The second letter of the message was enciphered using the second shifted alphabet, etc.Īlberti's cipher disk implemented the same scheme. Trithemius's idea was to encipher the first letter of the message using the first shifted alphabet, so A became B, B became C, etc. Each alphabet was shifted one letter to the left from the one above it, and started again with A after reaching Z, like this: He started with a tabula recta, a square with 26 alphabets in it (Trithemius, writing in Latin, used 24 alphabets). Unlike Alberti's cipher, which switched alphabets at random intervals, Trithemius switched alphabets for each letter of the message. Johannes Trithemius, in a book published after his death, invented a progressive key polyalphabetic cipher. Alberti also invented a decoder device, his encryption disk, which implemented a cipher equivalent to the one published later by Johannes Trithemius.

Alberti used a Caesar cipher to encrypt a message, but whenever he wanted to he would switch to a different alphabet, indicating that he had done so by capitalizing the first letter encrypted with the new alphabet. The first published polyalphabetic cipher was invented by Leon Battista Alberti around 1467. The Vigenère's cipher is probably the best-known example of a polyalphabetic cipher, though it is only a special case.

A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets.

Polyalphabetic cipher