1. MD5 is one of the most popular hash algorithms. It processes an input file or message into a "unique" 128-bit fingerprint. This fingerprint is believed to be "unique"; while it is theoretically possible that two inputs could hash to the same fingerprint, it is nearly statistically impossible. Contrast: Compared to other hash algorithms, MD5 is extremely popular. It is the most popular hashing algorithm, used in SSL, PGP, HTTP authentication, Tripwire, and many other places. MD5 is one of the faster hash algorithms. However, a theoretical weakness has been found such that an attacker may be able to create two separate messages that hash to the same value. Therefore, most use of MD5 is simply for backwards compatibility. History: MD5 was written by Ron Rivest as an enhanced version of the earlier MD4. MD4 is part of many standards, but is considered completely broken by today's standards (and MD5 itself is now considere to have some weaknesses). See also: integrity From Hacking-Lexicon