A message digest is the results you get when you run text (such as an e-mail message) through a hash algorithm. A message digest concisely represents a longer message or document. You can think of a message digest as the "digital fingerprint" of a larger document. A message digest is used to create a digital signature that's unique to a particular document.

A message digest does not reveal the contents of a document. That is, if you can view the message digest, you cannot figure out what the original message was.

MD2, MD4 and MD5 (MD stands for Message Digest) are widely used hash functions designed specifically for cryptographic use. They produce 128-bit digests and there is no known attack faster than exhaustive search.