Message Digests (HASH)

Message digests or hashes are commonly 128 bits to 160 bits in length and provide a digital identifier for each digital file or document. Message digest functions also called hash functions, are used to produce digital summaries of information called message digests. Message digest functions are mathematical functions that process information to "produce a different message digest for each unique document". Identical documents have the same message digest; but if even one of the bits for the document changes, the message digest changes.

Figure. Example of the Message Digest Process

Because message digests are much shorter than the data from which the digests are generated and the digests have a finite length, duplicate message digests called collisions can exist for different data sets. However, good message digest functions use one-way functions to ensure that it is mathematically and computationally infeasible to reverse the message digest process and discover the original data.

Message digests are commonly used in conjunction with public key technology to create digital signatures or "digital thumbprints" that are used for authentication, integrity, and nonrepudiation. Message digests also are commonly used with digital signing technology to provide data integrity for electronic files and documents.