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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:

  1. It is computationally infeasible to find a message that corresponds to a given message digest.
  2. It is computationally infeasible to find two different messages that produce the same message digest.
  3. Any change to a message (including single bit changes) will, with an exceedingly high probability, result in a completely 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.

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. An open competition for a new SHA-3 function was formally announced in the American Federal Register on November 2, 2007. "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)." Submissions are due October 31, 2008 and the proclamation of a winner and publication of the new standard are scheduled to take place in 2012.

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