Sorry, without detailed output of gpg we can't help you here. This is definitely not a GnuPG bug because too many people are using mutt and gnupg. You should also "set crypt_use_gpgme" -it works far better.

Packet 20 is the new AEAD packet which GnuPG 2.3 can generate and does generate if all recipients have new keys generated with such a versions. However, the version of gpg you use now does not support AEAD and thus fails.

• You created a key with a new version of gpg (2.3.x) and thus listing AEAD as its preferences
• You sent a mail to that key using gnupg 2.3 and AEAD was used
• You used GnuPG 2.2 to decrypt this message encrypted to your own AEAD capable key. However that particular 2.2 has not even read-only support for AEAD.

The general rule for downgrading to an older versions is to adjust the algorithm preferences for keys generated with the newer version.

Thanks @werner

1. So, firstly, can we get an error message that states something to that effect AND can also be displayed by Mutt?

2. Secondly, are you saying that I should simply recreate the keys with the 'lowest' gpg package used?

Not sure what to do with the keys that are on the openpg.org keyservers but I'll investigate if I can update those.

Use a gpg 2.3 version:

$ gpg --edit-key YOUR_KEY
[...]
gpg> pref 
[ unknown] (1). aead-eax@test.gnupg.org
     S9 S8 S7 S2 A1 H10 H9 H8 H11 H2 Z2 Z3 Z1 [mdc] [aead] [no-ks-modify]
gpg> setpref  S9 S8 S7 S2 H10 H9 H8 H11 H2 Z2 Z3 Z1
Set preference list to:
     Cipher: AES256, AES192, AES, 3DES
     AEAD: 
     Digest: SHA512, SHA384, SHA256, SHA224, SHA1
     Compression: ZLIB, BZIP2, ZIP, Uncompressed
     Features: MDC, Keyserver no-modify
Really update the preferences? (y/N) y
[...]
gpg> save

In T5886#156407, @TonyBarganski wrote:

2. As things stand right now, someone with a Public key created on gpg version 2.3 on a macOS cannot privately communicate with someone using a Linux server, news group or Linux Desktop.

I'm pretty sure that this claim is false. Above Werner wrote "Packet 20 is the new AEAD packet which GnuPG 2.3 can generate and does generate if all recipients have new keys generated with such a versions."

Your problem is that you created a key with gpg 2.3 (or a newer version of gpg 2.2) and then copied this key to another machine with a very old version of gpg 2.2.

Despite Werner's comment "Do not use a lower minor version on one of your systems or change the preferences of your new keys if you need to use outdated versions.", I'd go for the pragmatic workaround to "recreate the keys with the 'lowest' gpg package used".

@ikloecker thanks for your reply.

RE: "I'd go for the pragmatic workaround to 'recreate the keys with the 'lowest' gpg package used'"

1. If I create entirely NEW keys, how do I update my old keys on the 'keys.openpgp.org' keyserver?

2. Can we get some documentation in PLAIN ENGLISH on this topic?

I found @werner language very PGP specific and I couldn't understand it, and I'm a techie.

3. This suggestion/situation to me says that backward compatibility in PGP is broken!

Happy to be proven otherwise.

Thanks in advance.

Backward compatibility means that newer versions work with data created with older versions of a program. What you are asking for is forward compatibility, i.e. you want older versions of a program to work with data created with newer versions of a program. In the extreme that would mean that gpg must not use modern encryption algorithms because old versions of gpg cannot deal with them. It should be obvious that this doesn't make any sense.

To get compatibility with older versions of gpg you have to tell the newer gpg that it should create data (e.g. keys or encrypted data) that is compatible with older versions of gpg. That's what Werner is trying to tell you with "change the preferences of your new keys if you need to use outdated versions".

Regarding

If I create entirely NEW keys, how do I update my old keys on the 'keys.openpgp.org' keyserver?

one possibility is to revoke the old keys and then upload the revoked keys to the keyserver. This way you communicate to the world that the old keys shouldn't be used anymore.

@ikloecker Thanks for the clarification (appreciated).

• So (how) do we do that? How do I create keys that can be read by gpg 2.2?

Ubuntu 20.04 package management only supports this version. Its not feasible to recompile gpg on Ubuntu20.04.

• Is there some documentation you can point me to which explains this and what to do in such a situation?

Create the keys with gpg 2.2. I'm not aware of such documentation apart from the manual page of GnuPG. And, as I tried to explain, this situation isn't really different from any other software. If you create a document with the newest version of LibreOffice then you cannot expect it to look exactly the same with an older version of LibreOffice. It's your responsibility not to use new features of the new LibreOffice if you still need to use an older version on another machine.

Hi @werner
I had missed your earlier post quoted below on using setperf.

I've gone through the gpg man page but cannot see what those 1char + {1-2}digit preference codes refer to.

Where can I get a list of the correct option codes that I need to set please to turn off [aead]?

Tony

In T5886#156410, @werner wrote:

Use a gpg 2.3 version:

$ gpg --edit-key YOUR_KEY
[...]
gpg> pref 
[ unknown] (1). aead-eax@test.gnupg.org
     S9 S8 S7 S2 A1 H10 H9 H8 H11 H2 Z2 Z3 Z1 [mdc] [aead] [no-ks-modify]
gpg> setpref  S9 S8 S7 S2 H10 H9 H8 H11 H2 Z2 Z3 Z1
Set preference list to:
     Cipher: AES256, AES192, AES, 3DES
     AEAD: 
     Digest: SHA512, SHA384, SHA256, SHA224, SHA1
     Compression: ZLIB, BZIP2, ZIP, Uncompressed
     Features: MDC, Keyserver no-modify
Really update the preferences? (y/N) y
[...]
gpg> save

S9, etc. are short-hand IDs, for the cipher algorithms, digest algorithms, etc. Use showpref instead of pref to get the preference list in human-readable form (AES256, SHA512, etc.) instead of in expert form (cryptic IDs).

You can simply use the algorithm names instead of the IDs with setpref, e.g.

gpg> setpref AES256 AES192 AES 3DES SHA512 SHA384 SHA256 SHA224 SHA1 ZLIB BZIP2 ZIP

@werner
The setpref S9 S8 S7 S2 H10 H9 H8 H11 H2 Z2 Z3 Z1 worked!

But there is no documentation that I can find to tell me which setting took off the AEAD preference from my key.

This isn't very workable. I think there needs a ticket or two:

a) Create a description for the meaning of the unknown packet: type 20 error message and a link to documentation which discusses the options and how to get a working solution, i.e. change the key preferences to remove, in this case, AEAD capabilities from the key

b) Add documentation on how to change the preferences of a key and where to look up the available option codes - better still, have human readable options for AEAD

c) A writeup on the fact that this might happen and how to update the keyservers with one's updated keys