DES是一种典型的块密码—一种将固定长度的平文通过一系列复杂的操作变成同样长度的密文的算法。对DES而言，块长度为64位。同时，DES使用密钥来自定义变换过程，因此算法认为只有持有加密所用的密钥的用户才能解密密文。密钥表面上是64位的，然而只有其中的56位被实际用于算法，其余8位可以被用于奇偶校验，并在算法中被丢弃。因此，DES的有效密钥长度为56位，通常称DES的密钥长度为56位。-DES is a typical block-cipher- a fixed-length level Through a complex series of operations into the algorithm of the same length of the ciphertext. Terms of the DES, the block length of 64. Meanwhile, DES uses the key to customize the transformation process, so the algorithm that only holds encryption keys the user can decrypt the ciphertext. Key surface is 64, of which only 56 are actually used in the algorithm, however, the remaining eight bits can be used for parity, and in the algorithm are discarded. Therefore, effective key length of DES to 56, usually known as the DES key length of 56 bits.