Communication Theory of Secrecy Systems - Network Research Lab

The cryptogram used as “key” 8 :
Message S E N D S U P P L I E S · · ·
Key C O M E T U S Z H L O H · · ·
Cryptogram U S Z H L O H O S T S Z · · ·
4.10 Fractional Ciphers
In these, each letter is first enciphered into two or more letters or numbers
and these symbols are somehow mixed (e.g., by transposition). The result
may then be retranslated into the original alphabet. Thus, using a mixed 25letter
alphabet for the key, we may translate letters into two-digit quinary
numbers by the table:
0 1 2 3 4
0 L Z Q C P
1 A G N O U
2 R D M I F
3 K Y H V S
4 X B T E W
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.
4.11 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 SYSTEM
There are a number of different criteria that should be applied in estimating
the value of a proposed secrecy system. The most important of these are:
5.1 Amount 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 variations
in the amount of labor required to effect this solution and in the amount of
material that must be intercepted to make the solution unique.
8 This system is trivial from the secrecy standpoint since, with the exception of the first d letters, the
enemy is in possession of the entire “key”.
669