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In cryptography, a substitution cipher is a method of encoding by which units of plaintext are replaced with ciphertext, according to a fixed system; the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing the inverse substitution. Substitution ciphers can be compared with transposition ciphers. In a transposition cipher, the units of the plaintext are rearranged in a different and usually quite complex order, but the units themselves are left unchanged. By contrast, in a substitution cipher, the units of the plaintext are retained in the same sequence in the ciphertext, but the units themselves are altered. There are a number of different types of substitution cipher. If the cipher operates on single letters, it is termed a simple substitution cipher; a cipher that operates on larger groups of letters is termed polygraphic. A monoalphabetic cipher uses fixed substitution over the entire message, whereas a polyalphabetic cipher uses a number of substitutions at different positions in the message, where a unit from the plaintext is mapped to one of several possibilities in the ciphertext and vice versa. ==Simple substitution== Substitution of single letters separately—simple substitution—can be demonstrated by writing out the alphabet in some order to represent the substitution. This is termed a substitution alphabet. The cipher alphabet may be shifted or reversed (creating the Caesar and Atbash ciphers, respectively) or scrambled in a more complex fashion, in which case it is called a ''mixed alphabet'' or ''deranged alphabet''. Traditionally, mixed alphabets may be created by first writing out a keyword, removing repeated letters in it, then writing all the remaining letters in the alphabet in the usual order. Using this system, the keyword "zebras" gives us the following alphabets: A message of flee at once. we are discovered! ''enciphers to'' SIAA ZQ LKBA. VA ZOA RFPBLUAOAR! Traditionally, the ciphertext is written out in blocks of fixed length, omitting punctuation and spaces; this is done to help avoid transmission errors and to disguise word boundaries from the plaintext. These blocks are called "groups", and sometimes a "group count" (i.e., the number of groups) is given as an additional check. Five letter groups are traditional, dating from when messages used to be transmitted by telegraph: SIAAZ QLKBA VAZOA RFPBL UAOAR If the length of the message happens not to be divisible by five, it may be padded at the end with "nulls". These can be any characters that decrypt to obvious nonsense, so the receiver can easily spot them and discard them. The ciphertext alphabet is sometimes different from the plaintext alphabet; for example, in the pigpen cipher, the ciphertext consists of a set of symbols derived from a grid. For example: Such features make little difference to the security of a scheme, however – at the very least, any set of strange symbols can be transcribed back into an A-Z alphabet and dealt with as normal. In lists and catalogues for salespeople, a very simple encryption is sometimes used to replace numeric digits by letters. Example: MAT would be used to represent 120. 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「Substitution cipher」の詳細全文を読む スポンサード リンク
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