The Zodiac Killer terrorized the San Francisco Bay area during the 60’s and 70’s. Five murders were attributed to him although he has claimed many more. What really set him apart was his mysterious letters and ciphers that he sent to the local newspapers to taunt the police and the public. The first cipher was cracked by a couple of puzzle enthusiasts while the others remain unsolved. These cryptic messages have forever changed how we perceive the serial killer especially in fictional stories. Serial killers leave clues and codes for the police to follow and only the smartest detective is up to that task. Is it really that hard to make an unbreakable cipher though?
Even if someone has the key to break a cipher, it’s still a rather tedious task especially without a computer. Without knowing the key, it takes a patient person to look for patterns in the letters and words. From there you can solve each letter one by one as you recognize certain words and phrases. The couple that solved the first Zodiac cipher, called the 408 cipher due to there being 408 letters, did just this by correctly assuming that the Zodiac would use “I” in the first sentence and the word “kill”. Even then it took them about 20 hours to completely solve.
What they learned is that the Zodiac used a homophonic substitution cipher which suggests that he had at least studied cryptography. Homophonic substitution ciphers use more than one character for one or more letters for the message. The idea behind this is to hide certain patterns and trends in the encrypted message. E, T, A, O, I, and N are the most common letters used in the English language. By using more than one character to represent each of these, it can make frequency analysis useless. For the letter E alone, he had six characters.
It is believed that these messages were not truly meant to be decrypted. After his first message was solved, and relatively quickly, he probably felt that he needed to increase the difficulty of his cipher. Several factors can aid to really increase the difficulty and with minimal effort. His most famous cipher is the short 13 letter cipher that supposedly contains his name. By making the cipher so short and using a different key than previously, a lot of assumptions would have to be made and many mistakes or incorrect guesses have confused things. Did he actually give his name? Did he use his full name or middle name? The number of letters is roughly the average of most English names in the United States which leaves a lot of room for false matches. My name wouldn’t work no matter what I tried. So, rest assured I’m definitely not the Zodiac.
Other factors can make it more and more difficult to solve. Using symbols makes it more challenging especially when you don’t have a name in your head to associate with it. Not using spaces between words or punctuation for sentences really forces you to focus on common patterns in words. Misspellings can really mess with your confidence that what your solving is working. From what we see in the first cipher, all these tactics were employed.
So now we move on to the 340 cipher which was the last cipher sent to the press by the Zodiac. Many people claimed to have solved it but their decrypted messages is usually garbage. Even with powerful software designed to find the smallest patterns in codes and ciphers have failed to pick up any clues. For over fifty years it has gone unsolved, and in my opinion will never be solved. We cannot know if he even played fair in creating a cipher. He could have changed the key every line. He could have used a combination of ciphers on top of one another. He could have used a Vigenere cipher where the key word is also 408 characters long. He could have made a message out of complete gibberish.
One problem with making ciphers and codes is that you can quickly make it too difficult for other people to solve. The issue with this, is typically you want other people to solve it. During war you need to make your codes and cipher difficult enough so that the enemy cannot understand it but follow simple enough rules so that your ally can quickly get the message. So, intelligent and well-planned ciphers that are meant to be solved are where true skill and craft come into play. If you want to make something impossible to solve, all you have to do is make an impossible set of rules that follow no real logic, or make something so short that multiple solutions muddle the result.
To answer the question proposed in the title, I do not believe the Zodiac Killer was a genius. I believe he was egotistical, and it served his purposes to make it seem like he was smarter than the police. He clearly studied cryptography and purposefully used tactics that make it difficult or impossible to solve.
If you want to read more on the Zodiac ciphers, check out http://zodiackillerciphers.com and watch the videos presented by the American Cryptogram Association.
-J.C. Mystery Detective