Editing Tabula recta (section)

== Trithemius cipher ==
The Trithemius cipher was published by [[Johannes Trithemius]] in his book ''[[Polygraphia (book)|Polygraphia]]'', which is credited with being the first published printed work on [[cryptology]].<ref name="Poygraphia">{{cite book |last1=Kahn |first1=David |title=The Codebreakers |date=1996 |publisher=Scribner |isbn=978-0-684-83130-5 |page=133 |edition=2nd}}</ref>

Trithemius used the ''tabula recta'' to define a [[polyalphabetic cipher]], which was equivalent to [[Leon Battista Alberti]]'s [[Alberti cipher disk|cipher disk]] except that the order of the letters in the target alphabet is 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 the [[Caesar cipher]] can be described in terms of the ''tabula recta''.

The tabula recta uses a letter square with the 26 letters of the alphabet followed by 26 rows of additional letters, each shifted once to the left from the one above it. This, in essence, creates 26 different [[Caesar cipher]]s.<ref name="Salomon"/>

The resulting ciphertext appears as a random string or block of data. Due to the variable shifting, natural [[Letter frequency|letter frequencies]] are hidden. However, if a codebreaker is aware that this method has been used, it becomes easy to break. The cipher is vulnerable to attack because it lacks a [[Key (cryptography)|key]], thus violating [[Kerckhoffs's principle]] of cryptology.<ref name="Salomon"/>

=== Improvements ===
In 1553, an important extension to Trithemius's method was developed by [[Giovan Battista Bellaso]], now called the [[Vigenère cipher]].<ref>Salomon, Coding for Data, page 249</ref> Bellaso added a key, which is used to dictate the switching of cipher alphabets with each letter. This method was misattributed to [[Blaise de Vigenère]], who published a similar [[autokey cipher]] in 1586.

The classic Trithemius cipher (using a shift of one) is equivalent to a Vigenère cipher with ABCDEFGHIJKLMNOPQRSTUVWXYZ as the key. It is also equivalent to a Caesar cipher in which the shift is increased by 1 with each letter, starting at 0.