3.8.3 What are MD2, MD4 and MD5?

  MD2, MD4 and MD5 (MD stands for Message Digest) are widely used hash functions designed by Ron Rivest specifically for cryptographic use. They produce 128-bit digests and there is no known attack faster than exhaustive search.

MD2 is the slowest of the three; MD4 is the fastest. MD5 has been dubbed ``MD4 with safety belts'' by Rivest, since it has a more conservative design than MD4; the design gives it increased security against attack, but at a cost of being approximately 33% slower than MD4. MD5 is the most commonly used of the three algorithms. MD4 and MD5 are publicly available for unrestricted use; MD2 is available for use with PEM (see Question 3.8.7). Details of MD2, MD4, and MD5 with sample C code are available in Internet RFCs (Requests For Comments) 1319, 1320, and 1321, respectively.

No feasible attacks on any of the MD algorithms have been discovered, although some recent theoretical work has found some interesting structural properties.