Cryptographic Hash Function

is a mathematical function that creates a unique number that represents the contents of a block of data or text. In digital signature processing the data or text that the user is digitally signing is called the message. The number generated by the cryptographic hash function is called the message digest. To verify a copy of the message, the cryptographic hash function is applied to both the original message and the copy of the message, and the resulting message digests are compared. If they are both the same, then the copy is valid.

There is a number of cryptographic hash functions used in digital signature processing. All major development environments such as Microsoft and Adobe support the most commonly used hash algorithm family, SHA-1, SHA-256, SHA-384, SHA-512 hash algorithms which were developed by National Security Agency (NSA).