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Cryptography or cryptology; from Greek ''kryptós'', "hidden, secret"; and ''graphein'', "writing", or ''-logia'', "study", respectively is the practice and study of techniques for secure communication in the presence of third parties (called adversaries). These adversaries are often referred to as Eve in cryptography, while the sender and recipient of messages are called Alice and Bob respectively. More generally, cryptography is about constructing and analyzing protocols that block Eve (or adversaries); various aspects in information security such as data confidentiality, data integrity, authentication, and non-repudiation〔 are central to modern cryptography. Modern cryptography exists at the intersection of the disciplines of mathematics, computer science, and electrical engineering. Applications of cryptography include ATM cards, computer passwords, and electronic commerce. Cryptography prior to the modern age was effectively synonymous with ''encryption'', the conversion of information from a readable state to apparent nonsense. The originator of an encrypted message (Alice) shared the decoding technique needed to recover the original information only with intended recipients (Bob), thereby precluding unwanted persons (Eve) from doing the same. Since World War I and the advent of the computer, the methods used to carry out cryptology have become increasingly complex and its application more widespread. Modern cryptography is heavily based on mathematical theory and computer science practice; cryptographic algorithms are designed around computational hardness assumptions, making such algorithms hard to break in practice by any adversary. It is theoretically possible to break such a system, but it is infeasible to do so by any known practical means. These schemes are therefore termed computationally secure; theoretical advances, e.g., improvements in integer factorization algorithms, and faster computing technology require these solutions to be continually adapted. There exist information-theoretically secure schemes that cannot be broken even with unlimited computing power—an example is the one-time pad—but these schemes are more difficult to implement than the best theoretically breakable but computationally secure mechanisms. The growth of cryptographic technology has raised a number of legal issues in the information age. Cryptography's potential for use as a tool for espionage and sedition has led many governments to classify it as a weapon and to limit or even prohibit its use and export.〔 In some jurisdictions where the use of cryptography is legal, laws permit investigators to compel the disclosure of encryption keys for documents relevant to an investigation.〔 Cryptography also plays a major role in digital rights management and piracy of digital media.〔 ==Terminology== Until modern times, cryptography referred almost exclusively to ''encryption'', which is the process of converting ordinary information (called plaintext) into unintelligible text (called ciphertext).〔 Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext. A ''cipher'' (or ''cypher'') is a pair of algorithms that create 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 (ideally known only to the communicants), usually a short string of characters, which is needed to decrypt the ciphertext. Formally, a "cryptosystem" is the ordered list of elements of finite possible plaintexts, finite possible cyphertexts, finite possible keys, and the encryption and decryption algorithms which correspond to each key. Keys are important both formally and in actual practice, as ciphers without variable keys can be trivially broken with only the knowledge of the cipher used and are therefore useless (or even counter-productive) for most purposes. Historically, ciphers were often used directly for encryption or decryption without additional procedures such as authentication or integrity checks. There are two kinds of cryptosystems: symmetric and asymmetric. In symmetric systems the same key (the secret key) is used to encrypt and decrypt a message. Data manipulation in symmetric systems is faster than asymmetric systems as they generally use shorter key lengths. Asymmetric systems use a public key to encrypt a message and a private key to decrypt it. Use of asymmetric systems enhances the security of communication. Examples of asymmetric systems include RSA (Rivest-Shamir-Adleman), and ECC (Elliptic Curve Cryptography). Symmetric models include the commonly used AES (Advanced Encryption System) which replaced the older DES (Data Encryption Standard). 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, "wallaby" 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 also are better adapted to computers. Cryptanalysis is the term used for the study of methods for obtaining the meaning of encrypted information without access to the key normally required to do so; i.e., it is the study of how to crack encryption algorithms or their implementations. Some use the terms ''cryptography'' and ''cryptology'' interchangeably in English, while others (including US military practice generally) 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.〔Oded Goldreich, ''Foundations of Cryptography, Volume 1: Basic Tools'', Cambridge University Press, 2001, ISBN 0-521-79172-3〕 English is more flexible than several other languages in which ''cryptology'' (done by cryptologists) is always used in the second sense above. RFC 2828 advises that steganography is sometimes included in cryptology. The study of characteristics of languages that have some application in cryptography or cryptology (e.g. frequency data, letter combinations, universal patterns, etc.) is called cryptolinguistics. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cryptography」の詳細全文を読む スポンサード リンク
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