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The Vigenère cipher is a method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of a keyword. It is a simple form of polyalphabetic substitution. The Vigenère ((:viʒnɛːʁ)) cipher has been reinvented many times. The method was originally described by Giovan Battista Bellaso in his 1553 book ''La cifra del. Sig. Giovan Battista Bellaso''; however, the scheme was later misattributed to Blaise de Vigenère in the 19th century, and is now widely known as the "Vigenère cipher". Though the cipher is easy to understand and implement, for three centuries it resisted all attempts to break it; this earned it the description le chiffre indéchiffrable (French for 'the indecipherable cipher'). Many people have tried to implement encryption schemes that are essentially Vigenère ciphers. Friedrich Kasiski was the first to publish a general method of deciphering a Vigenère cipher. ==History== The first well documented description of a polyalphabetic cipher was formulated by Leon Battista Alberti around 1467 and used a metal cipher disc to switch between cipher alphabets. Alberti's system only switched alphabets after several words, and switches were indicated by writing the letter of the corresponding alphabet in the ciphertext. Later, in 1508, Johannes Trithemius, in his work ''Poligraphia'', invented the tabula recta, a critical component of the Vigenère cipher. The Trithemius cipher, however, only provided a progressive, rigid and predictable system for switching between cipher alphabets. What is now known as the Vigenère cipher was originally described by Giovan Battista Bellaso in his 1553 book ''La cifra del. Sig. Giovan Battista Bellaso''. He built upon the tabula recta of Trithemius, but added a repeating "countersign" (a key) to switch cipher alphabets every letter. Whereas Alberti and Trithemius used a fixed pattern of substitutions, Bellaso's scheme meant the pattern of substitutions could be easily changed simply by selecting a new key. Keys were typically single words or short phrases, known to both parties in advance, or transmitted "out of band" along with the message. Bellaso's method thus required strong security for only the key. As it is relatively easy to secure a short key phrase, say by a previous private conversation, Bellaso's system was considerably more secure. Blaise de Vigenère published his description of a similar but stronger autokey cipher before the court of Henry III of France, in 1586. Later, in the 19th century, the invention of Bellaso's cipher was misattributed to Vigenère. David Kahn in his book ''The Codebreakers'' lamented the misattribution by saying that history had "ignored this important contribution and instead named a regressive and elementary cipher for him () though he had nothing to do with it". The Vigenère cipher gained a reputation for being exceptionally strong. Noted author and mathematician Charles Lutwidge Dodgson (Lewis Carroll) called the Vigenère cipher unbreakable in his 1868 piece "The Alphabet Cipher" in a children's magazine. In 1917, ''Scientific American'' described the Vigenère cipher as "impossible of translation". This reputation was not deserved. Charles Babbage is known to have broken a variant of the cipher as early as 1854; however, he didn't publish his work. Kasiski entirely broke the cipher and published the technique in the 19th century. Even before this, though, some skilled cryptanalysts could occasionally break the cipher in the 16th century.〔 The Vigenère cipher is simple enough to be a field cipher if it is used in conjunction with cipher disks.〔(Codes, Ciphers, & Codebreaking ) (The Rise Of Field Ciphers)〕 The Confederate States of America, for example, used a brass cipher disk to implement the Vigenère cipher during the American Civil War. The Confederacy's messages were far from secret and the Union regularly cracked their messages. Throughout the war, the Confederate leadership primarily relied upon three key phrases, "Manchester Bluff", "Complete Victory" and, as the war came to a close, "Come Retribution". Gilbert Vernam tried to repair the broken cipher (creating the Vernam-Vigenère cipher in 1918), but, no matter what he did, the cipher was still vulnerable to cryptanalysis. Vernam's work, however, eventually led to the one-time pad, a provably unbreakable cipher. 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「Vigenère cipher」の詳細全文を読む スポンサード リンク
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