In 1991, Ron Rivest released the next version of his message digest algorithm, which he called MD5. It also processes 512-bit blocks of the message, but it uses four distinct rounds of computation to produce a digest of the same length as the earlier MD2 and MD4 algorithms (128 bits). MD5 implements security features that reduce the speed of message digest production significantly. Unfortunately, recent cryptanalytic attacks demonstrated that the MD5 protocol is subject to collisions, preventing its use for ensuring message integrity.