The MD6 Message-Digest Algorithm is a cryptographic hash function. It uses a Merkle tree-like structure to allow for immense parallel computation of hashes for very long inputs. Authors claim a performance of 28 cycles per byte for MD6-256 on an Intel Core 2 Duo and provable resistance against differential cryptanalysis. The source code of the reference implementation was released under MIT license. Speeds in excess of 1 GB/s have been reported to be possible for long messages on 16-core CPU architecture. In December 2008, Douglas Held of Fortify Software discovered a buffer overflow in the original MD6 hash algorithm's reference implementation. This error was later made public by Ron Rivest on 19 February 2009, with a release of a corrected reference implementation in advance of the Fortify Report. MD6 was submitted to the NIST SHA-3 competition. However, on July 1, 2009, Rivest posted a comment at NIST that MD6 is not yet ready to be a candidate for SHA-3 because of speed issues, a "gap in the proof that the submitted version of MD6 is resistant to differential attacks", and an inability to supply such a proof for a faster reduced-round version, although Rivest also stated at the MD6 website that it is not withdrawn formally. MD6 did not advance to the second round of the SHA-3 competition. In September 2011, a paper presenting an improved proof that MD6 and faster reduced-round versions are resistant to differential attacks was posted to the MD6 website.

MD6 hash test vectors

MD6("The quick brown fox jumps over the lazy dog") = 977592608c45c9923340338450fdcccc21a68888e1e6350e133c5186cd9736ee A change in even a single bit of the message will, with overwhelming probability, result in a completely different message digest due to the avalanche effect: MD6("The quick brown fox jumps over the lazy cog") = 85fe717a5896a085a31be5d9457b4da75a6ebc003eded96d7cb0ff1737235bba The hash of the zero-length string is: MD6("") = bca38b24a804aa37d821d31af00f5598230122c5bbfc4c4ad5ed40e4258f04ca