Abstract
We describe the background of the Zodiac killer’s cipher, and present a strategy for how to attack the unsolved Z340 cipher. We present evidence that there is a high degree of non-randomness in the sequence of ciphertext symbols in this cipher, suggesting it has been constructed in a systematic way. Next, we use this information to design a tool for solving the Zodiac ciphers. Using this tool we are able to re-solve the known Z408 cipher.