Collision attack

In cryptography, a collision attack on a cryptographic hash tries to find two inputs producing the same hash value, i.e. a hash collision. This is in contrast to a preimage attack where a specific target hash value is specified.

There are roughly two types of collision attacks:

Classical collision attack

Find two different messages m₁ and m₂ such that hash(m₁) = hash(m₂).

More generally:

Chosen-prefix collision attack

Given two different prefixes p₁ and p₂, find two suffixes s₁ and s₂ such that hash(p₁ ∥ s₁) = hash(p₂ ∥ s₂), where ∥ denotes the concatenation operation.