MD5 hashing method
Published by Evil Bee Sunday, 29 June 2008 13:36
The algorithm takes as input a message of arbitrary length and produces as output a 128-bit "fingerprint" or "message digest" of the input. It is conjectured that it is computationally infeasible to produce two messages having the same message digest, or to produce any message having a given prespecified target message digest. The MD5 algorithm is intended for digital signature applications, where a large file must be "compressed" in a secure manner before being encrypted with a private (secret) key under a public-key cryptosystem such as RSA.
The MD5 algorithm is an extension of the MD4 message-digest algorithm. MD5 is slightly slower than MD4, but is more "conservative" in design. MD5 was designed because it was felt that MD4 was perhaps being adopted for use more quickly than justified by the existing critical review; because MD4 was designed to be exceptionally fast, it is "at the edge" in terms of risking successful cryptanalytic attack. MD5 backs off a bit, giving up a little in speed for a much greater likelihood of ultimate security. It incorporates some suggestions made by various reviewers, and contains additional optimizations.
The MD5 algorithm is a block-chained hashing algorithm. The first block is hashed with an initial seed, resulting in a hash. The hash is summed with the seed, and that result becomes the seed for the next block. When the last block is computed, it's "next-seed' value becomes the hash for the entire stream. Thus, the seed for block depends on both the hash and the seed of its preceding block. As a result, blocks cannot be hashed in parallel.