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The Playfair cipher or Playfair square is a manual symmetric encryption technique and was the first literal digraph substitution cipher. The scheme was invented in 1854 by Charles Wheatstone, but bears the name of Lord Playfair who promoted the use of the cipher. The technique encrypts pairs of letters (''digraphs''), instead of single letters as in the simple substitution cipher and rather more complex Vigenère cipher systems then in use. The Playfair is thus significantly harder to break since the frequency analysis used for simple substitution ciphers does not work with it. The frequency analysis of digraphs is possible, but considerably more difficult. With 600〔No duplicate letters are allowed, and one letter is omitted (Q) or combined (I/J), so the calculation is 600 = 25×24.〕 possible digraphs rather than the 26 possible monographs, a considerably larger cipher text is required in order to be useful. ==History== It became known as the Playfair cipher after Lord Playfair, who heavily promoted its use, despite its invention by Wheatstone. The first recorded description of the Playfair cipher was in a document signed by Wheatstone on 26 March 1854. It was rejected by the British Foreign Office when it was developed because of its perceived complexity. Wheatstone offered to demonstrate that three out of four boys in a nearby school could learn to use it in 15 minutes, but the Under Secretary of the Foreign Office responded, "That is very possible, but you could never teach it to attachés." It was used for tactical purposes by British forces in the Second Boer War and in World War I and for the same purpose by the British and Australians during World War II. This was because Playfair is reasonably fast to use and requires no special equipment - just a pencil and some paper. A typical scenario for Playfair use would be to protect important but non-critical secrets during actual combat. By the time that the enemy cryptanalysts could break the message, the information would be useless to them. During the Second World War, the Government of New Zealand used it for communication among New Zealand, the Chatham Islands, and the coastwatchers in the Pacific Islands.〔http://rnzncomms.org/ourhistory/chapter8/ "A History of Communications Security in New Zealand By Eric Mogon"], Chapter 8〕 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「Playfair cipher」の詳細全文を読む スポンサード リンク
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