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 message m1, find a message m2 such that hash(m2) = hash(m1).

Due to the similarity between these two cases a method for attacking one can normally be applied to attacking the other.^*

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 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 $2^\{n/2\}$ operations.

Cryptographic attacks

Preimage-Angriff