| Hash Collision |
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| cryptographic hash functions | |
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Most hash functions have potential collisions, but with good hash functions they occur less often than with bad ones. In certain specialized applications where a relatively small number of possible inputs are all known ahead of time it is possible to construct a Perfect Hash Function which maps all inputs to different outputs. But in a function which can take input of arbitrary length and content and returns a hash of a fixed length (such as MD5 ), there will always be collisions, because any given hash can correspond to an infinite number of possible inputs. IN SEARCHING See Also: Hash table An efficient method of searching can be to process a lookup key using a hash function, then take the resulting hash value and then use it as an index into an array of data. The resulting data structure is called a '' Hash Table ''. As long as different keys map to different indices, lookup can be performed in constant time. When multiple lookup keys are mapped to identical indices, however, a hash collision occurs. The most popular ways of dealing with this are ''chaining'' (building a Linked List of values for each array index), and ''open addressing'' (searching other array indices nearby for an empty space)... IN CRYPTOGRAPHY One desirable property of Cryptographic Hash Function s is that it is computationally infeasible to find a collision. The value of a hash function can be used to certify an input is unchanged by publishing the Signed value of the hash ''if'' it is not feasible to produce a collision. ''Feasible'' in this context refers to any method which is faster than a brute-force Birthday Attack . The process of finding two arbitrary values whose hashes collide is called a Collision Attack ; the process of finding one arbitrary value whose hash collides with another, given hash is called a Preimage Attack . A successful preimage attack is considered a much more serious break than a successful collision attack. |
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