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.