<Dd> For brevity, the following description omits the exact transformations and permutations which specify the algorithm; for reference, the details can be found in DES supplementary material . </Dd> <P> DES is the archetypal block cipher--an algorithm that takes a fixed - length string of plaintext bits and transforms it through a series of complicated operations into another ciphertext bitstring of the same length . In the case of DES, the block size is 64 bits . DES also uses a key to customize the transformation, so that decryption can supposedly only be performed by those who know the particular key used to encrypt . The key ostensibly consists of 64 bits; however, only 56 of these are actually used by the algorithm . Eight bits are used solely for checking parity, and are thereafter discarded . Hence the effective key length is 56 bits . </P> <P> The key is nominally stored or transmitted as 8 bytes, each with odd parity . According to ANSI X3. 92 - 1981 (Now, known as ANSI INCITS 92 - 1981), section 3.5: </P> <P> One bit in each 8 - bit byte of the KEY may be utilized for error detection in key generation, distribution, and storage . Bits 8, 16,..., 64 are for use in ensuring that each byte is of odd parity . </P>

A symmetric block cipher that uses a 56-bit key and encrypts data in 64-bit blocks