CH. XIV] CRYPTOGRAPHS AND CIPHERS 301 



characters, were used freely, and successfully baffled the in- 

 genuity of the enemy, into whose hands they fell. 



A common cipher of this type is made by using the actual 

 letters of the alphabet, but in a non-natural sense as indicating 

 other letters. Thus we may use each letter to represent the one 

 immediately following it in the natural order of the alphabet— 

 the letters being supposed to be cyclically arranged — a standing 

 for b wherever it occurs, b standing for c, and so on, and finally 

 z standing for a: this scheme is said to have been used by the 

 Carthaginians and Romans. 



More generally we may write the letters of the alphabet in 

 a line, and under them re-write the letters in any order we like. 

 For instance 



abcdefghijklmn opqrstuvwxyz 

 olkmazsqxeufy r thcwbvnidgjp 



In such a scheme, we must in our communication replace a 

 by o, b by I, etc. The recipient will prepare a key by re- 

 arranging the letters in the second line in their natural order 

 and placing under them the corresponding letter in the first 

 line. Then whenever a comes in the message he receives 

 he will replace it by an e; similarly he will replace b by s, 

 and so on. 



A cipher of this kind is not uncommonly used in military 

 signalling, the order of the letters being given by the use of 

 a key word. Ciphers of this class were employed by the British 

 forces in the Sudan and South African campaigns. If, for 

 instance, Pretoria is chosen as the key word, we write the letters 

 in this order, striking out any which occur more than once, 

 and continue with the unused letters of the alphabet in their 

 natural order, writing the whole in two lines thus : 



pretoiabc dfg h 

 z y xwvus q n ml k j 



Then in using the cipher p is replaced by z and vice versa, 

 r by y, and so on. A long message in such a cipher would be 

 easily discoverable, but it is rapidly composed by the sender 

 and read by the receiver, and for some purposes may be useful, 



