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In cryptography, a one-way compression function is a function that transforms two fixed-length inputs into a fixed-length output.〔Handbook of Applied Cryptography by Alfred J. Menezes, Paul C. van Oorschot, Scott A. Vanstone. Fifth Printing (August 2001) page 328.〕 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 can be inverted exactly (lossless compression) or approximately (lossy compression) to the original data. 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.) == Compression == A compression function mixes two fixed length inputs and produces a single fixed length output of the same size as one of the inputs. This can also be seen as that the compression function transforms one large fixed-length input into a shorter, fixed-length output. For instance, ''input A'' might be 128 bits, ''input B'' 128 bits and they are compressed together to a single output of 128 bits. This is the same thing as if one single 256-bit input is compressed together to a single output of 128 bits. Some compression functions have different size of the two inputs but the output usually is the same size as one of the inputs. For instance, ''input A'' might be 256 bits, ''input B'' 128 bits and they are compressed together to a single output of 128 bits. That is, a total of 384 input bits are compressed together to 128 output bits. The mixing is done in such a way that full avalanche effect is achieved. That is, every output bit depends on every input bit. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「One-way compression function」の詳細全文を読む スポンサード リンク
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