The MD5 Message-Digest Algorithm is a widely used cryptographic hash function that produces a 128-bit (16-byte) hash value. Specified in RFC 1321, MD5 has been utilized in a wide variety of security applications, and is also commonly used to check data integrity. MD5 was designed by Ron Rivest in 1991 to replace an earlier hash function, MD4. An MD5 hash is typically expressed as a hexadecimal number, 32 digits long.

However, it has since been shown that MD5 is not collision resistant;[3] as such, MD5 is not suitable for applications like SSL certificates or digital signatures that rely on this property. In 1996, a flaw was found with the design of MD5, and while it was not a clearly fatal weakness, cryptographers began recommending the use of other algorithms, such as SHA-1—which has since been found to be vulnerable as well. In 2004, more serious flaws were discovered in MD5, making further use of the algorithm for security purposes questionable—specifically, a group of researchers described how to create a pair of files that share the same MD5 checksum.[4][5] Further advances were made in breaking MD5 in 2005, 2006, and 2007.[6] In December 2008, a group of researchers used this technique to fake SSL certificate validity,[7][8] and CMU Software Engineering Institute now says that MD5 "should be considered cryptographically broken and unsuitable for further use",[9] and most U.S. government applications now require the SHA-2 family of hash functions.[10]

MD5 is one in a series of message digest algorithms designed by Professor Ronald Rivest of MIT (Rivest, 1994). When analytic work indicated that MD5's predecessor MD4 was likely to be insecure, MD5 was designed in 1991 to be a secure replacement. (Weaknesses were indeed later found in MD4 by Hans Dobbertin.)