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Tabula recta
Tabula recta is a cryptographic term invented by Johannes Trithemius[?] in 1518. The tabula recta was a square table of alphabets, each one made by shifting the previous one to the left, like this:
|ABCDEFGHIJKLMNOPQRSTUVWXYZ -+-------------------------- A|ABCDEFGHIJKLMNOPQRSTUVWXYZ B|BCDEFGHIJKLMNOPQRSTUVWXYZA C|CDEFGHIJKLMNOPQRSTUVWXYZAB D|DEFGHIJKLMNOPQRSTUVWXYZABC E|EFGHIJKLMNOPQRSTUVWXYZABCD F|FGHIJKLMNOPQRSTUVWXYZABCDE G|GHIJKLMNOPQRSTUVWXYZABCDEF H|HIJKLMNOPQRSTUVWXYZABCDEFG I|IJKLMNOPQRSTUVWXYZABCDEFGH J|JKLMNOPQRSTUVWXYZABCDEFGHI K|KLMNOPQRSTUVWXYZABCDEFGHIJ L|LMNOPQRSTUVWXYZABCDEFGHIJK M|MNOPQRSTUVWXYZABCDEFGHIJKL N|NOPQRSTUVWXYZABCDEFGHIJKLM O|OPQRSTUVWXYZABCDEFGHIJKLMN P|PQRSTUVWXYZABCDEFGHIJKLMNO Q|QRSTUVWXYZABCDEFGHIJKLMNOP R|RSTUVWXYZABCDEFGHIJKLMNOPQ S|STUVWXYZABCDEFGHIJKLMNOPQR T|TUVWXYZABCDEFGHIJKLMNOPQRS U|UVWXYZABCDEFGHIJKLMNOPQRST V|VWXYZABCDEFGHIJKLMNOPQRSTU W|WXYZABCDEFGHIJKLMNOPQRSTUV X|XYZABCDEFGHIJKLMNOPQRSTUVW Y|YZABCDEFGHIJKLMNOPQRSTUVWX Z|ZABCDEFGHIJKLMNOPQRSTUVWXY
Trithemius used the tabula recta to define a polyalphabetic cipher which was equivalent to Leon Battista Alberti's cipher disk. The tabula recta is often referred to in discussing pre-computer ciphers, including the Vigenère Cipher and Vigenère's less well-known (but much stronger) autokey cipher. All polyalphabetic ciphers based on Caesar ciphers can be described in terms of the tabula recta.
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