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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 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 2n operations to be successful. On the other hand, due to the birthday attack, one can expect to find a collision between two arbitrary messages in an order of 2n/2 operations.
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