Secure Hash Algorithm (SHA Hash)

The Secure Hash Algorithm, or SHA Hash, is published by the National Institute of Standards and Technology (NIST) as a U.S. Federal Information Processing Standard - FIPS PUB 180-3, which specifies three flavors of the SHA Algorithm:

• SHA-0: No longer used.

• SHA-1: The most widely used version

• SHA-2: Comes in four different variants: SHA-224, SHA-256, SHA-384, and SHA-512

When a message of any length less than 2⁶⁴ bits (SHA-1, SHA-224, and SHA-256) or less than 2¹²⁸ bits (SHA-384 & SHA-512) is input to a hash algorithm, the result is an output called a message digest. The message digests range in length from 160 to 512 bits, depending on the algorithm.

The five hash algorithms specified in this standard are called secure because, for a given algorithm, it is computationally infeasible to find a message that corresponds to a given message digest, or to find two different messages that produce the same message digest. Any change to a message will, with a high probability, result in a different message digest. This will result in a verification failure when the secure hash algorithm is used with a digital signature algorithm or a keyed-hash message authentication algorithm.