If we envi­sion ser­i­al killers as fig­ures who taunt law enforce­ment with cryp­tic mes­sages sent to the media, we do so in large part because of the Zodi­ac Killer, who ter­ror­ized north­ern Cal­i­for­nia in the late 1960s and ear­ly 70s. Though he seems to have stopped killing more than half a cen­tu­ry ago, he remains an object of great fas­ci­na­tion (and even became the sub­ject of David Fincher’s acclaimed film Zodi­ac in 2007). As thor­ough­ly as the case has been inves­ti­gat­ed, much remains unknown — not least what he actu­al­ly said in some of his cod­ed let­ters. But just this month, a team of three cryp­tog­ra­phy enthu­si­asts man­aged to break one of the Zodi­ac’s ciphers, final­ly reveal­ing the con­tents of a 51-year old let­ter.

The Zodi­ac wrote this par­tic­u­lar com­mu­niqué in a trans­po­si­tion cipher, which, as Ars Tech­ni­ca’s Dan Good­in writes, uses “rules to rearrange the char­ac­ters or groups of char­ac­ters in the mes­sage.” In the case of the 340, named for the num­ber of sym­bols, the con­tent “was prob­a­bly rearranged by manip­u­lat­ing tri­an­gu­lar sec­tions cut from mes­sages writ­ten into rec­tan­gles.” For the past half-cen­tu­ry, nobody could suc­cess­ful­ly return the text to its orig­i­nal arrange­ment, but in 2020, there’s an app for that. Or rather, a soft­ware engi­neer named David Oran­chak, a math­e­mati­cian named Sam Blake, and a pro­gram­mer named Jarl Van Eycke made an app for that. Good­in quotes Oran­chak as say­ing the three had been “work­ing on and off on solv­ing the 340 since 2006.”

You can see Oran­chak explain how he and his col­lab­o­ra­tors final­ly cracked the 340’s cipher in the video at the top of the post, the final episode of his five-part series Let’s Crack the Zodi­ac. This was­n’t a mat­ter of sim­ply whip­ping up the right piece of arti­fi­cial intel­li­gence and let­ting it rip: they had to gen­er­ate hun­dreds of thou­sands of per­mu­ta­tions of the mes­sage as well as attempts at decryp­tions of those mes­sages. And even when rec­og­niz­able words and phras­es began to emerge in the results — “TRYING TO CATCH ME,” “THE GAS CHAMBER” — quite a bit of tri­al, error, and thought, remained to be done. It helped that Oran­chak knew his Zodi­ac his­to­ry, such as that some­one claim­ing to be the killer men­tioned not want­i­ng to be sent to the gas cham­ber when he called in to a local tele­vi­sion show on Octo­ber 20, 1969, two weeks before the 340 was received.

Was it real­ly him? The 340, when final­ly decod­ed — a process com­pli­cat­ed by the mis­takes the Zodi­ac made, not just in spelling but in exe­cut­ing his labo­ri­ous, ful­ly ana­log encryp­tion process — seems to pro­vide the answer:

I HOPE YOU ARE HAVING LOTS OF FUN IN TRYING TO CATCH ME
THAT WASNT 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 IS
LIFE WILL BE AN EASY ONE IN PARADICE DEATH

“The mes­sage does­n’t real­ly say a whole lot,” admits Oran­chak. “It’s more of the same atten­tion-seek­ing junk from Zodi­ac. We were dis­ap­point­ed that he did­n’t put any per­son­al­ly iden­ti­fy­ing infor­ma­tion in the mes­sage, but we did­n’t expect him to.” The Zodi­ac Killer remains uniden­ti­fied, and indeed remains one of recent his­to­ry’s more com­pelling vil­lains, not just to those with an inter­est in true crime, but to those with an inter­est in cryp­tog­ra­phy as well. For two more mes­sages still remain to be decod­ed, and in one of them he offers a short cipher that, he writes, con­tains his name — but then, if there’s any cor­re­spon­dent we should­n’t rush to take at his word, it’s this one.

Relat­ed Con­tent:

Arti­fi­cial Intel­li­gence May Have Cracked the Code of the Voyn­ich Man­u­script: Has Mod­ern Tech­nol­o­gy Final­ly Solved a Medieval Mys­tery?

The Enig­ma Machine: How Alan Tur­ing Helped Break the Unbreak­able Nazi Code

How British Code­break­ers Built the First Elec­tron­ic Com­put­er

The Ser­i­al Killer Who Loved Jazz: The Infa­mous Sto­ry of the Axe­man of New Orleans (1919)

The Grue­some Doll­house Death Scenes That Rein­vent­ed Mur­der Inves­ti­ga­tions

Based in Seoul, Col­in Mar­shall writes and broad­casts on cities, lan­guage, and cul­ture. His projects include the Sub­stack newslet­ter Books on Cities, the book The State­less City: a Walk through 21st-Cen­tu­ry Los Ange­les and the video series The City in Cin­e­ma. Fol­low him on Twit­ter at @colinmarshall, on Face­book, or on Insta­gram.