A group of hashing algorithms used in cryptography and developed by Rivest. The term “message digest” refers to a short string or hash value of fixed length that is computed from the longer variable-length message being hashed by the algorithm. The important message digest (MD) algorithms include MD2, MD4, and MD5, all of which produce a 128-bit hash value. The MD algorithms are commonly used to generate a digital signature from a message.
MD2 was developed in 1989 for 8-bit encoders. It pads the message to be encoded until it is a multiple of 16 bytes in length, appends a 16-byte checksum, and computes the hash.
MD4 was developed in 1990 for 32-bit encoders. It pads the message to be encoded until it is 56 bytes short of being a multiple of 512 bytes, appends an 8-byte message length value, and iteratively hashes the message in three rounds. MD4 can be broken fairly easily in a dedicated cryptographic attempt. It is implemented in the Microsoft Challenge Handshake Authentication Protocol (MS-CHAP) supported by the Remote Access Service (RAS) on Microsoft Windows NT.
MD5 was developed in 1991 for 32-bit encoders and is an extension of MD4. It uses four rounds of hashing instead of three. It is fairly difficult to crack. Windows NT RAS Client supports MD5-CHAP for connecting to third-party Point-to-Point Protocol (PPP) servers supporting MD5 authentication, but Windows NT RAS Server does not. However, Service Pack 3 for Windows NT provides limited support for MD5-CHAP PPP authentication.
MD5 is also used in Simple Network Management Protocol (SNMP) version 2 for authenticating the origin of SNMP messages and verifying their integrity. Another hashing algorithm used in generating digital signatures is the secure hash algorithm (SHA) developed by the Network Information System (NIS).