One-Way Compression Function

A one-way compression function is a cryptographic building block that takes two fixed-length inputs and produces a single fixed-length output in a way that is difficult to reverse: given an output, it is computationally hard to find inputs that produce it. It is unrelated to ordinary data compression, which is designed to be inverted exactly or approximately back to the original data. One-way compression functions are used inside the Merkle-Damgard construction, the design underlying cryptographic hash functions such as MD5 and SHA-1.