The SHA Hash Functions

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The SHA hash functions are five cryptographic hash functions designed by the National Security Agency (NSA) and published by the NIST as a U.S. Federal Information Processing Standard. SHA stands for Secure Hash Algorithm. Hash algorithms compute a fixed-length digital representation (known as a message digest) of an input data sequence (the message) of any length. They are called “secure” when (in the words of the standard), “it is computationally infeasible to:

   1. find a message that corresponds to a given message digest, or
   2. find two different messages that produce the same message digest.

Any change to a message will, with a very high probability, result in a different message digest.”

The five algorithms are denoted SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512. The latter four variants are sometimes collectively referred to as SHA-2. SHA-1 produces a message digest that is 160 bits long; the number in the other four algorithms' names denote the bit length of the digest they produce.

SHA-1 is employed in several widely used security applications and protocols, including TLS and SSL, PGP, SSH, S/MIME, and IPsec. It was considered to be the successor to MD5, an earlier, widely-used hash function.

The security of SHA-1 has been somewhat compromised by cryptography researchers. Although no attacks have yet been reported on the SHA-2 variants, they are algorithmically similar to SHA-1 and so efforts are underway to develop improved alternative hashing algorithms. "NIST is initiating an effort to develop one or more additional hash algorithms through a public competition, similar to the development process for the Advanced Encryption Standard (AES)." It's tentatively scheduled to proclaim winner and publish final standard in 2012.

Even a small change in the message will, with overwhelming probability, result in a completely different hash due to the avalanche effect.

SHA1("The quick brown fox jumps over the lazy dog")
  = 2fd4e1c6 7a2d28fc ed849ee1 bb76e739 1b93eb12

SHA1("The quick brown fox jumps over the lazy cog")
  = de9f2c7f d25e1b3a fad3e85a 0bd17d9b 100db4b3