The Trithemius cipher is a polyalphabetic cipher invented by the German author and monk Johannes Trithemius in the 15th century. The cipher was published in his book Polygraphia, which is credited with being the first published work on cryptology. 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.
This method removes the letter frequencies from the ciphertext, making it appear as a random string or block of data. However, 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' principle, a rule of cryptology.
Using a tabula recta, each alphabet is shifted one letter to the left from the one above it. This forms 26 rows of shifted alphabets, ending with Z (as shown in image). Data is encrypted by switching each letter of the message with the letter directly below, using the first shifted alphabet. The next letter is switched by using the second shifted alphabet, and this continues until you have encrypted the entire message.
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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