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+# sha1collisiondetection
+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.
+
+## Developers
+
+- Marc Stevens, CWI Amsterdam (https://marc-stevens.nl)
+- Dan Shumow, Microsoft Research (https://www.microsoft.com/en-us/research/people/danshu/)
+
+## About
+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
+
+## Compiling
+
+Run:
+```
+make
+```
+
+## 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).
+
+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 -
+```
+
+## 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];
+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");
+```
+
+## 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.
+
+- 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.
+
+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
+
+- SHA1DC_FORCE_LITTLEENDIAN / SHA1DC_FORCE_BIGENDIAN
+
+ Override the check for processor endianenss and force either
+ Little-Endian or Big-Endian.
+
+- SHA1DC_FORCE_UNALIGNED_ACCESS
+
+ 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).