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103 lines
4.4 KiB
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# sha1collisiondetection
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Library and command line tool to detect SHA-1 collisions in files
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Copyright 2017 Marc Stevens <marc@marc-stevens.nl>
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Distributed under the MIT Software License.
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See accompanying file LICENSE.txt or copy at https://opensource.org/licenses/MIT.
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## Developers
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- Marc Stevens, CWI Amsterdam (https://marc-stevens.nl)
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- Dan Shumow, Microsoft Research (https://www.microsoft.com/en-us/research/people/danshu/)
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## About
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This library and command line tool were designed as near drop-in replacements for common SHA-1 libraries and sha1sum.
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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.
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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:
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```
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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),
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I(46,2), I(47,2), I(48,2), I(49,2), I(50,2), I(51,2),
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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),
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II(46,2), II(49,2), II(50,2), II(51,2)
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```
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The possibility of false positives can be neglected as the probability is smaller than 2^-90.
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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.
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Colliding files will have the same SHA-1 hash, but will have different unpredictable safe-hashes.
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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.
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For the theoretical explanation of collision detection see the award-winning paper on _Counter-Cryptanalysis_:
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Counter-cryptanalysis, Marc Stevens, CRYPTO 2013, Lecture Notes in Computer Science, vol. 8042, Springer, 2013, pp. 129-146,
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https://marc-stevens.nl/research/papers/C13-S.pdf
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## Inclusion in other programs
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In order to make it easier to include these sources in other project
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there are several preprocessor macros that the code uses. Rather than
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copy/pasting and customizing or specializing the code, first see if
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setting any of these defines appropriately will allow you to avoid
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modifying the code yourself.
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- SHA1DC_NO_STANDARD_INCLUDES
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Skips including standard headers. Use this if your project for
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whatever reason wishes to do its own header includes.
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- SHA1DC_CUSTOM_INCLUDE_SHA1_C
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Includes a custom header at the top of sha1.c. Usually this would be
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set in conjunction with SHA1DC_NO_STANDARD_INCLUDES to point to a
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header file which includes various standard headers.
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- SHA1DC_INIT_SAFE_HASH_DEFAULT
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Sets the default for safe_hash in SHA1DCInit(). Valid values are 0
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and 1. If unset 1 is the default.
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- SHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_C
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Includes a custom trailer in sha1.c. Useful for any extra utility
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functions that make use of the functions already defined in sha1.c.
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- SHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_H
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Includes a custom trailer in sha1.h. Useful for defining the
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prototypes of the functions or code included by
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SHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_C.
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- SHA1DC_CUSTOM_INCLUDE_UBC_CHECK_C
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Includes a custom header at the top of ubc_check.c.
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- SHA1DC_CUSTOM_TRAILING_INCLUDE_UBC_CHECK_C
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Includes a custom trailer in ubc_check.c.
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- SHA1DC_CUSTOM_TRAILING_INCLUDE_UBC_CHECK_H
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Includes a custom trailer in ubc_check.H.
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This code will try to auto-detect certain things based on
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CPU/platform. Unless you're running on some really obscure CPU or
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porting to a new platform you should not need to tweak this. If you do
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please open an issue at
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https://github.com/cr-marcstevens/sha1collisiondetection
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- SHA1DC_FORCE_LITTLEENDIAN / SHA1DC_FORCE_BIGENDIAN
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Override the check for processor endianenss and force either
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Little-Endian or Big-Endian.
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- SHA1DC_FORCE_UNALIGNED_ACCESS
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Permit unaligned access. This will fail on e.g. SPARC processors, so
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it's only permitted on a whitelist of processors. If your CPU isn't
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detected as allowing this, and allows unaligned access, setting this
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may improve performance (or make it worse, if the kernel has to
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catch and emulate such access on its own).
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