CH. XIV] CRYPTOGRAPHS AND CIPHERS 307 



key number. The difficulty of discovering the key number is 

 increased if after every (say) mth letter (or word) a non-significant 

 letter is inserted. I suggest this as an improvement in the cipher. 



Here for instance is a communication written in this cipher 

 with the clue numbers 4276 and 7: atpzn hvaxu xhiep tvafwg 

 hzniy prpsi kbdkz yygkq prgez uytlk obldi febzm xlpog quyit 

 cmgxk ckuex vsqka ziagg sigay tnvvs styvu aslyw gjuzm csfct 

 qbpwj vaepf xhibw pxiul txlav vtqzo xwkvt uvvfh cqbxn pvism 

 phzmq tuwxj ylceev Itif. The recipient would begin by striking 

 out every eighth letter. He would then shift .back every letter 

 4, 2, 7, 6, 4, 2, &c, places respectively, and in reading it, would 

 leave out the letters j, q, and z as only marking the ends of words. 



With these modifications, this is an excellent cipher, and it has 

 the additional merit of not materially lengthening the message. 

 It can be rendered still more difficult by arranging that either 

 or both the clue numbers shall be changed according to some 

 definite scheme, and it may be further agreed that they shall 

 change automatically every day or week. 



A similar system, now known as the St Cyr Method, was 

 proposed by Wilkins *. He took a key word, such as prudentia, 

 and constructed as many alphabets as there were letters in 

 it, each alphabet being arranged cyclically and beginning 

 respectively with the letters p, r, u, d, e, n, t, i, and a. He thus 

 got a table like the following, giving nine possible letters 

 which might stand for any letter of the alphabet. Using this 

 we may vary the cipher in successive words or letters of the 

 communication. Thus the message The prisoners have mutinied 

 and seized the railway station would, according as the cipher 

 changes in successive words or letters, read as Hwt fhziedvhi 

 bupy pxwmqmhg erh ervmrq max zirteig station or as Hyy 

 svvlwnthm lehx uuhzgmiq tvd gvcciq mqe frcoanr atpkcrr. 



The name by which the method is known is derived from 

 the fact that it was taught at St Cyr under Napoleon. This 

 system is said to have been widely employed by both armies in 

 the Franco-German war in 1870 — 1871. The construction of 

 military ciphers must be so simple that messages can be rapidly 

 * Mercury, by J. Wilkins, London, 1641, pp. 59, 60. 



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