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From CryptoDox, The Online Encyclopedia on Cryptography and Information Security

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'''Hash functions''' are functions for summarizing or probabilistically identifying data. Such a summary is known as a hash value or simply a hash, and the process of computing such a value is known as hashing. A hash function takes an input string (message) of arbitrary size and reduces it to a short string. A typical cryptographic hash function takes any input string and makes a 256 bit string from it. The hash value of an input string is analogous to the fingerprint of a person. It is often also called a "message digest." Hash functions are used for [[digital signature]]s such as [[RSA]] and [[DSA]], but also for the construction of MACs (message authentication codes), the protection of passwords, and for the derivation of independent secret keys from a single master key. Cryptographic hash functions are an essential building block for applications that require data integrity, such as detectors of computer viruses, Internet security (for example [[PGP]] or [[IPSEC]]), and the security of electronic commerce and banking. A cryptographic hash function must be a one-way function, which means that finding an input corresponding to a given output string is difficult: Even an opponent who spends a significant amount of money, say $10 million, will have a negligible success probability. == Algorithms == There are various algorithms available for generating message-digests, or hashes. Some of these are listed below. An example of the output generated by hash functions is also shown in the figure. [[Image:HashFunctions.jpg|right]] Current recommended hash functions for cryptographic applications: * [[RIPEMD]] * [[WHIRLPOOL]] * SHA-2 of the [[SHA family]] Historically important hash functions: * [[Gost-Hash]] * [[HAS-160]] -- designed for use with the Korean KCDSA digital signature algorithm * [[HAVAL]] * [[MD2]] -- specified in RFC 1319. As of 2004, no longer considered secure. * [[MD4]] -- used in the ed2k URI scheme. As of 2004, no longer considered secure. * [[MD5]] -- specified in RFC 1321. Designed to replace MD4. Widely used to store passwords, and to check files for tampering. * [[MDC-2]] -- U.S. Patent 4,908,861, expires on August 28, 2007. * [[N-hash]] -- As of 1991, no longer considered secure. * [[Snefru]] * [[Tiger]] * [[SHA family]] -- required by law for use in certain U. S. Government applications * [[SHA-1]] -- used to hash objects in the git distributed revision control system. == References == * [http://www.ddj.com/documents/s=945/ddj9701b/ The RIPEMD-160 Cryptographic Hash Function] - Dr. Dobb's Journal January 1997 == Books of Interest == <amazon>3540417281</amazon> <amazon>038723473X</amazon> <amazon>0471117099</amazon> <amazon>0849385237</amazon> <amazon>0824704657</amazon> <amazon>3540430725</amazon> == External Links == *[http://www.partow.net/programming/hashfunctions/index.html General purpose hash function algorithms (C\C++\Pascal\Java\Ruby)] * [http://www.rsasecurity.com/rsalabs/node.asp?id=2176 What is a hash function?] * [http://serversniff.net/hash.php Online Hash Generators] [[Category:Hash Functions]]

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