---------------------------------- A ABCDEFGHIJKLMNOPQRSTUVWXYZ abcdefghijklmnopqrstuvwxyz ---------------------------------- B ABCDEFGHIJKLMNOPQRSTUVWXYZ bcdefghijklmnopqrstuvwxyza ---------------------------------- C ABCDEFGHIJKLMNOPQRSTUVWXYZ cdefghijklmnopqrstuvwxyzab ---------------------------------- etc. ---------------------------------- Y ABCDEFGHIJKLMNOPQRSTUVWXYZ yzabcdefghijklmnopqrstuvwx ---------------------------------- Z ABCDEFGHIJKLMNOPQRSTUVWXYZ zabcdefghijklmnopqrstuvwxy ---------------------------------- The large letters at the left are the letters of the key word. It will be noted that these letters correspond to the first letters of the cipher alphabets (in small letters) as in the Vigenere cipher. A much simpler arrangement of the Vigenere cipher is the use of a fixed and sliding alphabet. Either the fixed or sliding alphabet must be double in order to get coincidence for every letter when A is set to the letter of the key word. Fixed Alphabet of Text | | |ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZ | +-----------------------------------------------------+ | ABCDEFGHIJKLMNOPQRSTUVWXYZ | | | | Movable Alphabet of Cipher | +-------------------------------------------+ As shown here, A of the fixed or text alphabet coincides with T of the movable cipher alphabet. This is the setting where T is the letter of the key word in use. The lower movable alphabet is moved for each letter of the message and the A of the fixed alphabet is made to coincide in turn with each letter of the key before the corresponding letter of the text is enciphered. It is obviously only a step from this arrangement to that of a cipher disk, where the fixed alphabet, (a single one will now serve) is printed in a circle and the movable alphabet, also in a circle, is on a separate rotatable disk. Coincidence of any letter on the disk with A of the fixed alphabet is obtained by rotating the disk. The well known U. S. Army Cipher Disk has just such an arrangement of the fixed alphabet but the alphabet of the disk is reversed. This has several advantages in simplicity of operation but none in increasing the indecipherability of the cipher prepared with it. The arrangement of fixed and sliding alphabets which is equivalent to the U. S. Army cipher disk is this: