What is "md5"

MD5 (Message-Digest Algorithm 5) is a 128-bit unidirectional hash algorithm developed by RSA Data Security, Inc., described in RFC 1321, and widely used by peer-to-peer (P2P) software in the integrity check of files and logins.

It was developed in 1991 by Ronald Rivest to succeed the MD4 which had some security problems. Because it is a one-way algorithm, a md5 hash cannot be transformed back into the source text. The verification method is then done by comparing the two hash (one from the original trusted message and one from the received message). MD5 is also used to check the integrity of a file through, for example, the md5sum program, which creates the hash of a file. This can become very useful for large file downloads, for P2P programs that build the file through pieces and are subject to corruption of the same. How login authentication is used on multiple Unix operating systems and on many authenticated websites.