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