Zodiac Killer ciphers

The Zodiac ciphers are a series of four cryptograms mailed to California newspapers by an unidentified serial killer who operated in the San Francisco Bay Area between 1968 and 1969. Their fame comes from the violence of the claimed murders, the still-open mystery of the killer’s identity, and the sheer technical resilience of some of the ciphers — one of them held up for more than fifty years.

The four cryptograms

Z408 — mailed July 1969

• 408 symbols, split into three fragments sent to three different newspapers.
• Homophonic substitution: 53 unique symbols for 26 letters, with high-frequency plain letters (E, T, A) getting 3 to 4 codes.
• Cracked in a week by Donald and Bettye Harden, a schoolteacher couple from Salinas.
• The message opens with I LIKE KILLING PEOPLE… — menacing tone, distorted spelling.

Z340 — mailed November 1969

• 340 symbols, apparently similar in structure.
• Resisted every attempt for 51 years.
• Solved in December 2020 by David Oranchak, Sam Blake and Jarl Van Eycke, using a computer search that exploited a non-obvious diagonal-block transposition.
• Equally menacing tone, no clear hint about identity.

Z13 and Z32 — mailed April 1970

• 13 symbols (My name is…) and 32 symbols (a so-called treasure map marking a bomb).
• Still unsolved. Z13 is almost certainly too short for reliable cryptanalysis (not enough statistical material).

Why Z340 held so long

Z408 was a “flat” homophonic substitution — the Harden method (guessing on I LIKE KILLING, FORESTER, ELECTRIC) recovered the table by reasoned cribs. Z340 introduced two extra obstacles:

• Partial transposition — symbols aren’t read in the natural order; a diagonal read across blocks of three lines is needed.
• Errors or bluffs — the killer injected misplaced symbols (deliberately or by inattention), which poisoned every automated attack until 2020.

Oranchak’s team used a modern statistical solver (AZDecrypt) that jointly searches millions of reading orders to identify the one producing plausible English plaintext.

Why the Zodiac remains unidentified

Despite:

• Several cryptograms, two of them solved,
• Handwritten letters signed with a ∴ (three-dot triangle),
• Partial fingerprints recovered at crime scenes,
• A police sketch from a surviving witness,

the killer was never formally identified. The FBI officially reopened the file in March 2021, after Z340 fell. Arthur Leigh Allen, the prime suspect of the 1970s, died in 1992 without ever being charged.

Cryptanalytic interest

The Zodiac ciphers illustrate:

• The superiority of homophonic over straight substitution for short texts.
• The importance of reading-order structure: Z340 held partly because of the substitution, and partly because of how the symbols were laid out.
• The modern role of computers in historical cryptanalysis: what resisted 51 years by hand yields in days to a strong statistical solver.