📜 Security

Trithemius Cipher

Polyalphabetic cipher where letter n is shifted by n positions (similar to Vigenère with key ABCDEF…). Invented by Johannes Trithemius in the 16th century.

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Trithemius: the cipher that advances by itself

Johannes Trithemius published in 1508 the idea of using a different alphabet for each letter of the message, following a fixed progression: the first letter does not shift, the second shifts one place, the third two, and so on. It is the first polyalphabetic cipher described with a table, and the direct ancestor of Vigenère.

Paste the text and the page applies the progression. The practical effect is that identical plaintext letters become different ciphertext letters depending on position — AAA becomes ABC. That alone defeats naive frequency analysis, which was the only cryptanalytic tool available at the time.

Its weakness is having no key: the progression is always the same, so anyone who knows the method deciphers without needing a secret. Vigenère fixes exactly that by replacing the fixed progression with a keyword, which determines which alphabets are used and in what order. Trithemius is, in that sense, Vigenère with the key ABC — and that is how the relationship is usually explained.

Frequently asked questions

How does it differ from the Caesar cipher?

Caesar uses a single shift for the whole message. Trithemius uses a shift that grows with each letter. In practice that means the language's frequency distribution dissolves, which does not happen with Caesar — but at the cost of having no key to vary.

Does the shift start at zero or one?

In this implementation, at zero: the first letter comes out unchanged. That is the most common convention, and the one making the cipher equivalent to Vigenère with the key ABC. Some descriptions start at one, which merely shifts everything one place.

What happens after the twenty-sixth letter?

The progression wraps: the shift is taken modulo 26, so the letter at index 26 goes back to no shift. That makes the cipher periodic with period 26 — and it is that periodicity Kasiski analysis would spot easily in a long text.

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