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In classical cryptography, the running key cipher is a type of polyalphabetic substitution cipher in which a text, typically from a book, is used to provide a very long keystream. Usually, the book to be used would be agreed ahead of time, while the passage to be used would be chosen randomly for each message and secretly indicated somewhere in the message. == Example == The text used is ''The C Programming Language'' (1978 edition), and the ''tabula recta'' is the tableau. The plaintext is "Flee at once". Page 63, line 1 is selected as the running key: errors can occur in several places. A label has... The running key is then written under the plaintext: The message is then sent as "JCVSR LQNPS". However, unlike a Vigenère cipher, the message is extended, the key is not repeated; the key text itself is used as the key. If the message is extended, such as, "Flee at once. We are discovered", then the running key continues as before: To determine where to find the running key, a fake block of five ciphertext characters is subsequently added, with three denoting the page number, and two the line number, using A=0, B=1 etc. to encode digits. Such a block is called an indicator block. The indicator block will be inserted as the second last of each message. (It should be noted that many other schemes are possible for hiding indicator blocks.) Thus page 63, line 1 encodes as "AGDAB" (06301). This yields a final message of "JCVSR LQNPS YGUIM QAWXS AGDAB MECTO". 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「Running key cipher」の詳細全文を読む スポンサード リンク
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