Do you think you’ve solved the Zodiac 340 cipher? University of North Texas professor Ryan Garlick has written a handy guide that you should read. Once featured on the National Geographic documentary “Code Breakers“, Dr. Garlick has studied the Zodiac ciphers for many years, and has even recruited his students to try to solve them. He knows how to spot bad solutions, and his guide shows you how you can easily spot them, too. Bad solutions usually have common symptoms, such as symbols representing multiple letters, liberal use of anagramming, gibberish that needs to be highly interpreted, frequent changing of the cipher keys, and the use of fake patterns such as numerology. To his guide I would add my litmus test: If your approach can generate many different and equally compelling plaintext messages, then you can’t prove your approach is the one used by Zodiac to encrypt his message. The best example is the known solution for the 408-character cipher. There is only one overall solution that can be generated with that method, and it results in a coherent message requiring no elaborate manipulations of the key. You could quibble over some misspellings and mistakes, but they do not result in a brand new message that it totally different from the known solution.
Kevin Fagan recently wrote that many theories about the Zodiac Killer case are still being made, despite the case being 45 years old. If you have cracked the cipher, stand apart from all the bad ideas by making sure your solution does not show the problems mentioned in Garlick’s guide. Then maybe you are on the right track. Good luck!