October 13, 2026
The cipher that beat the FBI for 51 years — until three hobbyists cracked it on a laptop
The Z340 cipher as published (image via Ars Technica / David Oranchak)
In November 1969, a man calling himself the Zodiac mailed a 340-character cipher to the San Francisco Chronicle. For the next 51 years it defeated the FBI, the NSA, and the American Cryptogram Association. Then, in December 2020, three private citizens solved it — and the FBI confirmed they were right.
The cipher with a second trick
The code, known as Z340, used 63 different symbols — a homophonic substitution, where several symbols could stand for the same letter. That alone made it hard. But the real trick was underneath the symbols: before they were assigned, the message had been scrambled with a transposition. Every professional attack for half a century fed the text to standard tools. None of those tools tried the scramble.
The trio who cracked it: David Oranchak, a software developer in Virginia; Sam Blake, a mathematician in Australia; and Jarl Van Eycke, a Belgian programmer who writes cipher-solving software as a hobby.
Their breakthrough came from Van Eycke's program, AZdecrypt. After testing hundreds of thousands of permutations, they landed on the transposition scheme: start at the upper-left corner, move down one row and over two columns, wrap around at the edge, and repeat — then read the result. The message was never gibberish. It was just scrambled in a way nobody's tools had tried. They submitted the solution to the FBI on December 5, 2020; the bureau validated it and confirmed it publicly days later.
The decoded text contained no name and no identity. Out of respect for the victims, the FBI declined further comment and noted the case remains open. The killer was never identified.
It happened before
There is a precedent, and it is even better. The Zodiac's first cipher — 408 characters, sent in three parts to three newspapers earlier in 1969 — was cracked in days. Not by the FBI. By Donald and Bettye Harden, a schoolteacher couple in Salinas, working at their kitchen table. Bettye guessed the word "kill" would appear in the message — a classic codebreaker's crib — and the rest fell into place.
Twice now, the professionals were outrun by ordinary people who refused to stop.
What is still unsolved
Two of the four ciphers attributed to the Zodiac remain unsolved: a 13-symbol "my name is" cipher and a 32-symbol one. The killer's identity is still unknown, and the case is still open. The Z340 story is not a solution to the mystery — it is a solution to one of its puzzles, with the biggest one still waiting.
Somewhere out there might be another laptop, another kitchen table, another solution.
If this hooked you
The book Codebreaking: A Practical Guide by Elonka Dunin and Klaus Schmeh is the genuine article on exactly this craft — hands-on exercises, real historical ciphers, and the free tools professionals use. Dunin co-leads the team still working on the CIA's Kryptos cipher, which we covered here in October. It teaches the actual techniques: substitution, polyalphabetic systems, transpositions — the same family of tricks that finally gave way to three hobbyists and a laptop.
Image: the Z340 cipher as published (via Ars Technica / David Oranchak). Source: The Register and the Associated Press, December 2020.
Source: The Register — read the original report
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