(Message Digest 5) A popular one-way hash function developed by Ronald Rivest (the "R" in RSA) which is used to create a message digest for digital signatures. MD5 is faster than SHA-1, but is considered less secure. MD5 is similar to the previous MD4 method as both were designed for 32-bit computers, but MD5 adds more security since MD4 has been broken. The earlier MD2 function was designed for 8-bit computers. See one-way hash function.