The key numbers are used over and over until the entire message is enciphered. The key number can sometimes be expressed by a single word, as, for instance, "Grant," each letter having a value of tens in accordance with its position in the alphabet; that is, G, the seventh letter equals 70; R equals 180; A equals 10; N equals 140; and T equals 200. Or by preconcerted arrangement letters may represent units or hundreds. Security from translation by persons not having the key number is greater when the key numbers are used alternately additive and subtractive. If a cipher key word is used, it should be one of an odd number of letters, as, for instance, "Jones," the numbers corresponding to the positions of the letters in the alphabet. The first number should be additive, the second subtractive, etc. By this means the first letter of the key word is additive the first time it is used, subtractive the second, additive the third, and so on. In some instances the key number, when added to or subtracted from the code number, gives a resulting number exceeding the highest code number or less than unity. In cases of this kind it should be remembered in enciphering that unity follows the highest code number in addition, and that the highest code number follows unity in subtraction. In deciphering a message the process of enciphering is reversed.
THE WAR DEPARTMENT CODE.
As previously stated, the War Department Code is the technical military code and contains expressions numbered consecutively from 1 to 62,000. All the code words are composed of 6 letters, which are so arranged that the vowels and consonants invariably alternate. In the formation of code words the following 13 letters only are used, viz, A, B, D, E, F, G, I, K, M, N, S, U, and X. The body of the code book is arranged as follows:
(a) Army list, containing the name of every commissioned officer in the regular establishment.
(b) Military organizations, giving all batteries, companies, troops, etc.
(c) Military posts and stations, covering Alaska, Hawaii, Philippine Islands, Porto Rico, and the United States.
(d) United States naval stations and vessels.
(e) Geographical names.
(f) Miscellaneous tables as follows:
Numerals.
Arrivals and departures.
Dates.
Indorsements.
Letter acknowledgments.
Requisitions.
Telegram acknowledgments.
Mails, shipments, and transports.
Blanks for future additions as they may be needed.
Ranks and grades of officers and men in the Army.
Wireless stations of the Army and Navy.(g) Alphabetical list of code expressions arranged conveniently for use.
When it is desired to transmit some word or expression not to be found in the code and no suitable synonym can be discovered the word or expression should be sent in plain language or spelled out by the equivalents for letters and endings to be found on page 589.
Complete instructions for the use of the code either as a code or cipher are contained in the introductory pages of the book.
CIPHER CODE IN FIELD WORK.
The use of cipher code in enciphering field messages will usually be practicable only between the several headquarters and other large stations supplied with code books. This method, too, is prohibitive for urgent messages when the time of enciphering and deciphering is an important factor connected with delivery.
FIELD CIPHERS.
Description and use.—Field ciphers include all systems and the apparatus connected therewith which are ordinarily employed in enciphering and deciphering field messages. Field ciphers are intended for use when code books are not available, and hence the employment of cipher code is precluded. Some methods of field cipher employ simple forms of apparatus, while others require the use of no apparatus at all.