Security of cryptographic hash functions: Difference between revisions

Revision as of 13:47, 9 November 2010

Revision as of 15:39, 28 January 2011

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* '''Pre-image resistance''': given a hash h it should be ''hard'' to find any message m such that h = hash(m). This concept is related to that of one way function. Functions that lack this property are vulnerable to pre-image attacks.

* '''Second pre-image resistance''': given an input m1, it should be ''hard'' to find another input, m2 (not equal to m1) such that hash(m1) = hash(m2). This property is sometimes referred to as weak collision resistance. Functions that lack this property are vulnerable to second pre-image attacks.

* '''Collision resistance''': it should be ''hard'' to find two different messages m1 and m2 such that hash(m1) = hash(m2). Such a pair is called a (cryptographic) hash collision,. and thisThis property is sometimes referred to as strong collision resistance. It requires a hash value at least twice as long as what is required for pre-image resistance, otherwise collisions may be found by a [[birthday attack]].

==== The Meaning of "Hard" ====