CH. XIV] CRYPTOGRAPHS AND CIPHERS 311 



might betray the secret to an unauthorized person; such that 

 the key to it changes or can be changed at short intervals; 

 and such that it is not ambiguous. Many ciphers of the 

 second and third types fulfil these conditions; in particular 

 the Gronfeld or St Cyr Method, or the Playfair Cipher, may be 

 noted. A cipher written cryptographically, or a cryptograph 

 written in cipher, or a cipher again enciphered by another 

 process, is almost insoluble even by experts, unless accidents 

 reveal something in the construction, but it is troublesome to 

 make, and such elaborate processes are suited only for the study, 

 where the time spent in making them up and deciphering them 

 is not of much consequence. 



Cipher Machines. The use of instruments giving a cipher, 

 which is or can be varied constantly and automatically, has 

 been often recommended*. The possession of the key of the 

 instrument as well as a knowledge of the clue word is necessary 

 to enable anyone to read a message, but the risk of some instru- 

 ment, when set, falling into unauthorized hands must be taken 

 into account. Since equally good ciphers can be constructed 

 without the use of mechanical devices I do not think their 

 employment can be recommended. 



On the Solution of Cryptographs and Ciphers. Much in- 

 genuity has been shown in devising means for reading messages 

 written in cryptograph or cipher. It is a fascinating pursuit, 

 but I can find space for only a few remarks about it. 



In such problems we must begin by deciding whether 

 the message is a cryptograph or a cipher. If it is a combi- 

 nation of both, the problem is one of extreme difficulty, and is 

 likely to baffle anyone but a specialist, but such combinations 

 are unusual, and most secret messages belong to one class or 

 the other. 



If the scale of frequency of the letters agrees generally with 

 Conrad's Table, presumably the message is in cryptograph, 



* See, for instance, the descriptions of those devised by Sir Charles Wheat- 

 stone, given in his Scientific Papers, London, 1879, pp. 342—347; and by 

 Capt. Bazeries in Comptes Bendus, Association Francais pour VAvancment des 

 Sciences, vol. xx (Marseilles), 1891, p. 160 et seq. 



