Tabula recta

In cryptography, the tabula recta (from Latin ) is a square table of alphabets, each row of which is made by shifting the previous one to the left. The term was invented by the German author and monk Johannes Trithemius in 1508, and used in his Trithemius cipher.

The Trithemius cipher was published by Johannes Trithemius in his book Polygraphia, which is credited with being the first published work on cryptology.

Trithemius used the tabula recta to define a polyalphabetic cipher, which was equivalent to Leon Battista Alberti's cipher disk except that the alphabets are not mixed. The tabula recta is often referred to in discussing pre-computer ciphers, including the Vigenère cipher and Blaise de Vigenère's less well-known autokey cipher. All polyalphabetic ciphers based on Caesar ciphers can be described in terms of the tabula recta.

It uses a letter square with the 26 letters of the alphabet following 26 rows of additional letters, each shifted once to the left. This creates 26 different Caesar ciphers.

The resulting ciphertext appears as a random string or block of data. However, the letter frequencies are just shifted: If e is the most frequent letter in the cleartext and the shift is 3, then h will be the most frequent letter in the ciphertext. Especially if a person is aware that this method is being used, it becomes easy to break. The cipher is vulnerable to attack because it lacks a key, which is said to break Kerckhoffs's principle, a rule of cryptology.

In 1553, an important extension to Trithemius's method was developed by Giovan Battista Bellaso called the Vigenère cipher. Bellaso added a key to switch cipher alphabets every letter. This method was misattributed to Blaise de Vigenère, who published a similar autokey cipher in 1586.

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