(<)\l Ml MC \ri(>.\ lllh.OKV Oh' SECRECY SYSTEMS (;69 



rho (Tyi)t()^nim used as "key":** 



Messa^ro s E .V I) s r r r L I e s ■■■ 



Key C ME '^_^'J Z H L O II ■ 



CrypK.trram F s'YllL O II O S T S ••• 



10. I-'raclioiial Ciphers. 



In these, each letter is first enciphered into two or more letters or num- 

 bers and these symbols are somehow mixed (e.g. by transposition). The 

 result may then be retranslated into the original alphabet. Thus, using a 

 mixed 25-lettcr alphabet for the key, we may translate letters into two-digit 

 (juinary numbers by the table: 



12 3 4 



L Z Q C P 



1 A G N O U 



2 R D M I F 



3 K Y H V S. 

 4X5 TEW 



Thus B becomes 41. After the resulting series of numbers is transposed in 

 some way they are taken in pairs and translated back into letters. 



11. Codes. 



In codes words (or sometimes syllables) are replaced by substitute letter 

 groups. Sometimes a cipher of one kind or another is applied to the result. 



5. Valuations of Secrecy Systems 



There are a number of different criteria that should be applied in esti- 

 mating the value of a proposed secrecy system. The most important of 

 these are: 



1. AmoHiil of Secrecy. 



There are some systems that are perfect— the enemy is no better off after 

 intercepting any amount of material than before. Other systems, although 

 giving him some information, do not yield a unique "solution" to intercepted 

 cryptograms. Among the uniquely solvable systems, there are wide varia- 

 tions in the amount of labor required to effect this solution and in the 

 amount of material that must be intercej)ted to make the solution unique. 



8 This system is trivial from the secrecy slandpoint since, with the excci)li()n of the 

 first d letters, the enemy is in possession of the entire "key." 



