A code is a list or collection of arbitrary words or groups of letters to each of which some ordinary word, proper name, phrase, or sentence is assigned for meaning.

Ciphers embrace all means whereby writings may be transcribed into occult terms. All ciphers employ some distinct method for transcription, which method is termed a key. In practice the key is usually applied directly in enciphering and reversed in deciphering messages.

CODES IN USE.

The codes of the Western Union and Postal Telegraph companies are examples of well-known codes suited to general commercial use. Besides these, many special codes have been formulated, so as to embody technical expressions especially adapted to use in particular lines of industry. The War Department Code is a military code adapted to the special needs of the military establishment in peace and war.

EMPLOYMENT OF CODES.

Codes are primarily intended for economy, but they may also be readily employed to secure secrecy. When used solely for economy, the coded message is said to be plain code; that is, the word or phrases of the message are coded by direct reference to their respective code equivalents. Thus plain code is readily translatable to anyone in possession of a code book. When secrecy is desired, some method of enciphering or key is employed in such a way that only persons in possession of it can in conjunction with the code book decipher it. In such case the message is said to be in cipher code.

CIPHER CODE.

In all codes each expression and its equivalent in plain language is assigned a number. These numbers usually commence at unity and increase consecutively to any desired figure. Messages may be enciphered by means of a key number or series of numbers. An additive number, say 55 additive, requires that in enciphering a message, the fifty-fifth word numerically greater than the proper code word shall be used; if 55 subtractive is used, the fifty-fifth word numerically smaller than the proper code word is to be used. By agreement a single key number can be used alternately additive and subtractive, that is, first additive, second subtractive, third additive, etc.

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.

Forms of field cipher.—There are two general classes of field cipher. The first class employs the transposition or reversal of the letters or words of a message according to some preconcerted rule as a means of secrecy. The route cipher hereafter described is an example of this class. The method used in ciphers of the second class consists in the substitution of certain letters or symbols for each of the individual letters composing the words of the message. Both classes of cipher can be rendered more efficient by a judicious use of inversions and by the concealment of terminations.

Inversions.—By the inversions of the whole or certain parts of messages, according to some preconcerted arrangement, the complications of cipher can be greatly increased. If a message is to be inverted, either as a whole or by clauses, it should be inverted before the cipher letters are written over it. Messages may be further complicated by sending the letters of each word backward in various other prearranged combinations.

Concealment of terminations.—To evade the discovery of the key or keys employed, it is most important that the termination of the words of a message should be concealed. The best method to conceal the beginning, and at the same time the termination of words, is to divide them into arbitrary groups of four or five letters each. This procedure will add immeasurably to the strength of the cipher and should in no way confuse one in possession of the key. For instance, the words "sufficient time" would be divided "suff" "icie" "ntti" "me," and such blind letters as may be agreed upon to fill the last two spaces of the last group. All such artifices as this will surely delay a translator not in possession of the key.

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:

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."

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:

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.

THE MATHEMATICAL CIPHER.

This cipher is a highly efficient one for the purpose of secrecy and at the same time requires no apparatus whatever attendant upon its use. The cipher is constructed as follows: Commit to memory the alphabet by numbers, viz, A, 1; B, 2; etc. Take any key word, phrase, or sentence desired; for example, "A discovery." Suppose the message to be enciphered is "Send me powder tonight." The enciphering of the message using the key given above will be as follows:

To encipher, first write out the key, letter by letter, placing the message letter by letter beneath it. Then reduce the letters of the key and the message to the numeral alphabetical equivalents. Add the individual columns and subtract unity from each. From any result thus found, which exceeds the number of letters in the alphabet, the number 26 must be subtracted. The final totals reduced to letters by numerical alphabetical equivalents will then give the cipher.

Now add the columns and subtract unity from each. If any result so found exceeds the number of letters in the alphabet 26 must be subtracted from it.

In the example given the numerical totals are as follows:

Translation of cipher is had by reversing the processes described.

THE ROUTE CIPHER.

This is a cipher in which the words or a message are retained unchanged, but are so disarranged by preconcerted rules that the sense becomes unintelligible. The message as received seems to be a number of disconnected words and without meaning, but by arrangement in proper order in accordance with certain rules can be easily read. Messages enciphered in this manner may be translated by persons not in possession of the key, and therefore the information contained therein should only be of such a character as to be of little value to the enemy unless acted upon immediately. The usual method employed in arranging a message for this cipher is to write the words in vertical columns. The number of words in each column should always equal the number of columns, being made so, if necessary, by the addition of sufficient "blind" words. A preconcerted route is agreed upon, as up to the first column, down the third, up the second, etc. The message is then transmitted without reference to the columns, but is deciphered at the receiving station by column arrangement and perusal along the original route.

For example, to encipher the message "Move daylight. Enemy approaching from north. Prisoners say strength one hundred thousand. Meet him as planned," arrange as follows:

Here the route is down the first column, up the fourth, down the second, and up the third.

CIPHER DETECTION.

General instructions.—In deciphering a message in which the same cipher letter or symbol is uniformly used to represent the same text letter, the following data will be of assistance.

The proportion of occurrence of letters of the alphabet in English words is as follows: For every 2 of the letter Q there are 4 of the letter X, 8 of K, 16 of B, 13 of C, 80 of I, N, O, and S; 85 of A, 90 of T, and 120 of letter E.

The compounds most frequently met with are NG EE LL MM TT DD and NN.

The order of frequency in which the letters of the alphabet occur as initial letters in words is as follows:

Employment of Cipher Disk.

If messages are enciphered by a mere transposition of the letters of the alphabet, the cipher disk can be used to quickly decipher the message, as the following example will show: Assuming that F is used to represent A, G to represent B, H to represent C, I to represent D, J to represent E, etc., in regular sequence, and that the message to be enciphered is: "We are short of rifle ammunition; send 30,000 rounds at once."

This would be enciphered if divided into groups of four letters as follows:

Place "a" of the upper cipher disk under B of the lower disk and notice whether the cipher letters jbfo—the first group—are intelligible. They give "sawn," continue this for "saw," the first three letters, may be the text word. Now the next group is B N Y R and these give A O D K. We know that A does not represent B because the first 8 cipher letters give the meaningless letters "sawnaodk." Turn "a" to C and we have for the first group T B X O, which is without meaning. Turning "a" to D we get U C Y P, a meaningless jumble. Turn "a" to E and we get V D Z Q, which is meaningless. Now turn "a" under F and we find that JBFO mean "Wear," which, so far at least, gives us a part of a word, or the word "We" and part of another word. We continue to the next group B N Y R, which gives us "esho." We now have these letters "Wearesho," which at a glance we read "We are sho;" continuing to the next group O M R A the cipher disk gives us "rtof," and we read "We are short of" and know we have found the key letter, and the information hidden in the cipher is ours. Continue deciphering with "a" under F until the end of the message. Sometimes the key letter is changed after two, three, or four letters.

It is a matter of minutes only to run through the alphabet and learn the meaning of a message so enciphered.