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Security Dictionary
Can't tell 'smtp' from 'snmp'? Find the precise meaning of these and hundreds of other security-related terms in our convenient and up-to-date Security Dictionary.
hash function
(I) An algorithm that computes a value based on a data object (such as a message or file; usually variable-length; possibly very large), thereby mapping the data object to a smaller data object (the "hash result") which is usually a fixed-size value. (See: checksum, keyed hash.)

(O) "A (mathematical) function which maps values from a large (possibly very large) domain into a smaller range. A 'good' hash function is such that the results of applying the function to a (large) set of values in the domain will be evenly distributed (and apparently at random) over the range." [X509]

(C) The kind of hash function needed for security applications is called a "cryptographic hash function", an algorithm for which it is computationally infeasible (because no attack is significantly more efficient than brute force) to find either (a) a data object that maps to a pre-specified hash result (the "one-way" property) or (b) two data objects that map to the same hash result (the "collision-free" property). (See: MD2, MD4, MD5, SHA-1.)

(C) A cryptographic hash is "good" in the sense stated in the "O" definition for hash function. Any change to an input data object will, with high probability, result in a different hash result, so that the result of a cryptographic hash makes a good checksum for a data object.

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