Message digests

 

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Message digests are fixed size numeric representations of the contents of messages, which are inherently variable in size. A message digest is computed by a hash function, which is a transformation that meets two criteria:

• The hash function must be one-way. It must not be possible to reverse the function to find the message corresponding to a given message digest, other than by testing all possible messages.

• It must be computationally infeasible to find two messages that hash to the same digest.

A message digest is also known as a Message Authentication Code (MAC), because it can provide assurance that the message has not been modified. The message digest is sent with the message itself. The receiver can generate a digest for the message and compare it with the sender's digest. If the two digests are the same, this verifies the integrity of the message. Any tampering with the message during transmission almost certainly results in a different message digest.