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In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is ''encipherment''. To encipher or encode is to convert information into cipher or code. In common parlance, 'cipher' is synonymous with 'code', as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially classical cryptography. Codes generally substitute different length strings of characters in the output, while ciphers generally substitute the same number of characters as are input. There are exceptions and some cipher systems may use slightly more, or fewer, characters when output versus the number that were input. Codes operated by substituting according to a large codebook which linked a random string of characters or numbers to a word or phrase. For example, "UQJHSE" could be the code for "Proceed to the following coordinates." When using a cipher the original information is known as plaintext, and the encrypted form as ciphertext. The ciphertext message contains all the information of the plaintext message, but is not in a format readable by a human or computer without the proper mechanism to decrypt it. The operation of a cipher usually depends on a piece of auxiliary information, called a key (or, in traditional NSA parlance, a ''cryptovariable''). The encrypting procedure is varied depending on the key, which changes the detailed operation of the algorithm. A key must be selected before using a cipher to encrypt a message. Without knowledge of the key, it should be extremely difficult, if not impossible, to decrypt the resulting ciphertext into readable plaintext. Most modern ciphers can be categorized in several ways * By whether they work on blocks of symbols usually of a fixed size (block ciphers), or on a continuous stream of symbols (stream ciphers). * By whether the same key is used for both encryption and decryption (symmetric key algorithms), or if a different key is used for each (asymmetric key algorithms). If the algorithm is symmetric, the key must be known to the recipient and sender and to no one else. If the algorithm is an asymmetric one, the enciphering key is different from, but closely related to, the deciphering key. If one key cannot be deduced from the other, the asymmetric key algorithm has the public/private key property and one of the keys may be made public without loss of confidentiality. ==Etymology== "Cipher" is alternatively spelled "cypher"; similarly "ciphertext" and "cyphertext", and so forth. The word "cipher" in former times meant "zero" and had the same origin: Middle French as ''フランス語:cifre'' and Medieval Latin as ''cifra,'' from the Arabic صفر ''ṣifr'' = zero (see Zero—Etymology). "Cipher" was later used for any decimal digit, even any number. There are many theories about how the word "cipher" may have come to mean "encoding". *Encoding often involved numbers. *The Roman number system was very cumbersome because there was no concept of zero (or empty space). The concept of zero (which was also called "cipher"), which is now common knowledge, was alien to medieval Europe, so confusing and ambiguous to common Europeans that in arguments people would say "talk clearly and not so far fetched as a cipher". Cipher came to mean concealment of clear messages or encryption. * *The French formed the word "フランス語:chiffre" and adopted the Italian word "". * *The English used "zero" for "0", and "cipher" from the word "ciphering" as a means of computing. * *The Germans used the words "Ziffer" (digit) and "Chiffre". * *The Dutch still use the word "cijfer" to refer to a numerical digit. * *The Serbians use the word "cifra", which refers to a digit, or in some cases, any number. Besides "cifra", they use word "broj" for a number. * *The Italians and the Spanish also use the word "cifra" to refer to a number. * *The Swedes use the word "siffra" which refers to a digit and "nummer" to refer to a combination of "siffror". Ibrahim Al-Kadi concluded that the Arabic word ''sifr'', for the digit zero, developed into the European technical term for encryption.〔Ibrahim A. Al-Kadi, "Cryptography and Data Security: Cryptographic Properties of Arabic", proceedings of the Third Saudi Engineering Conference. Riyadh, Saudi Arabia: Nov 24-27, Vol 2:910-921., 1991.〕 As the decimal zero and its new mathematics spread from the Arabic world to Europe in the Middle Ages, words derived from ''ṣifr'' and ''zephyrus'' came to refer to calculation, as well as to privileged knowledge and secret codes. According to Ifrah, "in thirteenth-century Paris, a 'worthless fellow' was called a , i.e., an 'arithmetical nothing'." Cipher was the European pronunciation of sifr, and cipher came to mean a message or communication not easily understood.〔The Muslim next door : the Qur'an, the media, and that veil thing, Sumbul Ali-Karamali, 2008, pp. 240-241〕 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「cipher」の詳細全文を読む スポンサード リンク
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