Hi Shirpelu,

Secure
Hash Algorithm
“This standard specifies a Secure Hash Algorithm (SHA), which is necessary to ensure the security of the Digital Signature Algorithm (DSA). When a message of any length < 264 bits is input, the SHA produces a 160-bit output called a message digest. The message digest is then input to the DSA, which computes the signature for the message. Signing the message digest rather than the message often improves the efficiency of the process, because the message digest is usually much smaller than the message. The same message digest should be obtained by the verifier of the signature when the received version of the message is used as input to SHA. The SHA is called secure because it is designed to be computationally infeasible to recover a message corresponding to the message digest. Any change to the message in transit will, with a very high probability, result in a different message digest, and the signature will fail to verify. The SHA is based on principles similar to those used by Professor Ronald L. Rivest of MIT when designing the MD4 message digest algorithm, and is closely modelled after that algorithm.”  (Proposed Federal Information Processing Standard for Secure Hash Standard,” Federal Register, v. 57, n. 177, 11 Sep 1992, p. 41727)