Decipherment

An Excerpt from The Structure of Philosophy: A Cybernetic Interpretation

Ronald K. Hoeflin

In The Labyrinth of Reason, William Poundstone says (p. 219):

A convincing demonstration of the correctness of decipherment has four steps:
First, you specify the cipher system and its key. Here ``key'' can mean whatever minimum amount of information must be remembered in order to apply the cipher, whether it takes the form of a printed key or not.
Second, you reverse the cipher system, producing an alleged plaintext [i.e., text produced by decipherment] from the ciphertext [i.e., text to be deciphered].
Third, the resulting plaintext is a sensible message, not nonsense.
Fourth, the key can be specified concisely. Examples of concise descriptions of keys are: "Use Caesarian cipher `J' throughout"; "The key is the block of letters on a slip of paper found among Roger Bacon's effects"; [etc.].
The fourth requirement is necessary to prevent decipherers from working backward from a hoped-for plaintext. . . . The decipherer must provide some reason to believe that the key existed before he started working on the cipher.

Poundstone mentions the need for a concise key in both his first and fourth points, which creates some unnecessary redundancy. I eliminate the redundancy by assigning the need for a ciphertext to his first point and restricting the need for a concise key, known or knowable before decipherment begins, to his fourth point alone.

My classification would then be as follows:

Phase  

Aspect of verifying a decipherment

D  

(1) The ciphertext is specified

A  

(4) A concise key is specified

G  

(2) The plaintext is produced

Q  

(3) The plaintext is sensible

The ciphertext is the problem, which drives, D, the decipherment process forward.

The key is the means of anticipating, A, how to convert ciphertext to plaintext. A "Caesarian" cipher (so called because it was used by Julius Caesar) simply replaces each letter in a plaintext with another letter a constant distance forward in the alphabet. The Caesarian J key moves each letter 9 letters forward: A to J, B to K, C to L, etc., and Z to I, since at the end of the alphabet the entire alphabet is repeated. More sophisticated ciphers vary the number of digits each letter is moved forward using some familiar text or a pre-arranged code pad. For example, if the key's text, such as a passage written by Roger Bacon, begins with the word "THE," then if the first word of the plaintext is "BUT," the "B" would be shifted forward 19 letters (the distance of "T" to "A") to "U"; the "U" in "BUT" would be moved forward 7 letters (the distance of "E" from "A") to "X." This yields UBX as the first 3 letters of the ciphertext.

The plaintext is the goal of the decipherment, i.e., the text produced by applying the key to the ciphertext.

And the sensibleness of the plaintext is its quiescent property, Q, of being understandable, just as when a hungry man bites into an apple he wants to experience the quiescent taste, smell, feel, etc., of a real apple, not a wax or fake apple.

Copyright 1998 by Ronald K. Hoeflin. All rights reserved.