Hypothesis Testing

Introduction

The Zodiac Killer's 340-character cryptogram (referred to here as Z340) has many things in common with his 408-character cryptogram (Z408), but still remains unsolved. It seems rather evident that Z340 is not constructed using the same method as Z408, otherwise it would have been solved by now. Within the last 10 years or so, many software tools have been built to effectively solve homophonic substitution ciphers that have the same properties as Z408. If Z340 was a simple homophonic substitution cipher, it would have been solved by now by one of the numerous manual or automatic attempts.

Since Z340 remains unsolved, we must consider the possibility that some other scheme is used in its construction. How do we rule them out? There are many schemes to choose from. People have tried many different schemes, but there is not yet a comprehensive study over which schemes are feasible. Moreover, it is not clear if a particular scheme can be broken if Z340 uses it. We need a comprehensive way to explore the feasibility of the scheme, and to determine how effectively ciphers created under the scheme can be cracked.

Methodology

The strategy presented here is the following:

• First, define a hypothesis for the encipherment scheme. The hypothesis is a collection of factual statements about properties of the plaintext, and the operations performed on the plaintext to turn it into ciphertext.
• Create at least 100 test ciphers under the scheme.
• Guide the generation of test ciphers so that they contain statistical properties that are very similar to Z340.

Base Hypothesis

= Hypothesis 1: Z340 is a monoalphabetic homophonic substitution cipher.

= Hypothesis 2: Columnar transposition was applied to the plaintext prior to homophonic substitution

= Hypothesis 2: Scytale transposition was applied to the plaintext prior to homophonic substitution