A Message Digest is a digitally created hash (fingerprint) created from a plaintext block. All the information of the message is used to construct the Message Digest hash, but the message cannot be recovered from the hash. For this reason, Message Digests are also known as one way hash functions.

The size of a Message Digest is always the same, independent of the size or content of the message from which it was created. Generally, the size of a Message Digest is fairly short ( 1024 bits). The ideal Message Digest algorithm would possibly alter 50% of the bits in the resulting hash if one bit was altered in the plaintext message. 

Creating a Message Digest from a Plaintext message

See also: Message Digest code example