Encrypt or Decrypt Tritheme Cipher

What is Tritheme Cipher ?

Tritheme Cipher (or Trithemius in Latin) is a polyalphabetic cipher. That means one letter of the plain text can be encrypted into several different letters in the ciphered text. This avoid, or at least makes it more difficult, to break the code by a simple frequency analysis.

This cipher had been created during Renaissance period by a German Abbot, Thritheme. More precisly, he wrote a book "Polygraphiae" (which talks about cryptography) around the year 1500. This book presents, among other subjects, a table (the Tritheme table) called "substitution table" which shows in absciss and ordinate the alphabet letters (like the Vigenere cipher for instance). Tritheme then refers to this table to encrypt the letters of plain text.

The Trithem cipher is actually very simple, since it is just a sequence of Caesar cipher applied to each letter separetely with a different shift (actually a lot like Gronsfeld cipher but in a more predictive way). The shifts just grow after each letter, which means - starting with 0 - the firest letter won't move, the second one will move of one position in the alphabet, the third will move of two positions, etc. One can point out that encrypt a word of - for instance - ten letters using Tritheme cipher is actually the same than encrypting it using Gronsfeld cipher with the key "0123456789".

The Tritheme cipher is very easy to break, as you just have to invert the encrypting process, that is, starting from 0, shift the letters incrementaly but from Z to A instead of A to Z. Just as other polyalphabetic ciphers, the frequency analysis allows (under some circumstances by repetition of certain letters groups that were shifted in the same way) to retrieve the plain text, thus breaking the code. However, the Tritheme cipher is very simple, so doesn't need frequency analysis.

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