (AES)",
November 2001,
<http://csrc.nist.gov/publications/fips/fips197/
fips-197.{ps,pdf}>.
[BLOWFISH]
Schneier, B., "Description of a New Variable-Length Key,
64-Bit Block Cipher (Blowfish)", Fast Software Encryption,
Cambridge Security Workshop Proceedings, Springer-Verlag,
1994, pp191-204, December 1993,
<http://www.counterpane.com/bfsverlag.html>.
[BZ2] Seward, J., "The Bzip2 and libbzip2 home page",
<http://www.bzip.org/>.
[EAX] Bellare, M., Rogaway, P., and D. Wagner, "A Conventional
Authenticated-Encryption Mode", April 2003.
[ELGAMAL] Elgamal, T., "A Public-Key Cryptosystem and a Signature
Scheme Based on Discrete Logarithms,", IEEE Transactions
on Information Theory v. IT-31, n. 4, 1985, pp. 469-472, .
[FIPS180] National Institute of Standards and Technology, U.S.
Department of Commerce, "Secure Hash Standard (SHS), FIPS
180-4", August 2015,
<http://dx.doi.org/10.6028/NIST.FIPS.180-4>.
[FIPS186] National Institute of Standards and Technology, U.S.
Department of Commerce, "Digital Signature Standard (DSS),
FIPS 186-4", July 2013,
<http://dx.doi.org/10.6028/NIST.FIPS.186-4>.
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[FIPS202] National Institute of Standards and Technology, U.S.
Department of Commerce, "SHA-3 Standard: Permutation-Based
Hash and Extendable-Output Functions, FIPS 202", August
2015, <http://dx.doi.org/10.6028/NIST.FIPS.202>.
[HAC] Menezes, A., Oorschot, P., and S. Vanstone, "Handbook of
Applied Cryptography", 1996.
[I-D.irtf-cfrg-eddsa]
Josefsson, S. and I. Liusvaara, "Edwards-curve Digital
Signature Algorithm (EdDSA)", draft-irtf-cfrg-eddsa-02
(work in progress), January 2016.
[IDEA] Lai, X., "On the design and security of block ciphers",
ETH Series in Information Processing, J.L. Massey
(editor), Vol. 1, Hartung-Gorre Verlag Konstanz,
Technische Hochschule (Zurich), 1992.
[ISO10646]
International Organization for Standardization,
"Information Technology - Universal Multiple-octet coded
Character Set (UCS) - Part 1: Architecture and Basic
Multilingual Plane", ISO Standard 10646-1, May 1993.
[JFIF] Eric Hamilton, C-Cube Microsystems, Milpitas, CA, "JPEG
File Interchange Format (Version 1.02).", September 1996.
[PKCS5] RSA Laboratories, "PKCS #5 v2.0: Password-Based
Cryptography Standard", March 1999.
[RFC1950] Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data Format
Specification version 3.3", RFC 1950, May 1996.
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144,
May 1997.
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[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
"MIME Security with OpenPGP", RFC 3156, August 2001.
[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard
(AES) Key Wrap Algorithm", RFC 3394, September 2002.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3713] Matsui, M., Nakajima, J., and S. Moriai, "A Description of
the Camellia Encryption Algorithm", RFC 3713, April 2004.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC5639] Lochter, M. and J. Merkle, "Elliptic Curve Cryptography
(ECC) Brainpool Standard Curves and Curve Generation", RFC
5639, DOI 10.17487/RFC5639, March 2010,
<http://www.rfc-editor.org/info/rfc5639>.
[RFC5870] Mayrhofer, A. and C. Spanring, "A Uniform Resource
Identifier for Geographic Locations ('geo' URI)", RFC
5870, DOI 10.17487/RFC5870, June 2010,
<http://www.rfc-editor.org/info/rfc5870>.
[SCHNEIER]
Schneier, B., "Applied Cryptography Second Edition:
protocols, algorithms, and source code in C", 1996.
[SP800-56A]
Barker, E., Johnson, D., and M. Smid, "Recommendation for
Pair-Wise Key Establishment Schemes Using Discrete
Logarithm Cryptography", NIST Special Publication 800-56A
Revision 1, March 2007.
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[SuiteB] National Security Agency, "NSA Suite B Cryptography",
March 2010,
<http://www.nsa.gov/ia/programs/suiteb_cryptography/>.
[TWOFISH] Schneier, B., Kelsey, J., Whiting, D., Wagner, D., Hall,
C., and N. Ferguson, "The Twofish Encryption Algorithm",
1999.
18.2. Informative References
[KOBLITZ] Koblitz, N., "A course in number theory and cryptography,
Chapter VI. Elliptic Curves, ISBN: 0-387-96576-9,
Springer-Verlag", 1997.
[RFC1423] Balenson, D., "Privacy Enhancement for Internet Electronic
Mail: Part III: Algorithms, Modes, and Identifiers", RFC
1423, February 1993.
[RFC1991] Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message
Exchange Formats", RFC 1991, August 1996.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, February 2011.
[SEC1] Standards for Efficient Cryptography Group, "SEC 1:
Elliptic Curve Cryptography", September 2000.
[SP800-57]
NIST, "Recommendation on Key Management", NIST Special
Publication 800-57, March 2007,
<http://csrc.nist.gov/publications/nistpubs/800-57/
SP800-57-Part{1,2}.pdf>.
Appendix A. Test vectors
To help implementing this specification a non-normative example for
the EdDSA algorithm is given.
A.1. Sample EdDSA key
The secret key used for this example is:
D: 1a8b1ff05ded48e18bf50166c664ab023ea70003d78d9e41f5758a91d850f8d2
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Note that this is the raw secret key used as input to the EdDSA
signing operation. The key was created on 2014-08-19 14:28:27 and
thus the fingerprint of the OpenPGP key is:
C959 BDBA FA32 A2F8 9A15 3B67 8CFD E121 9796 5A9A
The algorithm specific input parameters without the MPI length
headers are:
oid: 2b06010401da470f01
q: 403f098994bdd916ed4053197934e4a87c80733a1280d62f8010992e43ee3b2406
The entire public key packet is thus:
98 33 04 53 f3 5f 0b 16 09 2b 06 01 04 01 da 47
0f 01 01 07 40 3f 09 89 94 bd d9 16 ed 40 53 19
79 34 e4 a8 7c 80 73 3a 12 80 d6 2f 80 10 99 2e
43 ee 3b 24 06
A.2. Sample EdDSA signature
The signature is created using the sample key over the input data
"OpenPGP" on 2015-09-16 12:24:53 and thus the input to the hash
function is:
m: 4f70656e504750040016080006050255f95f9504ff0000000c
Using the SHA2-256 hash algorithm yields the digest:
d: f6220a3f757814f4c2176ffbb68b00249cd4ccdc059c4b34ad871f30b1740280
Which is fed into the EdDSA signature function and yields this
signature:
r: 56f90cca98e2102637bd983fdb16c131dfd27ed82bf4dde5606e0d756aed3366
s: d09c4fa11527f038e0f57f2201d82f2ea2c9033265fa6ceb489e854bae61b404
The entire signature packet is thus:
88 5e 04 00 16 08 00 06 05 02 55 f9 5f 95 00 0a
09 10 8c fd e1 21 97 96 5a 9a f6 22 01 00 56 f9
0c ca 98 e2 10 26 37 bd 98 3f db 16 c1 31 df d2
7e d8 2b f4 dd e5 60 6e 0d 75 6a ed 33 66 01 00
d0 9c 4f a1 15 27 f0 38 e0 f5 7f 22 01 d8 2f 2e
a2 c9 03 32 65 fa 6c eb 48 9e 85 4b ae 61 b4 04
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Appendix B. ECC Point compression flag bytes
This specification introduces the new flag byte 0x40 to indicate the
point compression format. The value has been chosen so that the high
bit is not cleared and thus to avoid accidental sign extension. Two
other values might also be interesting for other ECC specifications:
Flag Description
---- -----------
0x04 Standard flag for uncompressed format
0x40 Native point format of the curve follows
0x41 Only X coordinate follows.
0x42 Only Y coordinate follows.
Appendix C. Changes since RFC-4880
o Applied errata 2270, 2271, 2242, 3298.
o Added Camellia cipher from RFC 5581.
o Incorporated RFC 6637 (ECC for OpenPGP)
o Added draft-atkins-openpgp-device-certificates
o Added draft-koch-eddsa-for-openpgp-04
o Added Issuer Fingerprint signature subpacket.
o Added a V5 key and fingerprint format.
o Added OIDs for brainpool curves and Curve25519.
o Marked SHA2-256 as MUST implement.
o Marked Curve25519 and Ed25519 as SHOULD implement.
o Marked SHA-1 as SHOULD NOT be used to create messages.
o Marked MD5 as SHOULD NOT implement.
{ Informational rfcs: [RFC1423] }
Appendix D. The principal authors of RFC-4880 are as follows:
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Jon Callas
EMail: jon@callas.org
Lutz Donnerhacke
EMail: lutz@iks-jena.de
Hal Finney
David Shaw
EMail: dshaw@jabberwocky.com
Rodney Thayer
EMail: rodney@canola-jones.com
Author's Address
Werner Koch
Email: wk@gnupg.org
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Event Timeline

• werner added a comment.Jul 25 2017, 9:06 PM

This is a rendered version of the ID as of commit 584fd6c795530b9d3b290781ef09fd58a2fe0cf3 of the rfc4880bis repo.