Zodiac 340 Cipher Solved: How Codebreakers Decoded the 51-Year-Old Mystery in 2020

On November 8, 1969, the self-proclaimed Zodiac Killer mailed a complex cryptogram consisting of 340 symbols arranged in a 17-column by 20-row grid to the San Francisco Chronicle. For fifty-one years, the cipher—designated by cryptanalysts as the Z-340 cipher—resisted decryption attempts by the FBI’s Cryptanalysis and Racketeering Records Unit (CRRU), the National Security Agency (NSA), and thousands of amateur and professional codebreakers worldwide. On December 5, 2020, an international team of codebreakers comprised of American software programmer David Oranchak, Belgian programmer Jarl Van Eycke, and Australian mathematician Sam Blake successfully solved the cipher, providing the first verified Zodiac message decrypted since 1969.

The Historical Context of the 340 Cipher

Between December 1968 and October 1969, the Zodiac Killer murdered at least five victims and critically injured two others across Benicia, Vallejo, Lake Berryessa, and the Presidio Heights neighborhood of San Francisco. In August 1969, the killer mailed his first cryptogram, the 408-character cipher (Z-408), which was promptly solved within a week by high school teacher Donald Harden and his wife Bettye using simple homophonic substitution techniques.

Embarrassed by the rapid solution of his first cipher, the Zodiac constructed the 340 cipher with significantly higher cryptographic complexity. When the cipher arrived at the Chronicle in November 1969, it was accompanied by a threatening letter. For decades, cryptographers assumed the Z-340 used the same substitution cipher system as the Z-408, but standard frequency analysis and automated computer decoders continually generated nonsensical anagrams.

The Cryptographic Architecture: Transposition & Manipulation

The breakthrough in 2020 came from the realization that the Zodiac had not merely substituted symbols for letters, but had applied a complex diagonal transposition system across divided sections of the message. The cipher was constructed in three distinct blocks: two blocks of nine rows (17×9) and one final block of two rows (17×2).

To read the plaintext, the solver had to apply a specific reading rule: start at the top-left corner, read down one row and over two columns (a “knight’s move” diagonal vector), and wrap around when hitting the edge of the 17-column grid. Furthermore, during cipher construction, the Zodiac made a transcription error on row 10, shifting a character out of sequence and confounding traditional automated cryptanalysis algorithms for over five decades.

The Role of AZdecrypt & The Decryption Process

David Oranchak, who had spent over fifteen years researching the cipher, collaborated with mathematician Sam Blake, who generated over 650,000 algorithmic reading directions and transposition manipulations. These mathematical variations were fed into AZdecrypt, a high-performance C-based substitution solver written by Belgian cryptographer Jarl Van Eycke.

On December 3, 2020, AZdecrypt produced a partial plaintext sequence containing the phrases: "HOPE YOU ARE HAVING LOTS OF FUN" and "THAT WASNT ME ON THE TV SHOW". Recognizing an immediate historical match to the October 1969 Jim Dunbar television talk show—where an imposter claiming to be Zodiac called into the broadcast—the team refined the transposition rules to decipher the entire message.

The Complete Decrypted Plaintext of the 340 Cipher

The verified plaintext of the 340 cipher reads as follows (preserving the killer’s original spelling and absence of punctuation):

FBI Verification & Modern Criminal Impact

On December 5, 2020, David Oranchak submitted the complete mathematical proof and plaintext reconstruction to the FBI’s Cryptanalysis and Racketeering Records Unit in Quantico, Virginia. On December 11, 2020, the FBI San Francisco Field Office released an official statement confirming the validity of the decryption:

“The FBI is aware that a cipher attributed to the Zodiac Killer was recently solved by private citizens. The Zodiac Killer case remains an ongoing investigation for the FBI San Francisco division and our local law enforcement partners.”

While the decrypted message did not reveal the killer’s true legal name, it provided vital behavioral and psychological confirmation: it proved conclusively that the caller to the Jim Dunbar television show was an impostor, and re-emphasized the killer’s persistent delusions regarding an afterlife in “paradice.” Two shorter Zodiac ciphers (the 13-character “My name is” cipher and the 32-character “Map” cipher) remain unverified due to their extremely short length.

Timeline of the Zodiac 340 Cipher

Mathematical Mechanics of the 17×20 Diagonal Transposition Matrix

The breakthrough that enabled David Oranchak, Jarl Van Eycke, and Sam Blake to solve the Zodiac 340 cipher in December 2020 rested on unraveling the mathematical structure of the grid. For over fifty years, cryptanalysts had attempted to solve the cipher as a pure homophonic substitution system—where multiple symbols represent single English letters. However, the Zodiac incorporated a second cryptographic layer: a periodic diagonal transposition system.

The 340-character grid consists of 17 columns and 20 rows. The message was split into three discrete blocks: Block 1 (rows 1 to 9, 153 symbols), Block 2 (rows 10 to 18, 153 symbols), and Block 3 (rows 19 to 20, 34 symbols). Within each 17×9 block, the characters were written along a diagonal vector moving down one row and over two columns (a knight’s move in chess). When the diagonal hit the bottom or right margin of the block, it wrapped around to the opposite edge according to precise modular arithmetic rules.

The Transcription Error on Row 10 & The AZdecrypt Solver

The primary reason computer programs failed to decrypt the 340 cipher for five decades was a human transcription error committed by the Zodiac himself. When beginning the second block on row 10, the killer accidentally skipped a column, shifting the alignment of the remaining characters in that block by one position. This single error corrupted the mathematical symmetry of the transposition matrix, causing standard statistical solvers to discard valid decryption candidates as statistical noise.

Belgian programmer Jarl Van Eycke modified his specialized solver, AZdecrypt, to handle multi-directional transpositions and dynamic offset shifts. When combined with Sam Blake’s massive batch tests of over 650,000 geometric reading directions, AZdecrypt unlocked the underlying English plaintext, producing the historic breakthrough that was subsequently verified and confirmed by the FBI’s Cryptanalysis and Racketeering Records Unit in Quantico, Virginia.

The Cryptographic Community & Open-Source Forensic Collaboration

The cracking of the Zodiac 340 cipher in December 2020 represented a landmark victory for open-source intelligence and citizen science. David Oranchak, Sam Blake, and Jarl Van Eycke made their complete mathematical proofs, Python decryption scripts, and AZdecrypt source code publicly available to cryptographers and researchers worldwide.

By documenting every step of the 17×20 diagonal transposition and error-correction matrix, the team provided a rigorous scientific template for analyzing other historical ciphers. The FBI’s formal verification solidified the role of crowdsourced computational power in assisting modern federal law enforcement agencies with complex historical cold cases.

The Statistical Validation of AZdecrypt & N-Gram Language Models

The algorithmic verification of the Zodiac 340 plaintext utilized 6-gram and 7-gram English language models within AZdecrypt. By evaluating character transition probabilities across hundreds of millions of randomized permutations, Jarl Van Eycke and David Oranchak proved that the likelihood of the decrypted English text appearing by random coincidence was less than 1 in 10 to the 40th power.

This mathematical certainty provided the FBI CRRU with the definitive scientific proof required to formally validate the solution, cementing the achievement as a masterclass in modern cryptanalysis.

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