Fixed Alphabet | | | ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZ | +------------------------------------------------------+ | ZYXWVUTSRQPONMLKJIHGFEDCBA | | | | Movable Alphabet | +------------------------------------------+ It will be noticed that with this arrangement of running the alphabets in opposite directions, it becomes immaterial which alphabet is used for the text and which for the cipher for if A = G then G = A. This is not true of the Vigenere cipher. It is perfectly feasible to substitute a card for the U. S. Army cipher disk. It would have this form: ABCDEFGHIJKLMNOPQRSTUVWXYZ 1 AZYXWVUTSRQPONMLKJIHGFEDCB 2 BAZYXWVUTSRQPONMLKJIHGFEDC 3 CBAZYXWVUTSRQPONMLKJIHGFED etc. 25 YXWVUTSRQPONMLKJIHGFEDCBAZ 26 ZYXWVUTSRQPONMLKJIHGFEDCBA The first horizontal line is the alphabet of the text. The other twenty-six lines are the cipher alphabets each corresponding to the letter of the key word which is at the left of the line. One of the ciphers of Porta was prepared with a card of this kind: ----------------------------- AB ABCDEFGHIJKLM NOPQRSTUVWXYZ ----------------------------- CD ABCDEFGHIJKLM ZNOPQRSTUVWXY ----------------------------- EF ABCDEFGHIJKLM YZABCDEFGHIJK ----------------------------- etc. ----------------------------- WX ABCDEFGHIJKLM PQRSTUVWXYZNO ----------------------------- YZ ABCDEFGHIJKLM OPQRSTUVWXYZN ----------------------------- In this cipher the large letters at the left correspond to the letters of the key and, in each alphabet, the lower letter is substituted for the upper and vice versa. For example, with key BAD to encipher WILL we would get JVXY. Note that with either B or A as the key letter, the first alphabet would be used. A combination of the Vigenere and Porta ciphers is this: