Communication Theory of Secrecy Systems - Network Research Lab

line labeled 1, etc. From each possible message there must be exactly one
line emerging for each different key. If the same is true for each cryptogram,
we will say that the system is closed.
A more common way of describing a system is by stating the operation
one performs on the message for an arbitrary key to obtain the cryptogram.
Similarly, one defines implicitly the probabilities for various keys by describing
how a key is chosen or what we know of the enemy’s habits of key choice.
The probabilities for messages are implicitly determined by stating our a priori
knowledge of the enemy’s language habits, the tactical situation (which
will influence the probable content of the message) and any special information
we may have regarding the cryptogram.
M1
M2
M3
M4
2
1 3
2
1
3
3
2
1
1
3
2
M1
M2
CLOSED SYSTEM NOT CLOSED
1
2
2 1
Fig. 2. Line drawings for simple systems
4 SOME EXAMPLES OF SECRECY SYSTEMS
In this section a number of examples of ciphers will be given. These will
often be referred to in the remainder of the paper for illustrative purposes.
4.1 Simple Substitution Cipher
In this cipher each letter of the message is replaced by a fixed substitute,
usually also a letter. Thus the message,
M = m1m2m3m4· · ·
where m1, m2, · · · are the successive letters becomes:
E = e1e2e3e4· · · = f(m1)f(m2)f(m3)f(m4)· · ·
where the function f(m) is a function with an inverse. The key is a permutation
of the alphabet (when the substitutes are letters) e.g. X G U A C D T
B F H R S L M Q V Y Z W I E J O K N P . The first letter X is the
substitute for A, G is the substitute for B, etc.
665
3
3
E 1
E 2
E 3