Field service pocket book, 1914 / General Staff, War Office.

Date:
[1914]
Catalogue details

Licence: In copyright

Credit: Field service pocket book, 1914 / General Staff, War Office. Source: Wellcome Collection.

    286/372 (page 254)
    Ill the first case each letter is represented in cipher by thatwliieh stands below it, and tlie bottom letter by the top one of the same column, e.g., F V would be represented by N S. In tlic second case each letter would be represented by the letter on its right and in the same line, and tlie letter on the extreme right by that on the extreme left, e.g., R C would be represented by A E. In the third case the two ietters are represented by those on the other diagonal of the rectangle, each by that which is in the same horizontal line, e.g., S G would be represented by 0 F. If, on dividing th(! letters of the text into pairs, a pair is found to be com- posed of a double letter, a dummy letter such as Q, X. Y or /, which is not likely to mislead the receiver of the message, should be introduced, care being taken that the dummy letter employed is constantly varied. If tlie message contains an uneven number of letters a dummy letter, should be added to complete a pair. Example.—If the message—“ Enemy attacked at noon and were repulsed *' —were to be enciphered with keyword “ Soldier (vide preceding table), the letters would be divided into pairs as follows :— EX—E.M—YA—TY—TA—Clv—ED—AT— NO—OX—AN—D\V—ER--ER—£P—EL- SE—DE—NE—SX. The pairs of letters w’ould then be enciphered thus : FV—CF—XB—YD—QB—BM—US-BQ—PS—SP— EQ—0 Y—R A—ll.A—RX—QI —E F—SB—TS—L V. The cryptogram, when grouped for transmission by telegraph or signalling would be— FVCFX—BYDQB—BMBSB—QPSSP—EQOYR—ARARN— QIEFS—BTSLV. 25. In order to decipher the cryptogram, the receiver divides it into pairs, and from this table finds the equivalents for each pair; taking the letters next above each when they arc in the same vertical column, those next on the left when they are in the same horizontal column, or those at the opposite angles of the rectangle. 26. In the solution of a cryptogram a knowledge of the comparative recurrence of the letters of the alphabet is very important. The frequency of recurrence is practically constant for all messages, ifcc., of 10,000 letters or over. The following table is applicable to military cryptograms for the solution of which the ordinary table of frequency requires modification :— Letter. English. German. French. Italian. Portu- guese. (*1) Spanish. A 7-7 6-2 8-0 11-7 14-0 13-4 B 1-5 1-8 O-i; 0-6 0-6 1-0 C 2-9 3-1 3-3 4-5 3-4 4-0 Ch , , 0-2 I) 4-1 5-i 4-0 *3-1 4-0 4-9 E 12-5 17-3 19-7 12-6 14-2 13-3 F o . o ^ *4 21 0-9 1-0 1-2 0-8 G 1-8 4-2 0-7 1-7 1-0 1-3 II 6-9 4-1 0-6 0-6 1-0 0-5 I 6-7 8-1 0-5 11-4 5-9 6-6 .T 0-5 0-1 0-3 t 0-5 0-4 IC 0-8 1-0 L 4-7 2-8 4-9 7-2 3-2 4-9 LI , , . . 0-5 M 2-9 2-6 3-1 3-0 4 (5 2-8