CIPHER APPARATUS.
The cipher disk.—The cipher disk is composed of two disks of cardboard, leather, or other material joined concentrically, the upper disk revolving upon the lower. The alphabet, reading from left to right, and such other signals, numerals, or combinations of letters, as may be desired, are printed around the circumference of the lower disk. On the upper disk are printed the alphabet and such other signals, numerals, or combinations of letters as are printed on the lower disk. On the lower disk they are printed from left to right, while on the upper disk they are printed from right to left. If it is desired to encipher a message, the key letter or the first letter of the key word or words is set opposite "A." Let us assume it to be "J." The cipher letters to be written are those opposite the text letter when the letter "a" on the upper disk is set opposite "J" on the lower disk. For example, "Send powder" would be written "rfwg uvngfs."
Having a cipher disk as above described, this mere transposition of letters would delay but a short time the deciphering of a message by one not knowing the key letter, as it would be necessary only to place, in turn, opposite "a" each of the letters of the alphabet beginning with "b" and noting the letters opposite the enciphered letters. But this simple disk can be used with a cipher word, or preferably, cipher words known only to the correspondents, and it is entirely improbable that a message so enciphered could be deciphered in time to be of any value to the enemy. Using the key words "permanent body" to encipher the message "Reenforcements will reach you at daylight," we would proceed as follows: Write out the message to be enciphered and above it write the key word or key words, letter over letter, thus:
| P | E | R | M | A | N | E | N | T | B | O | D | Y | P | E | R | M | A | N | E | N | T | B | O | D | Y | P | E | R | M | A | N | E | N | T | B | |
| R | e | e | n | f | o | r | c | e | m | e | n | t | s | w | i | l | l | r | e | a | c | h | y | o | u | a | t | d | a | y | l | i | g | h | t | |
| y | a | n | z | v | z | n | l | p | p | k | q | f | x | i | j | b | B | p | w | a | n | r | u | q | p | e | p | l | o | m | c | c | w | h | m | i |
Now bring the "a" of the upper disk under the first letter of the key word on the lower disk, in this case "P." The first letter of the message to be enciphered is "R." "Y" is found to be the letter connected with "R" and it is put down as the first cipher letter. The letter "a" is then brought under "E," which is the second letter of the key word. "E" is to be enciphered and "a" is found to be the second cipher letter. Then bring "a" to "R" and the cipher letter will represent "e," the third text letter of the message. Proceed in this manner until the last letter of the cipher words is used, and, beginning again with the letter "P," so continue until all letters of the message have been enciphered. Divided into groups of four letters, it will be as follows: "yanz vznl ppkq fxij bpwa nruq pepl omcc whmi."
Fig. 19.—Cipher disk.
To decipher the message, reverse the proceedings above described; thus the letter "a" on the upper disk is brought under the first letter of the key word "P." Following these instructions, we find the first cipher letter of the message; "a" is then brought to the next letter of the key word. In this case "E" is, of course, the next letter of the text. "R" is the next letter in the key and "a" is brought over it. The cipher letter "n" gives us the next text letter, which is "e," and so on until the completion of the message. If the letters of the key word or phrase are exhausted, begin again with the first letter and so continue until the entire message is deciphered.
With a key word, or, preferably, a key phrase of three or four words, the deciphering of a message is extremely difficult.
In a military cipher message, it may be desired to transmit numerals, the spelling out of which would require considerable time. This can be done by an arrangement of the cipher disk so that the numerals of which will appear in the same order as and follow the letters of the alphabet. Thus on the lower disk 1 is placed opposite A; 2 opposite B; 3 opposite C; 4 opposite D; 5, 6, 7, 8, 9, and 0 opposite E, F, G, H, I, and J, respectively.
On the upper disk the above numerals also appear, beginning numeral 1 opposite A; 2 opposite B, etc., 0 being opposite J.
The arbitrary sign XX will be used to indicate "numerals follow" and "numerals end." Supposing then we wish to send the following message: "Send 6,000 cavalry at once," and that the key word was "Washington." Following the instructions heretofore given for enciphering, we would place the words as follows:
| W | A | S | H | I | N | G | T | O | N | W | A | S | H | I | N | G | T | O | N | W | A | S | H | I |
| S | E | N | D | X | X | 6 | 0 | 0 | 0 | X | X | C | A | V | A | L | R | Y | A | T | O | N | C | E |
| E | W | F | E | L | Q | B | K | F | E | Z | D | Q | H | N | N | Y | C | Q | N | D | M | F | F | E |
In place of a disk means may be extemporized by taking two strips of paper, on one of which the alphabet, numerals, etc., are twice written in succession. On the other, with equal spacing, the alphabet, etc., are written once, but in reverse order. By sliding these strips in juxtaposition with each other they will replace the disk.
Cipher disks should never be allowed to fall into the hands of the enemy or of anyone unauthorized to have and use them; to insure this, special instructions should be issued for their care and keeping.