The ADFGVX cipher was developed by Colonel Fritz Nebel and introduced in March 1918. Germany used it as a field cipher during WWI. It was named the ADFGVX cipher for the letters used to create ciphertext, chosen for their distinctiveness in morse code. Its cryptanalysis requires two keys- a key matrix and a keyword. The key matrix is a 6x6 square with the letters A, D, F, G, V, and X above and to the side- populated with letters from the plaintext message and numbers 0-9. The first step of encoding is done through substitution- each letter of the plaintext is replaced by its two intersecting ADFGVX letters. The next step includes writing the enciphered text under the keyword in horizontal rows. A columnar transposition is then performed- the keyword is rearranged into alphabetical order and the vertical columns with each letter of the key word. The final ciphertext is formed by reading off the columns in vertical fashion. To decipher the ciphertext, you reverse the steps- using the same keyword and key matrix.
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Fritz Nebel , “ADFGVX Cipher,” Ciphers and Encryption, accessed August 1, 2021, https://crypto.omeka.net/items/show/20.