Zodiac Cipher
340 characters. 51 years unsolved. Cracked during a pandemic lockdown.
Oranchak, Blake & Van Eycke (2020)
Why This Matters
Z-340 remained unsolved for 51 years. Its 2020 solution combined a search for transposition patterns with homophonic-substitution solving and manual investigation. The result illustrates how an incorrect model can obstruct an attack, even when the individual cipher techniques are familiar.
Between 1969 and 1970, an unidentified serial killer called the Zodiac sent four encrypted messages to San Francisco Bay Area newspapers, taunting police and demanding publication. The killer claimed the ciphers contained his identity.
The first cipher, Z-408 (408 characters), was solved in 1969 by Donald and Bettye Harden. It contained no name. The second cipher, Z-340 (340 characters), defied cryptanalysts, amateur solvers, and FBI investigators for 51 years.
On December 11, 2020, David Oranchak, mathematician Sam Blake, and software developer Jarl Van Eycke publicly announced the solution. They had submitted it to the FBI on December 5; the FBI independently verified the decryption. They had worked remotely during the COVID-19 pandemic lockdown.
The Z-340 uses a layered cipher combining homophonic substitution with diagonal transposition — the key insight that eluded solvers for half a century.
Cipher type: Homophonic substitution + diagonal transposition
Step 1: The Z-340 uses a homophonic alphabet —
common letters map to multiple symbols
to flatten frequency patterns
Step 2: Undo the recovered section-specific
transposition — the first section reads
diagonally rather than left-to-right
Z-408 first row (17 symbols — the row used by
the interactive demo below):
HER>pl^VPk|1LTG2d
The diagonal transposition is why the cipher
resisted 51 years of analysis — solvers were
looking for patterns in the wrong reading order
The Z-408 cipher was solved in 1969. Below are the first 17 symbols of the real ciphertext. Click a symbol, then type its letter in the box to build the plaintext — or press Reveal Answer to see the known solution.
Ciphertext symbols (first 17 of Z-408) Plaintext (your decryption)Oranchak had suspected a transposition component since 2012. Working with Blake (who explored candidate transpositions) and Van Eycke (who built the AZdecrypt cipher-solving tool), the team identified the specific diagonal reading pattern through computational search. Candidate routes, recognizable fragments, homophonic search and correction of section-specific disruptions worked together. The exact physical procedure used to encipher the message remains uncertain.
The solution expresses taunts about the investigation and the killer’s claimed beliefs about an afterlife. It does not disclose the killer’s identity.
The Zodiac's identity was not revealed.
| Concept from Zodiac Cipher | Modern Evolution |
|---|---|
| Homophonic substitution (multiple symbols per letter) | Compare randomized or nonce-based encryption; padding alone does not hide repeated plaintexts |
| Layered cipher (transposition + substitution) | Modern cipher analysis studies how operations interact; layering alone does not establish security |
| 51 years + short ciphertext = hard | Ciphertext length matters: more data = more statistical signal for an attacker |
| Solved by n-gram scoring + computation | Language-scored search can attack classical ciphers; success depends on the cipher, data and search budget |
A short cipher message. Use what you just learned.
Hint: Not the Zodiac's cipher — this one is simpler. Sixteen letters gives limited frequency evidence, but there are only 25 shifts to try. Which one reads as English?
51 years, short ciphertext, unusual transposition.
| Exhibit | 35 of 172 |
| Era | 1969 · Solved 2020 |
| Trust | Broken |
| Author | Zodiac Killer (unidentified) |
| Year | 1969 |
| Broken By | Oranchak, Blake & Van Eycke |
The main Z-408 message and Z-340 are deciphered, but the meaning of Z-408’s final 18 characters remains uncertain and the Zodiac story is not closed. Two later messages remain open: Z-13, the short "My name is" cipher, and Z-32, usually interpreted as a location cipher. Their brevity is the main obstacle. At 13 and 32 characters, they do not provide enough depth for the kind of statistical leverage that broke Z-340. That is why Round 3 also routes them into Hall XII: Unsolved Ciphers as active research problems, even though this main exhibit focuses on the solved 408- and 340-character messages.
Follow the Evidence
Investigate the source, competing hypotheses and verification, then preserve your own readings in the Evidence Workbench.
Sources and Reading
David Oranchak, Sam Blake and Jarl Van Eycke, “The Solution of the Zodiac Killer’s 340-Character Cipher” (2024), §§1.3 and 5, for the distinction between the two messages, the remaining Z-408 tail and the solution process.