!!!Cryptology

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数据加密标准（DES）是一种经典的块密码算法，它通过固定长度的密钥对明文进行复杂运算转换为相同长度的密文。DES使用64位的密钥进行数据的加密和解密操作，尽管实际使用的有效密钥长度为56位，并且每个8字节中包含奇偶校验位，确保数据传输过程中的准确性。

Cryptography is the practice and study of techniques for secure communication in the presence of third parties.

Symmetric-key cryptography refers to encryption methods in which both the sender and receiver share the same key.

Data Encryption Standard

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.

The key is nominally stored or transmitted as 8 bytes, each with odd parity. According to ANSI X3.92-1981, section 3.5:

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.