CH. XIV] 



CRYPTOGRAPHS AND CIPHERS 



S09 



letter which occurs more than once) and the remaining letters 

 of the alphabet, thus : — 



Only 25 cells are available, so k has been used for both h and q. 

 The message is then divided into pairs of consecutive letters, 

 but to prevent any pair consisting of the same two letters, a 

 dummy letter like z is, when necessary, introduced. If both 

 letters of a pair appear in the same vertical (or horizontal) line 

 of the square, each of them is replaced by the letter in the 

 square immediately to its right (or below it) — the letters in 

 every line being treated as in cyclical order. If the letters in 

 a pair do not appear in the same line in the square they must 

 necessarily be at opposite angles of some rectangle, and they are 

 replaced by those at the other angles of the rectangle, each by 

 that which is in the same horizontal line. Thus the message 

 will meet you at noon would first be written wi Iz Im ez et yo 

 ua tn oz on; then be put in cipher as yfuw ka bv sr xp Ih gt ux 

 pc; and finally be sent as yfuwk abvsr xplhg tuxpc. It is curious 

 that this cipher is not used more extensively, for the discovery of 

 the key is difficult, even to specialists. 



Fourth Type of Ciphers. A cipher of the fourth type is one 

 in which each letter is always represented by the same symbol, 

 but more than one letter may be represented by the same 

 symbol. Such ciphers were not uncommon at the beginning of 

 the nineteenth century, and were usually framed by means of 

 a key sentence containing about as many letters as there are 

 letters in the alphabet. 



