In cryptography, a **one-way compression function** is a function that transforms a fixed-length input into a fixed-length output. The transformation is "one-way", meaning that it is difficult given a particular output to compute inputs which compress to that output. One-way compression functions are not related to data compression, which by definition is invertible.

One-way compression functions are for instance used in the Merkle–Damgård construction inside cryptographic hash functions.

One-way compression functions are often built from block ciphers. Some methods to turn any normal block cipher into a one-way compression function are **Davies–Meyer**, **Matyas–Meyer–Oseas**, **Miyaguchi–Preneel** (single-block-length compression functions) and **MDC-2/Meyer–Schilling**, **MDC-4**, **Hirose** (double-block-length compression functions). These methods are described in detail further down. (MDC-2 is also the name of a hash function patented by IBM.)

Read more about One-way Compression Function: Compression, One-way, The Merkle–Damgård Construction, Construction From Block Ciphers, Davies–Meyer, Matyas–Meyer–Oseas, Miyaguchi–Preneel, Hirose

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