312 CRYPTOGRAPHS AND CIPHERS [CH. XIV 



though we must allow for the possibility that dummy letters, 

 like j, k, x, and z, have been introduced either to separate words 

 or deliberately to confuse those not in the secret. A short 

 sentence of this kind may be read by an amateur, but only an 

 expert is likely to discover the key to a long and well con- 

 structed cryptographic message. 



If the message is long enough, say about 80 words, and 

 the scale of frequency of particular letters differs markedly 

 from Conrad's scale, there is a presumption that the message is 

 in cipher. If the numbers of the two scales agree generally, 

 probably a simple substitution alphabet has been used, i.e. it is 

 a cipher of the first type, and generally the discovery of the key 

 is easy. If it is not a cipher of this type, we must next try to 

 find whether it is of any of the other recognised types. The 

 majority of other ciphers are included in Gronfeld's number 

 (or the St Cyr word) system, and here I will confine myself to 

 a discussion of how such ciphers may be read. 



The discovery of a key to a cipher of this kind is best 

 illustrated by a particular case. I will apply the method to 

 the message cisvg vumya vijnp vgzsi ybpjp woiy. This is an 

 example of a Gronfeld's cipher with no additional complications 

 introduced, but the message is short, and it so happens that the 

 letters used are not in the normal scale of frequency ; yet it can 

 be read with ease and certainty. 



The first thing is to try to find the number of digits in the 

 key number. Now we notice that the pair of letters vg occurs 

 twice, with an interval of 12. If in each case these represent 

 the same pair of letters in the original message, the number of 

 digits in the key number must be 12 or a divisor of 12. Again 

 the pair of letters iy occurs twice, with an interval of 8, and 

 this suggests that the number of digits in the key number is 8 

 or a divisor of 8. Accordingly we conjecture that the key 

 number is one of either 2 or 4 digits: this conclusion is 

 strengthened by noting the intervals between the recurrences 

 of the same letters throughout the message. We may put 2 

 on one side till after we have tried 4, for anyone using Gron- 

 feld's method would be unlikely to employ a key number less 



