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copied!<p>from wikipedia: <a href="http://en.wikipedia.org/wiki/Cryptographic_hash_function">http://en.wikipedia.org/wiki/Cryptographic_hash_function</a></p> <blockquote> <p><em><strong>Properties</em></strong></p> <p>Most cryptographic hash functions are designed to take a string of any length as input and produce a fixed-length hash value. A cryptographic hash function must be able to withstand all known types of cryptanalytic attack. As a minimum, it must have the following properties:</p> <p><strong>Preimage resistance</strong> Given a hash <em>h</em> it should be difficult to find any message <em>m</em> such that <em>h = hash(m)</em>. This concept is related to that of one-way function. Functions that lack this property are vulnerable to preimage attacks.</p> <p><strong>Second-preimage resistance</strong> Given an input <em>m</em><sub>1</sub> it should be difficult to find another input <em>m</em><sub>2</sub> — where <em>m</em><sub>1</sub> != <em>m</em><sub>2</sub> — such that <em>hash</em>( <em>m</em><sub>1</sub> ) = <em>hash</em>( <em>m</em><sub>2</sub> ). This property is sometimes referred to as weak collision resistance, and functions that lack this property are vulnerable to second-preimage attacks.</p> <p><strong>Collision resistance</strong> It should be difficult to find two different messages <em>m</em><sub>1</sub> and <em>m</em><sub>2</sub> such that<br> <em>hash</em>( <em>m</em><sub>1</sub> ) = <em>hash</em>( <em>m</em><sub>2</sub> ). Such a pair is called a cryptographic hash collision. This property is sometimes referred to as strong collision resistance. It requires a hash value at least twice as long as that required for preimage-resistance, otherwise collisions may be found by a birthday attack.</p> </blockquote>

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