The ADFGVX encoding method was developed by Fritz Nebel, a colonel of the German
communications corps, and it was first employed in spring 1918 by the German
army. The procedure is an effective cascading cipher, consisting of a
substitution followed by a column transposition.
The key input is done in the dialog window key entry: ADFGVX.
The substitution is executed by means of a 6x6-matrix, containing the letters
'A' to 'Z' and ciphers '0' to '9' in random order. The rows and columns of the
matrix are identified by the letters A, D, F, G, V and X.
Since the substitution matrix is part of the key, it must be known to the
recipient of the message. The first encryption stage (substitution)
determines the position of each plaintext letter within the matrix and replaces
it by its row- and column-indicator. This way, every plaintext letter is
replaced by two ciphertext letters.
During the second encryption stage the ciphertext generated in the first stage is further
encoded by means of a column transposition. The basis of this transposition is
the transposition password, which must consist exclusively of the letters 'A'
to 'Z'; redundant characters are not allowed. Thus, the password length is
restricted to a maximum of 26 characters.
The transposition password forms the second part of the ADFGVX key. The
transposition again is realized through a matrix. The first row is filled with
the transposition password, which determines the number of columns. The stage 1
ciphertext is then written into the matrix line by line. Now the columns of the
matrix are swapped in such a way, that the letters of the transposition
password are arranged in alphabetical order. Finally, the stage 2 ciphertext is
extracted from the matrix column by column.
The security of this cipher bases on the fact that the two ciphertext letters
substituting every plaintext letter are separated by the transposition,
making a frequency analysis of the bigrams useless.
Thus, using the semi-manual analysis
included in CrypTool, the transposition has to be solved first, without having
a clue about the substitution key.