What a Mathematician Learned From Cracking the Zodiac Killer’s Code

What it was like decoding the anonymous serial killer’s 50-year-old cipher.
Gavin Butler
Melbourne, AU
zodiac killer, code breaking, ciphers
Photos: Bettmann / Contributor / Getty Images

A group of three hobbyist cryptographers last week cracked one of the most infamous ciphers created by the Zodiac Killer, more than half-a-century after the murderer claimed their first confirmed victim.

The unnamed Zodiac killed at least five people in Northern California between the late 1960s and early 1970s, and derived their pseudonym from a series of taunting letters sent to the San Francisco Bay Area press up until 1974. Those letters included four cryptic messages, known as “ciphers.” Up until a few days ago, only one of those ciphers had ever been convincingly decoded.


Cryptologist David Oranchak, who has been trying to crack the notorious “340 cipher” (it contains 340 characters) for more than a decade, made a crucial breakthrough earlier this year when applied mathematician Sam Blake came up with about 650,000 different possible ways in which the code could be read. From there, using code-breaking software designed by Jarl Van Eycke, the team’s third member, they came up with a small number of valuable clues that helped them piece together a message in the cipher:











Blake, along with Oranchak and Eycke, has since been officially recognised by the FBI for finally cracking the code. VICE World News spoke to him on the phone to hear more about how he became interested in the 340 cipher, what he learned about the Zodiac Killer through the process of decrypting it, and what this breakthrough could mean for the investigation going forward.


VICE: How did you become so heavily involved in decrypting the Zodiac Killer’s “340 cipher”?

Sam: Years and years ago I saw old documentaries from the early 1990s on the Zodiac Killer, and I thought this was a fascinating case. But one of the people I solved this with, David Oranchak, has been working on this cipher for 15 years, and one of the things he’s done to really popularize it and keep it in the interest people is he’s posted a number of videos on his efforts to solve the cipher.

When I watched those, I thought “This is a really good analytical approach that he’s taken to try to solve this.” And I sort of saw a couple of things that I thought might be interesting for him to try. So I reached out to him originally through a YouTube comment, and then we got chatting, and it went from there. I got serious about it in March of this year, and we spent a lot of time on it between March and now, just going through, having no success, no success, no success. And then Thursday, a week ago, we just started to piece it together.

Hobbyist Cryptographers Seemingly Solve Infamous Zodiac Killer Cipher

Did you learn anything about the Zodiac killer while cracking this code?

The process that we used was highly algorithmic. There’s been a lot of solutions in the past that have required artistic creativity and a lot of bending and massaging of the cipher in order to get it to make a few legible words… then something like a sentence, and often then the name of somebody who could be associated with the case. What we did was a very different approach to that. We looked at different possible ways you could read the cipher—what other reading directions could they have taken in terms of trying to write it out—and we then ran them through supercomputers and looked for a solution in that direction. 


In terms of trying to understand anything about the actual man himself who created that, the one thing you can say is that he sent in a cipher a couple of months previous to this one and it was solved within three or four days. He’s clearly wanting to make this one more difficult, but I’m not sure whether he realized just how difficult he had made it by writing it out this way. It’s taken 50 years and computing power that you could not imagine in 1969-1970 in order to crack this, purely because there’s just so many possible ways that you could write the cipher down onto a 20-by-17 grid.

So from my point of view, I feel like the way in which he’s created this cipher says more about him than the actual text that he’s put in it. This is a cipher that is very similar to the previous cipher he’s made; he’s gone to great lengths to disguise his text in his encryption; and that method of periodically writing down the diagonals of the message is something that I think he’s probably been taught at some stage, or he’s been exposed to through his work. If I was thinking about what would help to give me any insights into the person it would be that: trying to understand where he was exposed to that method of encryption.

Where might someone learn that particular method of encryption?

I mean there’s relatively similar schemes from a 1950s US Army cryptography manual that wasn't declassified until like 2014. Mind you he could have also gotten this from a book called “The Codebreakers,” which was a book that Robert Graysmith cited in his movie the Zodiac from 2007. Robert Graysmith thought he could have learned everything he needed from the Codebreakers book, which is true. But as far as I know—and I haven’t looked at it for a while—this method of writing diagonally I don’t think is present in that book. Although I’d have to go back and double check.


Do you think he made the code harder than he intended?

I mean, the stuff that he wrote in it was very contemporary, right? So he wrote in the cipher that it wasn’t him on the TV show, in reference to a TV show that aired about 10 days before he sent this, where a person called in claiming to be the Zodiac. And I believe everybody sort of had their suspicions that the person who was calling in wasn’t him, and then he’s confirmed that in the cipher. So that kind of seems like something he would have wanted people to have known fairly quickly, as opposed to 50 years later. 

If I was guessing, I don’t think he would have realized just how difficult this was to solve. 

What do you think that tells us about him? On the one hand we’ve got this incredibly hard cipher, which might indicate that the person who’s put this together has quite a sophisticated understanding of how codes and ciphers work. But on the other hand, because he’s made it so unintentionally difficult, could that indicate that he was a bit clumsy or wasn’t in full command of the cipher?

Well he made a mistake in the second segment of nine rows in the cipher, which made it a lot more difficult to decode. So he had a pattern in the way in which he was writing out where he would go one row down, two columns across, write a letter; then go one row down, two columns across and write a letter, and so on. And in that second segment, at some stage—it looks like an accident—instead of going one row down, two columns across he’s just gone one down, one across. And that broke the symmetry. 


He might have realized that mistake at the time, I have no idea. But he might not have understood the significance of how much more difficult that was going to make it to crack.

Apart from that, does the sophistication of this code say to you that this is someone who is quite mathematically savvy?

Reasonably. I think you could easily teach a high school student how to make code in the way in which he’s done it. But if he’s come up with it himself, that’s a different story. Then that would suggest he’s thought about this stuff a lot. 

I have no idea whether he’s been exposed to this stuff in the past or whether it’s just something that he’s come up with himself—but either way, it’s a lot easier to create one of these ciphers than it is to be able to crack it. The decryption process is so much more difficult, and I’d guess he probably didn’t understand how difficult it was.

You said you didn’t try to get into his head, but how did it feel for you, decoding something like this, when you started peeling back those layers?

I sort of had that moment where you’re like, “Wow, nobody has read this since this guy put it together 51 years ago or whatever it is.” So there was definitely that moment, of there being this small window of time where we’re the only people in the world who have seen this, and that was pretty special. And that was the pleasure of finding it out. But in terms of the actual content of the message, it was mostly just ramblings—or that’s how I’d interpret it as a non expert in terms of what serial killers would say.


What do we know now that we didn’t know before?

So it had been long conjectured that this cipher was just a random arrangement of letters and symbols designed to waste everybody’s time. And that conjecture had a reasonable amount of weight behind it, because it had gone this long without anybody solving it when it had been actively researched for 50 years. You had places like the FBI, NSA and different US military cryptography labs all trying to solve this over the years.

So what this does tell us is that he went to the effort to create a second cipher, and he did a very good job of making it difficult enough that it survived for all this time. But that’s probably as much as I could speak to on that.

What happens next?

I’m not sure. I think the FBI has released a statement saying something to the effect that they don’t comment on open cases beyond the fact that they’ve now verified a solution to the cipher. So I’m not sure what will happen with the case now, to be honest. 

I mean the Zodiac, if he was 30 when he was doing this, that would make him 80 years of age now, maybe older—so there’s a chance he’s still alive. But I actually have absolutely no idea what will be done with it from here.

Obviously this became something quite close to you. How does it feel now that it’s been taken back off your hands and is being carried on behind closed doors?

Our work is done: we have solved this thing, we are 100 percent confident in the solution, and the FBI has been able to verify what we’ve done. We are happy that it’s going to go off to investigators, and it would be fantastic if this would help narrow down who this person was. But I have no real idea as to how successful that will be. 

Any other ciphers or codes to be cracked within this case that you feel like you might turn your hand to?

There were two smaller ciphers that he sent through. One of them I think is called the Z-13 because it contains 13 characters. The 13 character cipher is so short that you can’t really prove you found a unique solution to it; it’s so short you could find multiple solutions to it. And then there’s another one that contains 32 characters. 

So they’re both very, very short, and unless we have other information they’re not solvable. But what we’ve been playing around with is trying to use the decryption keys from the 340 that we’ve just solved, and the original cipher that he sent through that was solved in 1969, the 408, to see if the 340 and the 408 would help us in solving either of those two ciphers. So that’s something that we’re looking into at the moment.

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