Zodiac Killer Cipher Solved!

Overview

On December 5, 2020, veteran code-breaker David Oranchak, applied mathematician (and fellow Melburnian) Sam Blake, and Belgian programmer Jarl Van Eycke solved the 340-symbol cipher sent by the Zodiac killer to the San Francsico Chronicle in November 1969.

Oranchak outlined their discovery in a YouTube video he posted on December 11, 2020, following the FBI's confirmation of their solution.

Oranchak posted a comprehensive description of the 340 cipher, and how Zodiac likely created it (equivalent to what is described below, which I and likely others shared with him), in a new January 29, 2021 YouTube installment. Anyone interested in the 340 cipher should watch this video (and any others that follow, as he promises at the end).

The cipher

This is what Zodiac sent to the Chronicle:

The key

Watch Oranchak's video for a description of how they discovered the key (the most difficult part of the process!). The following shows you which symbols (black) map to each original English letter (red):

The solution

As Oranchak explains, Zodiac broke the original message into three sections, with 9, 9, and 2 rows respectively. Within each of the first two sections, Zodiac wrote his message downwards, rather than across, and then rotated each row to the right by two, four, six … positions, with mistakes being made in the sixth row of the second section. Undoing those row rotations, after applying the above key, we can see the solution:

The message

Reading down and across through the first two sections, then through the third section (with some words reversed), and finally the separate words in purple (and fixing some encoding mistakes), we see the Zodiac's complete message:

I HOPE YOU ARE HAVING LOTS OF FUN IN TRYING TO CATCH ME

THAT WASN'T ME ON THE TV SHOW

WHICH BRINGS UP A POINT ABOUT ME

I AM NOT AFRAID OF THE GAS CHAMBER BECAUSE IT WILL SEND ME TO PARADICE ALL THE SOONER BECAUSE I NOW HAVE ENOUGH SLAVES TO WORK FOR ME WHERE EVERYONE ELSE HAS NOTHING WHEN THEY REACH PARADICE

SO THEY ARE AFRAID OF DEATH

I AM NOT AFRAID BECAUSE I KNOW THAT MY NEW LIFE WILL BE AN EASY ONE IN PARADICE

LIFE IS DEATH

The psychology

It's impossible to know for certain, but now that we have the solution to the 340, we can make a guess as to Zodiac's thought process in creating it.

Clearly, he was upset that his previous cipher was cracked so quickly, and wanted to make sure that this one wasn't so easy. On the other hand, he seemed to understand that if he made it too difficult, then it would never be solved at all, which would prevent him from having a laugh when people realized that it, again, contained nothing useful for identifying him. So he stuck with the same simple method, of replacing each letter with a set of symbols (roughly chosen so that popular letters in English had more symbols assigned to them, to prevent it from being too easy to recognize e.g. 'E' or 'T').

Here is where it becomes interesting. He clearly decided to make things more complicated by writing down the page, then across, rather than across and down. But he probably reasoned that that alone would not be enough to make the cipher difficult enough to crack. So he broke it into three sections, writing within the first before moving to the second.

He probably reasoned that that would still not be enough to prevent a rapid decryption. So he anchored "LIFE IS" at the end of the top row of the second section, in normal left-to-right text. He also switched to left-to-right in the third section, but reversed some of the words.

Had he stopped there, it may not have been 51 years before we had a solution to the 340. He probably realized that bigrams would still give a huge hint as to how he had encoded the message, and would point to his writing down the page rather than across. So he decided to rotate every row. One character of rotation would not quite do it, because someone scanning through diagonals might still pick up the pattern. So he rotated by two characters.

That final decision is what made the 340 effectively unbreakable. Even though, in hindsight, we can see that he didn't change his methods too much from the 408, that rotation was sufficient to make the space of possible methods enormous.

It is incredible that Oranchak, Blake, and Van Eycke did finally crack the uncrackable. It is difficult to know whether Zodiac was disappointed that he had made it just that much too difficult to crack in (likely) his lifetime, or whether he enjoyed the fact that, in hindsight, it was always so tantalizingly close. Maybe he intended to reveal the solution at some point in time, and died before doing it. We'll likely never know.