As part of my recent IDS project, I will be chronicling my ‘learning journey’ in the subject of cryptography and cryptoanalysis through a series of posts, each detailing a brief summary of what I have learned and/or accomplished.

In the last post, I talked about the earliest type of cryptography: the substitution cipher, and how to crack it. In this post, I will be detailing a more advanced cipher: the Vignere cipher.

This cipher originated much later than the substitution cipher, as it was invented in 1467 by Leon Alberti. The first version of the cipher used multiple cipher wheels, each with their own Caesar shift, and switched between them while writing the ciphertext. This essentially meant that the message was encrypted with not one key, but five, and while it could still be decoded by the intended recipient, performing frequency analysis on it was useless.

Later, in 1518, Johannes Trithemius published Polygraphiae, in which he conceptualized the tabula recta. The tabula recta is simply a grid with all the letters of the alphabet on each axis, with the key for a Caesar cipher marking each letter. The sender would use a predetermined keyword, and for each letter of the key, find the corresponding letter on the tabula recta, and use its caesar shift to encrypt one letter of the ciphertext, then repeat with the next letter.

This cipher puzzled cryptoanalysts for a long time, until Charles Babbage cracked it, basically by applying frequency analysis to every single letter.

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