Manual for the Solution of Military Ciphers - Parker Hitt

In this class belong the methods of Vigenere, Porta, Beaufort, St. Cyr, and many others. These methods date back several hundred years, but variations of them are constantly appearing as new ciphers. The Larrabee cipher, used for communication between government departments, is the Vigenere cipher of the 17th Century. The cipher disk method is practically the Vigenere cipher with reversed alphabets.

In using these ciphers, there is provided a number of different cipher alphabets, usually twenty-six, and each cipher alphabet is identified by a different letter or number. A key word or phrase (or key number) is agreed upon by the correspondents. The message to be enciphered is written in lines containing a number of letters which is a multiple of the number of letters of the key. The key is written as the first line. Then each column under a letter of the key is enciphered by the cipher alphabet pertaining to that letter of the key. For example, let us take the message, “All radio messages must hereafter be put in cipher,” with the key Grant, using the Vigenere cipher alphabets given below. Each of these alphabets is identified by the first or left hand letter which represents A of the text. We thus will use in turn the alphabets beginning with G, with R, with A, with N, and with T.

G	R	A	N	T	G	R	A	N	T

A	L	L	R	A	D	I	O	M	E
S	S	A	G	E	S	M	U	S	T
H	E	R	E	A	F	T	E	R	B
E	P	U	T	I	N	C	I	P	H
E	R

Using the alphabet indicated by G, we get

G	J
Y	Y
N	L
K	T
K

Continuing for the other alphabets, we get

G	C	L	E	T	J	Z	O	Z	X
Y	J	A	T	X	Y	D	U	F	M
N	V	R	R	T	L	K	E	E	U
K	G	U	G	B	T	T	I	C	A
K	I

This method of arranging the message into lines and columns and then enciphering whole columns with each cipher alphabet is much shorter than the method of handling each letter of the message separately. The chance of error is also greatly reduced.

All these cipher methods can be operated by means of squares containing the various alphabets, cipher disks or arrangements of fixed and sliding alphabets. For example, this was the original cipher of Vigenere: