340 characters. 51 years unsolved. Cracked during a pandemic lockdown.
The Zodiac Killer’s Z-340 cipher defied professional and amateur cryptanalysts for 51 years before being solved in 2020 — the most famous unsolved cipher in American criminal history and a testament to the difficulty of homophonic substitution analysis.
Between 1968 and 1969, 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 schoolteachers 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 5, 2020, David Oranchak, mathematician Sam Blake, and software developer Jarl Van Eycke announced the solution. 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: A diagonal transposition is applied
BEFORE encryption — the text reads
diagonally rather than left-to-right
Z-340 fragment (actual ciphertext):
HER>pl^VPk|1LTG2d
Kp-IlO+Pq4k5RZN<
The diagonal transposition is why the cipher
resisted 51 years of analysis — solvers were
looking for patterns in the wrong reading order
Oranchak had suspected a transposition component since 2012. Working with Blake (who wrote the statistical scoring software) and Van Eycke (who built the AZdecrypt cipher-solving tool), the team identified the specific diagonal reading pattern through computational search. Once the transposition was identified, the homophonic substitution fell to standard techniques.
The decoded message begins: "I hope you are having lots of fun in trying to catch me. I am not afraid of the gas chamber because it will send me to paradise all the sooner because I now have enough slaves to work for me..."
The Zodiac's identity was not revealed.
| Concept from Zodiac Cipher | Modern Evolution |
|---|---|
| Homophonic substitution (multiple symbols per letter) | Padding schemes: modern encryption adds randomness to prevent identical plaintexts producing identical ciphertexts |
| Layered cipher (transposition + substitution) | AES rounds: multiple operations per round make individual layers meaningless to attack |
| 51 years + short ciphertext = hard | Ciphertext length matters: more data = more statistical signal for an attacker |
| Solved by n-gram scoring + computation | Modern cryptanalysis: hill climbing with n-gram fitness functions breaks most classical ciphers |
A short cipher message. Use what you just learned.
Hint: Not the Zodiac's cipher — this one is simpler. What shift makes the most common letter = E?
51 years, short ciphertext, unusual transposition.
| Exhibit | 36 of 37 |
| Era | 1969 · Solved 2020 |
| Security | Solved 2020 |
| Author | Zodiac Killer (unidentified) |
| Year | 1969 |
| Broken By | Oranchak, Blake & Van Eycke |