- 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 thebirthday 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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