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In cryptology, a code is a method used to transform a message into an obscured form so it cannot be understood. Special information or a key is required to read the original message. The usual method is to use a ''codebook'' with a list of common phrases or words matched with a ''codeword''. Encoded messages are sometimes termed ''codetext'', while the original message is usually referred to as plaintext. Terms like ''code'' and ''cipher'' are often used to refer to any form of encryption. However, there is an important distinction between codes and ciphers in technical work; it is, essentially, the scope of the transformation involved. Codes operate at the level of meaning; that is, words or phrases are converted into something else. Ciphers work at the level of individual letters, or small groups of letters, or even, in modern ciphers, with individual bits. While a code might transform "change" into "CVGDK" or "cocktail lounge", a cipher transforms elements below the semantic level, i.e., below the level of meaning. The "a" in "attack" might be converted to "Q", the first "t" to "f", the second "t" to "3", and so on. Ciphers are more convenient than codes in some situations, there being no need for a codebook, with its inherently limited number of valid messages, and the possibility of fast automatic operation on computers. Codes were long believed to be more secure than ciphers, since (if the compiler of the codebook did a good job) there is no pattern of transformation which can be discovered, whereas ciphers use a consistent transformation, which can potentially be identified and reversed (except in the case of the one-time pad). ==One- and two-part codes== Codes are defined by "codebooks" (physical or notional), which are dictionaries of codegroups listed with their corresponding plaintext. Codes originally had the codegroups assigned in 'plaintext order' for convenience of the code designed, or the encoder. For example, in a code using numeric code groups, a plaintext word starting with "a" would have a low-value group, while one starting with "z" would have a high-value group. The same codebook could be used to "encode" a plaintext message into a coded message or "codetext", and "decode" a codetext back into plaintext message. However, such "one-part" codes had a certain predictability that made it easier for others to notice patterns and "crack" or "break" the message, revealing the plaintext, or part of it and, at the same time, gradually reconstruct the codebook. In order to make life more difficult for codebreakers, codemakers designed codes with no predictable relationship between the codegroups and the ordering of the matching plaintext. In practice, this meant that two codebooks were now required, one to find codegroups for encoding, the other to look up codegroups to find plaintext for decoding. Students of foreign languages work much the same way; for, say, a French-speaking person learning to speak English, there is need for both an English-French and a French-English dictionary. Such "two-part" codes required more effort to develop, and twice as much effort to distribute (and discard safely when replaced), but they were harder to break. 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「Code (cryptography)」の詳細全文を読む スポンサード リンク
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