Cryptography is the practice and study of hiding information. In modern times, cryptography is considered a branch of both mathematics and computer science, and is affiliated closely with information theory， computer security, and engineering. Cryptography is used in applications present in technologically advanced societies; examples include the security of ATM cards, computer passwords， and electronic commerce, which all depend on cryptography.密码学是信息隐藏的实践与研究。现代密码学被认为是数学和计算机科学的一个分支，它与信息论、计算机安全和工程密切相关。密码技术被应用于技术先进的社会中，例如ATM卡、计算机密码和电子商务的安全，这些都依赖于密码学。(1 )Terminology Until modem times, cryptography referred almost exclusively to encryption, the process of converting ordinary information (plaintext) into unintelligible gibberish (i.e., ciphertext). Decryption is the reverse, moving from unintelligible ciphertext to plaintext. A cipher (or cypher) is a pair of algorithms which creates the encryption and the reversing decryption. The detailed operation of a cipher is controlled both by the algorithm and, in each instance, by a key. This is a secret parameter (ideal以 known only to the communicants) for a specific message exchange context. Keys are important, as ciphers without variable keys are trivially breakable and therefore less than useful for most purposes. Historically, ciphers were often used directly for encryption or decryption, without additional procedures such as authentication or integrity checks. 直到近代，加密提到几乎完全加密，普通的转换过程的信息（明文）到不知所云胡言乱语（即密文）。解密是相反的，从难以理解的密文到明文。一个密码（或密码）是一个对算法产生的加密和解密的扭转。密码的详细操作由算法控制，并在每个实例中由一个键来控制。这是一个秘密参数（理想以只知道的圣餐）为某一特定的信息交流语境。钥匙是很重要的，因为密码没有变项trivially易碎，因此不到有用的用途。从历史上看，密码往往直接用于加密或解密，没有额外的程序，如认证、完整性检查。In colloquial use, the term code is often used to mean any method of encryption or concealment of meaning. However, in cryptography, code has a more specific meaning; it means the replacement of a unit of plaintext (i.e., a meaningful word or phrase) with a code word (for example, apple pie replaces attack at dawn). Codes are no longer used in serious cryptography-except incidentally for such things as unit designations (e.g., Bronco Flight or Operation Overlord), since properly chosen ciphers are both more practical and more secure than even the best codes, and better adapted to computers as well. 在口语使用中，“代码”一词常用来表示任何加密或隐藏意义的方法。然而，在密码学中，代码有一个更具体的意义；它意味着一个单位明文置换（即，一个有意义的单词或短语）与代码（例如，苹果派取代黎明进攻）。代码不再用于严重的密码-除了偶然的事情单位名称（例如，Bronco飞行或操作霸王）一因为适当的选择密码都更实用、更甚至比最好的代码的安全，并更好地适应电脑好。 Some use the terms cryptography and cryptology interchangeably in English, while others use cryptography to refer specifically to the use and practice of cryptographic techniques, and cryptology to refer to the combined study of cryptography and cryptanalysis.一些使用术语密码学和密码交替使用英语，而其他人使用的密码是特指使用密码技术的实践，和密码学指密码学和密码分析的综合研究。The study of characteristics of languages which have some application in cryptology, i.e.frequency data, letter combinations, universal patterns, etc. is called Cryptolinguistics. 对语言有一些应用密码学，即数据、字母组合，通用型的特性的研究，被称为密码学。(2) Modem cryptography the modem field of cryptography can be divided into several areas of study. The chief ones are discussed here; see Topics in Cryptography for more. 密码学的现代领域可以分为几个领域的研究。主要的是在这里讨论，看到更多的密码学主题。(3) Symmetric-key cryptography Symmetric-key cryptography refers to encrytion methods in which both the sender and receiver share the same key (or, less commonly, in which their keys are different, but related in an easily computable way). This was the only kind of encryption publicly known until June 1976. 对称密钥密码术是指发送方和接收方共享相同密钥（或者较不常见地，其中它们的密钥不同，但以容易计算的方式相关）的加密方法。 这是到1976年6月公开的唯一一种加密。One round (out of 8.5) of the patented IDEA cipher, used in some versions of PGP for high-speed encryption of, for instance, e-mail. 一轮（8.5个）的专利IDEA密码，在一些版本的PGP中用于高速加密，例如电子邮件。The modern study of symmetric-key ciphers relates mainly to the study of block ciphers and stream ciphers and to their applications. A block cipher is, in a sense, a modern embodiment of Albertis polyalphabetic cipher: block ciphers take as input a block of plaintext and a key, and output a block of ciphertext of the same size. Since messages are almost always longer than a single block, some method of knitting together successive blocks is required. Several have been developed, some with better security in one aspect or another than others. they are the mode of operations and must be carefully considered when using a block cipher in a cryptosystem. 对称密钥密码的现代研究主要涉及块密码和流密码及其应用的研究。 在某种意义上，分组密码是阿尔伯蒂的多重字母密码的现代实施例：分组密码采用明文和密钥块作为输入，并输出相同大小的密文块。 由于消息几乎总是比单个块长，因此需要一些将连续块编织在一起的方法。 已经开发了几种，一些在一方面具有更好的安全性或者另一方面比另一方面更好。它们是操作的模式，并且当在密码系统中使用块密码时必须仔细考虑。The Data Encryption Standard (DES) and the Advanced Encryption Standard (AES) are block cipher designs which have been designated cryptography standards by the US government (though DESs designation was finally withdrawn after the AES was adopted).Despite its deprecation as an official standard, DES (especially its still-approved