Library and command line tool to detect SHA-1 collisions in files

Copyright 2017 Marc Stevens [email protected]

Distributed under the MIT Software License.

See accompanying file LICENSE.txt or copy at https://opensource.org/licenses/MIT.

• Marc Stevens, CWI Amsterdam (https://marc-stevens.nl)
• Dan Shumow, Microsoft Research (https://www.microsoft.com/en-us/research/people/danshu/)

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.

• SHA1DC_NO_STANDARD_INCLUDES

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.