| commit | ca94024de3b8b1bfc73ffac004a8b14ebaf7bc7e | [log] [tgz] |
|---|---|---|
| author | Ævar Arnfjörð Bjarmason <avarab@gmail.com> | Thu May 18 21:38:43 2017 +0000 |
| committer | Marc Stevens <cr-marcstevens@users.noreply.github.com> | Sat May 20 12:49:39 2017 +0200 |
| tree | 09045cd6b06fa0b2acb5f47e1de237d9ffa74539 | |
| parent | 394e2f0f3ff51da9d7b6f3a6305006daadc85e6c [diff] |
Allow for defining SHA1DC_FORCE_UNALIGNED_ACCESS externally This allows for doing -DSHA1DC_FORCE_UNALIGNED_ACCESS on a platform/CPU that's not listed here, since the default is to fall back to aligned access unless except on a whitelist of CPUs. It would be nice e.g. for the git project if someone running on such a platform didn't have to monkeypatch the code, instead they can just provide a -DSHA1DC_FORCE_UNALIGNED_ACCESS argument
Library and command line tool to detect SHA-1 collisions in files
Copyright 2017 Marc Stevens marc@marc-stevens.nl
Distributed under the MIT Software License.
See accompanying file LICENSE.txt or copy at https://opensource.org/licenses/MIT.
This library and command line tool were designed as near drop-in replacements for common SHA-1 libraries and sha1sum. They will compute the SHA-1 hash of any given file and additionally will detect cryptanalytic collision attacks against SHA-1 present in each file. It is very fast and takes less than twice the amount of time as regular SHA-1.
More specifically they will detect any cryptanalytic collision attack against SHA-1 using any of the top 32 SHA-1 disturbance vectors with probability 1:
I(43,0), I(44,0), I(45,0), I(46,0), I(47,0), I(48,0), I(49,0), I(50,0), I(51,0), I(52,0),
I(46,2), I(47,2), I(48,2), I(49,2), I(50,2), I(51,2),
II(45,0), II(46,0), II(47,0), II(48,0), II(49,0), II(50,0), II(51,0), II(52,0), II(53,0), II(54,0), II(55,0), II(56,0),
II(46,2), II(49,2), II(50,2), II(51,2)
The possibility of false positives can be neglected as the probability is smaller than 2^-90.
The library supports both an indicator flag that applications can check and act on, as well as a special safe-hash mode that returns the real SHA-1 hash when no collision was detected and a different safe hash when a collision was detected. Colliding files will have the same SHA-1 hash, but will have different unpredictable safe-hashes. This essentially enables protection of applications against SHA-1 collisions with no further changes in the application, e.g., digital signature forgeries based on SHA-1 collisions automatically become invalid.
For the theoretical explanation of collision detection see the award-winning paper on Counter-Cryptanalysis:
Counter-cryptanalysis, Marc Stevens, CRYPTO 2013, Lecture Notes in Computer Science, vol. 8042, Springer, 2013, pp. 129-146, https://marc-stevens.nl/research/papers/C13-S.pdf
Run:
make
There are two programs bin/sha1dcsum and bin/sha1dcsum_partialcoll. The first program bin/sha1dcsum will detect and warn for files that were generated with a cryptanalytic SHA-1 collision attack like the one documented at https://shattered.io/. The second program bin/sha1dcsum_partialcoll will detect and warn for files that were generated with a cryptanalytic collision attack against reduced-round SHA-1 (of which there are a few examples so far).
Examples:
bin/sha1dcsum test/sha1_reducedsha_coll.bin test/shattered-1.pdf bin/sha1dcsum_partialcoll test/sha1reducedsha_coll.bin test/shattered-1.pdf pipe_data | bin/sha1dcsum -
See the documentation in lib/sha1.h. Here is a simple example code snippet:
#include <sha1dc/sha1.h>
SHA1_CTX ctx;
unsigned char hash[20];
SHA1DCInit(&ctx);
/** disable safe-hash mode (safe-hash mode is enabled by default) **/
// SHA1DCSetSafeHash(&ctx, 0);
/** disable use of unavoidable attack conditions to speed up detection (enabled by default) **/
// SHA1DCSetUseUBC(&ctx, 0);
SHA1DCUpdate(&ctx, buffer, (unsigned)(size));
int iscoll = SHA1DCFinal(hash,&ctx);
if (iscoll)
printf("collision detected");
else
printf("no collision detected");
In order to make it easier to include these sources in other project there are several preprocessor macros that the code uses. Rather than copy/pasting and customizing or specializing the code, first see if setting any of these defines appropriately will allow you to avoid modifying the code yourself.
Skips including standard headers. Use this if your project for whatever reason wishes to do its own header includes.
SHA1DC_CUSTOM_INCLUDE_SHA1_C
Includes a custom header at the top of sha1.c. Usually this would be set in conjunction with SHA1DC_NO_STANDARD_INCLUDES to point to a header file which includes various standard headers.
SHA1DC_INIT_SAFE_HASH_DEFAULT
Sets the default for safe_hash in SHA1DCInit(). Valid values are 0 and 1. If unset 1 is the default.
SHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_C
Includes a custom trailer in sha1.c. Useful for any extra utility functions that make use of the functions already defined in sha1.c.
SHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_H
Includes a custom trailer in sha1.h. Useful for defining the prototypes of the functions or code included by SHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_C.
SHA1DC_CUSTOM_INCLUDE_UBC_CHECK_C
Includes a custom header at the top of ubc_check.c.
SHA1DC_CUSTOM_TRAILING_INCLUDE_UBC_CHECK_C
Includes a custom trailer in ubc_check.c.
SHA1DC_CUSTOM_TRAILING_INCLUDE_UBC_CHECK_H
Includes a custom trailer in ubc_check.H.