After The History Channel’s recent season of “The Hunt For The Zodiac Killer” programmes (episodes 1, 2, 3, 4 and 5), I thought it was time to get back to some non-fake-news codebreaking research.

In particular, I want to suggest an approach we might follow to try to solve the Z340 that (hopefully) won’t need a brain the size of a planet to run it. But first I’m going to talk about the Z13 cipher, because I think it tells us a lot about what is hidden inside the Z340 and indeed why the Z340 was written at all…

The Z13 Cipher

The text just above the Zodiac Killer’s Z13 cipher (20th April 1970) clearly and unambiguously refers back to a ‘name’ supposedly in the Z340 cipher (8th November 1969), though as far as I can see the “Dripping Pen” note that arrived with the Z340 didn’t mention a name at all:

An oft-repeated account for this is that the Z13 had been constructed in response to a kind of cryptographic ‘taunt’ that appeared six months previously in the Examiner newspaper on 22nd October 1969, as detailed here. In the Examiner piece, entitled “Cipher Expert Dares Zodiac To ‘Tell’ Name“, the President of the American Cryptogram Association issued a direct challenge to the Zodiac Killer to reveal his name in a cipher.

However, if you put all these pieces together, it seems highly likely to me that it was instead the Z340 cipher that had been constructed as a response to President Marsh’s taunt (it appeared a mere seventeen days later). Hence it seems entirely reasonable to conclude that the Z340 indeed contains a specific name for us to decrypt – though, as always, it seems highly unlikely that this will contain the Zodiac Killer’s actual name.

Cryptanalytically, though, the Z13 couldn’t be further removed from the homophonic world of the (cracked) Z408 (and presumably the Z340), in that it has shape repeats and internal structure aplenty. In fact, if you colour all the Z13’s repeated cipher shapes (once again, using Dave Oranchak’s neat-o-rama Cipher Explorer), this is what you see:

Much as I love “Sarah The Horse” and “Clara Cataract” as elegant literary plaintexts for this, it’s important to note that these are homophonic solutions for something whose many repeats point to its actually being a monoalphabetic substitution cipher. Dave Oranchak’s “Laura Catapult”, and glurk’s “Gary Lyle Large” are fine examples of how it is possible to construct name-like phrases to fit: but these are relatively rare examples in a surprisingly sparse, errm, name-space.

In many ways, whereas the problem with the Z340 is that it has too many shapes, the problem with the Z13 is arguably that it has too few shapes. So there would seem to be something a little odd going on here, cryptanalytically speaking: something feels wrong.

In his 2017 book “Unsolved!”, Craig Bauer praised a possible crack of the Z13 cipher which I hadn’t previously heard of, and credits p.128 of Robert Graysmith’s (2002) “Zodiac Unmasked: The Identity of America’s Most Elusive Serial Killer Revealed” as the source (though Graysmith talks about it as if the suggestion were as old as the [Hollywood] Hills):

Now, even though this doesn’t quite fit the pattern (the N cipher shape shouldn’t be shared between plaintext F and M), I think Bauer was completely right to give this his imprimatur, because it seems exceptionally close. Giving MAD Magazine’s “Alfred E. Neuman” as his name feels like this exactly the kind of thing the Zodiac Killer would do, in that it is taunting, unhelpful, superior, nasty, satirical, self-centred, and narcissistic in all the right ways.

For ALFREDENEUMAN to be the Z13’s plaintext, the only concession you would need to make is that a single letter was misenciphered: and as starting points go for a ciphertext that already feels as though it has too few shapes, this is not half as big a step as almost all other solutions I’ve seen proposed. Even though I completely accept that this isn’t cast-iron proof, I do think it suggests that it is well worth considering as a conditional piece of evidence to work with.

And Now, The Z340 Cipher…

For me, the big (if not ‘huge’) question the above leads to is this: if this ALFREDENEUMAN Z13 decryption is actually correct, might the Zodiac Killer have included exactly the same name in his Z340 cipher? And if so, might we be able to use the name as a known-plaintext crib into the Z340? (AKA a block-paradigm match. 🙂 )

Assuming the Z340 does use some kind of homophonic cipher, there are (340 – 12) possible positions the Z13 crib could be positioned at: however, we should be able to eliminate any position containing a cipher shape repeat within the 13-shape stretch that does not match a repeat in the ALFREDENEUMAN crib, because that would mean that the same homophonic cipher shape would have been used to encipher two different plaintext letters.

For example, because Z340 line #4 begins “S99…”, the “99” part could not be any part of the Z13 crib because there are no doubled letters in “ALFREDENEUMAN”: this is also true for the “++” pairs in lines #4, #14, and #18. Similarly, the +..+ repeat on line #9 and the W..W repeat on line #18 both cannot be in the crib, because no plaintext letter is repeated three steps apart in “ALFREDENEUMAN”. If you run this against the most widely used Z340 transcription, there are – according to the vanilla C test I put together (below, which you can actually run for yourself by clicking on [Run]) – exactly 197 valid crib positions. So we can eliminate (340-12-197) = 131 candidate positions. Which is nice. 🙂

What I find interesting is that locking a set of fixed set of letters to an (albeit still hypothetical) crib should enable us to use a homophonic solver on far smaller subsections of the Z340 than we would normally be able to do. I’ve written before about how the top half and the bottom half of the Z340 have quite different (but subtly overlapping) properties: for example, how top-half ‘+’ characters seems to work differently to bottom-half ‘+’ characters. As a result, I think it would make sense to try to solve lines #1 to #9 separately from lines #11 to #19.

But there are other results, that point out how lines #1 to #3 seem to work quite differently from lines #4 to #6, and so on. So the ability to try to solve even smaller blocks of lines may well be a critically useful string for our cryptological bow.

Unfortunately, I’m not (yet) a zkdecrypto-lite power-user, so I don’t know how to automate this kind of search Anyone who would like to collaborate on doing this, please feel free to step forward: or if you want to take the idea and do what you like with it, that’s fine by me too. Can you blame me if I want to see this solved before they start shooting Season #2? 😉

Just One Last Thing…

There is, of course, one other possibility that should be investigated… it’s just that those cold, creepy eyes in the famous Zodiac poster remind me of someone, can’t think who it is, but the name might come to me soon, who was it…?

C: Crib Matching Code

17 thoughts on “The Zodiac Killer’s Z13 cipher meets his Z340 cipher…

  1. @ Nick & other ‘code-breakers’ : A lot of the “Zodiac” killer’s messages were his triangulations of San Francisco Bay for targeting his next victims. Mount Tamalpais , Lake Herman, Alameda, Alcatraz Island….. Berkeley , El Cerrito, Montclair district of Oakland, and Haight Street and Market St. San Francisco.

  2. john sanders on December 17, 2017 at 10:49 am said:

    Nick: I can’t for the life of me see what was disrespectful or off-putting in my last responsive post to Bdid1der […]

  3. John Sanders: when commenters inadvertently or deliberately troll other commenters, I delete the comment.

  4. john sanders on December 17, 2017 at 2:27 pm said:

    Nick: Now you are seeming to be disrespectful to my integrity. Trolls to my old fashioned beliefs are are obnoxious creatures that sit under bridges and entice adventurous kids with intentions of doing them harm. I’m no troll and I resent your non commital, though nonetheless snide insinuation that I might fit into that category. The double deletion is noted and I respectfully request satisfactory explanation of what you think that I may have said to offend your rules; rules that I have allways done my utmost to adhere to. Perhaps you might re-consider the deleted posts and see what others have to say about their trollish characteristics, including insult to one of our most senior and respected commenters, namely the one and only bdid1der.

  5. John Sanders: my advice would be to take it on the chin and move on, all other options are too boring for words.

  6. bdid1dr on December 17, 2017 at 4:00 pm said:

    @ Nick : I first met John Sanderson at a USO dance. At that time I was working at Pitney-Bowes, Inc. I came back from lunch one day to find the staff members in tears and hugging themselves while listening to a radio report — President John F. Kennedy had been shot to death.

    For many months afterwards, but before the ‘Zodiac’ began his rampage, San Francisco and Oakland were somewhat discombobulated. So, in recent years, I have been going back (in my memory) to see if there is any connection/reaction to President Kennedy’s assassination which would initiate the ‘Zodiac” murders.

  7. bdid1dr on December 17, 2017 at 4:19 pm said:

    My memory bank is getting more blank with every year. However, I still think it was my first husband who went completely bonkers after his experience as the Radio Shack operator/codiologist on the Ticonderoga — during the stand-off in Tonkin Bay.

  8. Definitely not alfred e. neuman on December 17, 2017 at 5:38 pm said:

    I wonder if “What, me worry?” is in the plaintext.

  9. bdid1dr on December 17, 2017 at 9:27 pm said:

    Thanx, Nick for all of your patient reviews/moderation of sometimes goofy contributions from u no who . Make that two uno who’s — besides me !

  10. john sanders on December 18, 2017 at 12:16 am said:

    bdid1dr: Alfred E Newman, with his toothy nonchalant smile always put me in mind of Jack Kennedy’s ambitious younger rudder Bobby who dreamt things that never were and said “why not” followed by the not so frequently quoted “What me worry”. NB: Most of the popular images also show him to be sporting the larger cymba ears, which is said to affect only 2 percent of the population and puts me in mind of some body, once important though now of little status.

  11. Davidsch on December 19, 2017 at 11:59 am said:

    Nick, although your programming skills seem quite alright, there’s also the possibility, within the same boundaries, that his name is “zodiac!”.
    If you then suggest that the last m can not represent two different letters at the same time: it does not need to be a homophonic cipher where one letter at a time is taken.

  12. Milton Kim on December 20, 2017 at 12:32 am said:


    When the Z13 cipher text was rotated to start with NAM…, it gives the text in the format of NAMAENOsXsYsZ. In Japanese, NAMAE literally means ‘name’ and NO is ‘of ~’. In other words, those 7 letters literally can be translated as ‘name of ~’. The Zodiac symbol ‘O’ with a cross in it can be regarded as a disguise of the letter ‘O’ (oh). The letter that looks like an eight ball can be regarded as a symbol or cipher for a space character. Three letters X, Y, and Z is highly likely to be a cipher for the initial of the person’s name. The carefully rearranged structure of Z13 cipher indicates that the person was trying to reveal something more in such a plain way that it can be deciphered as ‘NameOf X Y Z’. It still could be just a coincidence, but I would like to say it is an interesting one.

  13. Isaiah Engelke on December 29, 2017 at 2:27 pm said:

    I have so far figured out 5 letters in the z340. Notice how maybe the letters that are themselves, ARE themselves. I got HERE IT so far.

  14. Isaiah Engelke: errrm… you may wish to read this web-page, which describes Craig Bauer’s (now-infamous) attempted decryption of the Z340, which begins “HERE IT IS…”, and for very much the same reasons that inspired you.

  15. Isaiah Engelke on December 29, 2017 at 2:33 pm said:

    Next was I KILL BOTH

  16. Isaiah Engelke on December 29, 2017 at 2:49 pm said:

    Who is Craig Bauer

  17. Isaiah: he’s editor of arguably the world’s foremost cipher history journal, Cryptologia. But, like the rest of us, he is allowed to make mistakes from time to time. 🙂

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