A copy of sha1collisiondetection by Marc Stevens for use by the git.git project

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  1. cc46554 Skip temporary variable for SHA1DC_ALLOW_UNALIGNED_ACCESS by Ævar Arnfjörð Bjarmason · 9 weeks ago master
  2. 56e9ea3 Fix misformatted code added in db45d67 by Ævar Arnfjörð Bjarmason · 9 weeks ago
  3. b0d20a0 README: Document the macros you can set to override CPU detection by Ævar Arnfjörð Bjarmason · 9 weeks ago
  4. ca94024 Allow for defining SHA1DC_FORCE_UNALIGNED_ACCESS externally by Ævar Arnfjörð Bjarmason · 9 weeks ago
  5. 394e2f0 Remove trailing spaces in code by Ævar Arnfjörð Bjarmason · 9 weeks ago


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




Command-line usage

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).


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 -

Library usage

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];

/** 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");
    printf("no collision detected");

Inclusion in other programs

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.


    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.


    Sets the default for safe_hash in SHA1DCInit(). Valid values are 0 and 1. If unset 1 is the default.


    Includes a custom trailer in sha1.c. Useful for any extra utility functions that make use of the functions already defined in sha1.c.


    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.


    Includes a custom header at the top of ubc_check.c.


    Includes a custom trailer in ubc_check.c.


    Includes a custom trailer in ubc_check.H.

This code will try to auto-detect certain things based on CPU/platform. Unless you're running on some really obscure CPU or porting to a new platform you should not need to tweak this. If you do please open an issue at https://github.com/cr-marcstevens/sha1collisiondetection


    Override the check for processor endianenss and force either Little-Endian or Big-Endian.


    Permit unaligned access. This will fail on e.g. SPARC processors, so it‘s only permitted on a whitelist of processors. If your CPU isn’t detected as allowing this, and allows unaligned access, setting this may improve performance (or make it worse, if the kernel has to catch and emulate such access on its own).