摘 要
DES（Data Encryption Standard）是IBM公司于上世纪1977年提出的1种数据加密算法。在过去近310年的应用中，还无法将这种加密算法完全、彻底地破解掉。而且这种算法的加解密过程非常快，至今仍被广泛应用，被公认为安全的。在国内，随着3金工程尤其是金卡工程的启动，DES 算法在POS、ATM、磁卡及智能卡（IC 卡）、加油站、高速公路收费站等领域被广泛应用，以此来实现关键数据的保密，如信用卡持卡人的PIN 码加密传输，IC 卡与POS 间的双向认证、金融交易数据包的MAC 校验等，均用到DES 算法。DES加密体制是ISO颁布的数据加密标准.
本论文通过对DES算法的详细分析，提出了用C语言具体实现数据加密标准DES的加密和解密算法，以及对各个部分的输入和输出检测,通过循环,将简单的基本运算(例如左移、模2 加法等) 和变换(选择函数、置换函数) 构造成数据流的非线性变换(加密变换或解密变换) ,从而实现对计算机数据进行密码保护。着重介绍了用C语言实现DES的主要过程，以及生成1个微型的加密系统，可以用来对计算机中的数据文件进行加解密。
关键词:密码学；加密；解密；分组密码；数据加密标准DES；C 语言；函数；文件操作
Abstract
AS the popularly application of computer and Communications Network, the safety of information is not restricted to the field of politics, military and diplomacy, and it also has great relation with people’s daily life. The theory and technology of cryptography has been developed fast, and it is an important research filed of the science and technology of cryptography. there are 2 great invents of the cryptography research in modern times. The first is that NIST announced the American data encryption standard--- DES, and it announced its arithmetic of encryption, which was allowed to use on safe communication of ordinary units and commerce, from then on, the secret mask of cryptography was taken out; The second is that the article -《the new direction of cryptography》introduced the thought of public key cryptography suited to the secure communication in network, and started the prelude of the research on public key cryptography.
DES（Data Encryption Standard）is a data cryptographic algorithm supported by IBM in 1977.。In the past 30 years’ application, there was no way to decrypt it completely. Its encryption process is very fast, and it is still used and thought to be safe by the public. As the start of three-gold project, DES arithmetic is widely used on POS, ATM, IC card, gas station and highway toll station and so on, it is used to protect the security of key data, for example, the encryption transfers of PIN code of the user of credit card, the two-way attestation between IC card and POS, the verifying of MAC in the data packet of financial transaction and so on. DES encryption system is a data encryption standard announced by ISO.
By detailed analysis on DES arithmetic, this paper points out that using C programming language to realize DES, and tests each functional module, and how to make simple operation(left move,2-modulus add)and change(chosen change function, position change function) to be non-linear change by circulating, so realize the security protection for computer data. There are two important points in this paper. First, realize DES by C programming language; Second, a micro encryption system for encryption and decryption of small data’s file in computer.
keywords: Cryptography; Encryption; Decryption; Block cipher; DES;C; Function; File’s operation.
