304 



CRYPTOGRAPHS AND CIPHERS 



[CH. XIV 



numbers in an order like this, which suggests their meaning, 

 but it will serve to illustrate the principle. 



The cipher can be improved by introducing after every (say) 

 eleventh digit a non-significant digit. If this is done the 

 recipient of the message must erase every twelfth digit before 

 he begins to read the message. With this addition the difficulty 

 of discovering the key is considerably increased. 



The same principle is sometimes applied with letters instead 

 of numbers. For instance, if we take a word (say) of n letters, 

 preferably all different, and construct a table as shown below 

 of n 2 cells, each cell is defined by two letters of the key word. 

 Thus, if we choose the word smoking-cap we shall have 100 



cells, and each cell is determined uniquely by the two letters 

 denoting its row and column. If we fill these cells in order 

 with the letters of the alphabet we shall have a system similar 

 to that explained above, where a will be denoted by ss or og 

 or no, and so for the other letters. The last 22 cells may be 

 used to denote the first 22 letters of the alphabet, or better, 

 three or four of them may be used after the end of a word to 

 show that it is ended, and the rest may be used to denote words 

 or sentences which are likely to occur frequently. The statement 

 in cipher however is twice as long as when it is in clear. 



Like the similar cipher with numbers this can be improved 



