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:
S, C, P, A, D, I, F, B, L, T.
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:
| jbfo | bnyr | omra | oxub | fuls | xmxr | snbs | cmjb | smhm | yrln |
| fsco | rlsc | nfmr | sdb. |