A serial killer who murdered in Northern California in the late 1960s did something almost no other killer has done: he turned his crimes into a cryptography contest, mailing newspapers and police a series of coded messages and demanding they be printed or he would kill again. The most notorious of these, the 340-character cipher sent to the San Francisco Chronicle in November 1969, sat unsolved for fifty-one years — taunting the FBI, the NSA's codebreakers, and an army of amateurs who burned decades on it. Then, on 3 December 2020, three men who had never been in the same room solved it.

Start with what is verified rather than legend. The Zodiac's first cipher, the 408-character message split across three newspapers in 1969, was cracked within days — not by professionals but by a schoolteacher named Donald Harden and his wife Bette, who guessed the killer's ego would make him open with the word "I." That solution decoded to a rambling boast about killing people being more fun than killing wild game. It proved the ciphers were real and breakable. It also set the trap: everyone assumed the harder 340 used the same kind of system. It did not.

The 340's solution is one of the best-documented codebreaking events in modern true crime, because the man who led it, software developer David Oranchak, recorded the whole arc on his "Let's Crack Zodiac" YouTube series. Working with Australian applied mathematician Sam Blake and Belgian programmer Jarl Van Eycke — the author of the open-source solver AZdecrypt — the team finally understood the trick: the 340 was not a straightforward substitution read left to right. It was a homophonic cipher written on a diagonal, encoded in slanting lines and broken into pieces, which is exactly why every prior attempt to read it in normal order produced garbage. Blake's software generated and tested hundreds of thousands of possible transposition schemes; when the right one surfaced, recognizable English fell out, including the lines "I hope you are having lots of fun in trying to catch me" and a reference to "the gas chamber." The FBI's Cryptanalysis and Racketeering Records Unit confirmed the solution publicly.

The evidence that this is genuine and not pattern-matching wishful thinking is unusually strong. The decryption method is fully published, the software is open-source and anyone can re-run it, the diagonal key is specific and rigid rather than loose, and the resulting plaintext is grammatical English continuous across 340 characters — a result statistically impossible to produce by accident or by torturing the data. This is the rare paranormal-adjacent mystery that was closed not by belief but by reproducible mathematics.

Which makes the part that remains open all the stranger. Of the four ciphers attributed to the Zodiac, two are still unsolved — and one of them is the short 13-character cipher he mailed in April 1970 over the line "My name is —," claiming it spelled out his identity. Thirteen symbols. A schoolchild's worth of text. And it has never been cracked, for a brutal mathematical reason: a message that short, with that many distinct symbols, simply does not contain enough redundancy to be uniquely solved. Thousands of names fit it. There is no way, with the cipher alone, to know which one is real — or whether the killer was telling the truth at all.

The honest skeptical reading is that the 13-cipher may be unsolvable in principle rather than merely unsolved, and that the Zodiac, who delighted in dangling false promises, may have encoded nothing meaningful at all. That possibility is itself a kind of perfect crime: a man who killed at least five people, was never caught, and left behind a string of symbols that he said held his name, daring the world to read it forever.

The 340 fell to three strangers and an open-source program after half a century. The 13 still sits there. We solved the cipher meant to mock us and remain locked out of the one that supposedly tells us who he was. What did the Zodiac actually encode in those thirteen symbols — a name, a lie, or nothing at all?