SHA Secure Hash Algorithm

The Secure Hash Algorithm (English: Secure Hash Algorithm, abbreviated as SHA) is a family of cryptographic hash functions and is a FIPS-certified secure hash algorithm. It is an algorithm that can calculate a fixed-length string (also known as a message digest) corresponding to a digital message. And if the input messages are different, the probability that they correspond to different strings is very high.

The five algorithms of the SHA family are SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512, designed by the National Security Agency (NSA) of the United States and released by the National Institute of Standards and Technology (NIST); it is a US government standard. The latter four are sometimes collectively referred to as SHA-2. SHA-1 is widely used in many security protocols, including TLS and SSL, PGP, SSH, S/MIME, and IPsec, and was once regarded as the successor of MD5 (a hash function that was widely used earlier). However, the security of SHA-1 is now seriously questioned by cryptographers; although no effective attack on SHA-2 has yet appeared, its algorithm is basically still similar to SHA-1; therefore, some people have begun to develop other alternative hash algorithms.