The full name of md5 is message-digest algorithm 5, which was developed by computer science laboratory of Massachusetts Institute of Technology and ronald l. rivest of rsa data security inc in the early 1990s, and by md2, md3 and md4. Its function is to "compress" a large amount of information into a secure format (that is, convert a byte string of any length into a large integer of a certain length) before signing the private key with digital signature software. No matter md2, md4 or md5, it is necessary to get a random length of information and generate a 128-bit information summary. Although the structures of these algorithms are more or less similar, the design of md2 is completely different from that of md4 and md5, because md2 is optimized for 8-bit computers, while md4 and md5 are designed for 32-bit computers. The descriptions of these three algorithms and the source code of C language are described in detail in the Internet rfcs 132 1(/ set to openly solicit attacks specifically aimed at MD5), which was published on August 17, 2004: "China researchers found the collision of complete MD5 algorithms; Wang, Feng, Lai and Yu announced the collision of several Hash functions: MD5, MD4, HAVAL- 128, RIPEMD- 128. This is the most substantial research progress in the field of cryptography in recent years. Using their technology, MD5 conflicts can be found in a few hours. ..... Because of this landmark discovery, the MD5CRK project will end in the next 48 hours ".

MD5 uses hash function, and the irreversible encryption algorithms widely used in computer networks include MD5 algorithm invented by RSA company and secure hash algorithm SHA proposed by National Institute of Technical Standards.