Cryptographic hash function

A cryptographic hash function, also known as a collision resistant hash function, maps a message (of any length) to a digest (of a fixed length), and has the following three properties:

• Preimage resistance. It is too difficult to invert the function and find an original message from its digest.
• Second preimage resistance. It is too difficult to change the message without also changing its digest.
• Collision resistance. It is too difficult to find any two messages with the same digest.

With many algorithms, the message is a string of octets, and the digest has a fixed number of bits. For example, SHA-256 always computes a 256-bit digest. One can write this digest in hexadecimal.

Though algorithms like MD4 define digest for message of any bit length, many actual implementations assume that the bit length is a multiple of 8.

Algorithms

SHA-256 is the recommended stronger alternative to SHA-1 and RIPEMD-160, which in turn are stronger than MD5, which in turn is stronger than MD4.