Glossary
Collision Resistance
Collision resistance is the property that no one can find two different inputs with the same hash — the hardest core security property for a hash function.
Collision resistance is the property that it is computationally infeasible to find any two distinct inputs a ≠ b with H(a) = H(b). It is the strongest and hardest-to-achieve of the three core hash security properties, alongside preimage resistance.
Because of the birthday attack, collision resistance caps at about 2^(n/2) for an n-bit output — so collision resistance, not preimage resistance, is usually what dictates how large a digest must be.
A hash can lose collision resistance while remaining one-way: MD5 and SHA-1 still resist preimages in practice, yet both have efficient collision attacks (MD5, SHA-1). Modern functions like SHA-256 and SHA-3 remain collision resistant.
See how cryptographic hashing works for the full picture, or try the generator.