Assessing the security of cryptographic hash functions
The two most important types of attacks that can be performed on cryptographic hash functions are collision attacks and preimage attacks. A collision attack tries to find two messages that produce the same message digest, while a preimage attack aims to find a message that produces the predefined message digest. Performing a collision attack is easier than performing a preimage attack on the same hash function because the message search is not limited to a single target message digest.
The security level of a cryptographic hash function is the computational complexity of the collision attack. The security level is measured in security bits, similar to the security level of symmetric encryption algorithms. For example, if the complexity of the collision attack is 2,128 hash evaluations, then the security level is 128 bits.
The security level of a cryptographic hash function depends on the size of its output message digest...