# Preimage attack

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Preimage attack

In cryptography, a preimage attack on a cryptographic hash is an attempt to find a message that has a specific hash value. There are two types of preimage attacks:

* "First preimage attack": given a hash "h", find a message "m" such that "hash(m) = h".
* "Second preimage attack": given a fixed message "m1", find a different message "m2" such that "hash(m2) = hash(m1)".

A preimage attack differs from a collision attack in that there is a fixed hash or message that is being attacked. Optimally, a preimage attack on an "n"-bit hash function will take an order of 2"n" operations to be successful. On the other hand, due to the birthday attack, one can expect to find a collision between 2 arbitrary messages in an order of 2"n"/2 operations.

ee also

* Cryptographic hash function
* Collision attack
* Birthday attack
* Rainbow table

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