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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 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 2ⁿ 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 <math>2^{n/2}</math> operations.

[edit] See also

• Cryptographic hash function
• Collision attack
• Birthday attack

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