320 CRYPTOGRAPHS AND CIPHERS [CH. XIV 



insert a 9 after every seven digits we shall get a collection of 

 words (each represented by five digits), most of which would 

 have no connection with the original message, and probably 

 the number of digits used in the message itself would no longer 

 be a multiple of 5. Of course the receiver has only to reverse 

 the process in order to read the message. 



It is however unnecessary to use five symbols for each word. 

 For if we make a similar code with the twenty-six letters of 

 the alphabet instead of the ten digits, four letters for each word 

 or phrase would give us 26 4 , that is, 456976 possible variations. 

 Thus the message would be shorter and the power of the code 

 increased. Further, if we like to use the ten digits and the 

 twenty-six letters of the alphabet — all of which are easily 

 telegraphed — we could, by only using three symbols, obtain 36 s , 

 that is, 46656 possible words, which would be sufficient for all 

 practical purposes. 



This code, at any rate with these modifications, is unde- 

 cipherable by strangers, but it has the disadvantages that those 

 who use it must always have the code dictionary available, and 

 that it takes a considerable time to code or decode a com- 

 munication. For practical purposes its use would be confined 

 to communications which could be deciphered at leisure in an 

 office. 



NOTE. Mr C. H. Harrison has pointed out that an objection to the Play- 

 fair system described above on pages 308, 309 is that if a particular word is 

 repeated, its central letters can appear in the cipher in only two forms. If this 

 word is a long one and it is guessed correctly the key -word can generally be 

 found. 



Mr Harrison has devised a slide-rule cipher of the complex shifting substitu- 

 tion type but such that the recurrence of the shifts is non-cyclioal. I have not 

 space to describe it here, but it seems safer than most of those mentioned in 

 this chapter, and its use, in an office with intelligent clerks, would present no 

 difficulty. I am not however convinced that it would be easier or safer to use 

 than a simple cryptographic cipher. (See p. 293. ) 



