Estimated read time: 11 minutes | Category: Unsolved Mysteries | Last updated: June 2025
The Messages Nobody Can Read
Between July and November 1969, the Zodiac Killer sent four coded messages to Bay Area newspapers. He called them his ciphers. He implied they contained his identity. He taunted investigators with them, sending police and the public on a cryptographic chase that has lasted more than half a century.
Two of the four ciphers have been solved. The Z408 — a 408-character cipher sent in three parts — was decoded within a week of its publication by a high school history teacher and his wife using frequency analysis. The Z340 — a 340-character cipher sent in November 1969 — resisted decryption for 51 years before a team of three amateur codebreakers cracked it in December 2020.
Neither decoded cipher contained the Zodiac’s name.
The two remaining ciphers — the Z13 and the Z32 — have never been solved. The Z13 is prefaced by the words “My name is” and is only 13 characters long. If it encodes the Zodiac’s actual name, it is the most important unsolved cryptographic puzzle in American true crime history. The Z32 may contain location information. Both have defeated every cryptographer, codebreaker, and artificial intelligence system that has ever attempted them.
Here is everything we know.
What We Know For Certain
- [FACT] The Zodiac Killer sent four ciphers to Bay Area newspapers between July 1969 and November 1969. They are designated Z408, Z340, Z13, and Z32 based on their character counts.
- [FACT] The Z408 was sent in three parts to three newspapers on July 31, 1969. It was solved by Donald and Bettye Harden within a week using frequency analysis. It contained no name.
- [FACT] The Z340 was sent to the San Francisco Chronicle on November 8, 1969. It was solved on December 5, 2020 by David Oranchak, Sam Blake, and Jarl Van Eycke after 51 years. It contained no name.
- [FACT] The Z13 is a 13-character cipher sent with the letter “My name is ___” on April 20, 1970. The letter explicitly stated the cipher contained the Zodiac’s name.
- [FACT] The Z32 is a 32-character cipher sent with a map on June 26, 1970. The accompanying letter implied the cipher contained information about a bomb the Zodiac claimed to have planted.
- [FACT] Neither the Z13 nor the Z32 has been solved by any researcher, professional cryptographer, or AI system as of 2025.
- [FACT] The FBI’s Cryptanalysis and Racketeering Records Unit has examined both ciphers. Neither has been publicly solved by the Bureau.
The Two Solved Ciphers — Context
To understand why the Z13 and Z32 remain unsolved, it helps to understand what made the Z408 and Z340 solvable — and what those solutions revealed.
The Z408 — Solved in One Week
[FACT] The Z408 used a homophonic substitution cipher — a system in which each letter of the plaintext alphabet is represented by multiple different cipher symbols, making frequency analysis more difficult. The Zodiac used 54 different symbols to represent the 26 letters of the alphabet, with common letters represented by more symbols than uncommon ones.
[FACT] Donald Harden, a high school history teacher in Salinas, California, and his wife Bettye solved the Z408 using their knowledge that the message was in English and likely began with the word “I.” They worked backward from assumed word patterns to identify the substitution key. The decoded message was a rambling manifesto about killing being pleasurable and collecting slaves for the afterlife. It ended: “I will not give you my name because you will try to sloi down or stop my collecting of slaves for my afterlife.”
[FACT] The deliberate misspelling “sloi” — believed to be “slow” — and other spelling errors in the Z408 have been cited as evidence that the Zodiac was either poorly educated, deliberately introducing errors to confuse analysis, or writing quickly. The errors appear in the decoded text rather than in the cipher construction itself.
The Z340 — Solved After 51 Years
[FACT] The Z340 used a significantly more complex cipher system than the Z408 — incorporating a transposition element in addition to homophonic substitution, which scrambled the order of the symbols before encoding. This additional layer defeated every attempt at decryption for five decades.
[FACT] The breakthrough came when David Oranchak — an American software developer who had been working on the Z340 for 14 years — collaborated with mathematician Sam Blake and programmer Jarl Van Eycke to develop a systematic computer search approach. They identified that the cipher used a diagonal transposition pattern — symbols were read diagonally across the grid rather than horizontally — and once this transposition key was identified the underlying substitution cipher yielded to analysis.
[FACT] The decoded Z340 message read in part: “I hope you are having lots of fun in trying to catch me that wasnt me on the tv show because I am too clever for that.” It also referenced the Zodiac being angry at police for lying about the number of victims. It contained no name.
The Z13 — The Name Cipher
The Z13 is the most tantalising of the unsolved ciphers — and potentially the most significant document in the entire Zodiac case.
The Letter It Came With
[FACT] On April 20, 1970, the San Francisco Chronicle received a letter from the Zodiac that began: “My name is —” followed by the 13-character cipher. The letter continued with further taunts about the investigation and a count of claimed victims. The explicit framing — “My name is” — makes the Z13 unique among the Zodiac ciphers. Every other cipher’s content was implied or unknown. The Z13 was explicitly stated to contain the Zodiac’s name.
Why 13 Characters Is So Difficult
[FACT] The fundamental challenge of the Z13 is its length. Thirteen characters is an extremely short cipher text. In cryptographic analysis, longer texts provide more statistical data — more character frequencies, more pattern repetitions, more points of leverage for breaking an unknown system. Thirteen characters provides almost nothing to work with.
[ANALYSIS] Consider the mathematics: if the Z13 uses a simple substitution cipher, there are 26 possible values for each of 13 characters — producing an astronomical number of possible decodings, the vast majority of which produce nonsense. Without additional constraints — knowledge of the cipher system, knowledge of what name is being encoded, or cross-reference with other Zodiac materials — there is no reliable way to distinguish the correct decoding from the billions of incorrect ones.
[FACT] Researchers have attempted to use context to constrain the possibilities. If the Z13 encodes a name, it must be a human name. If it encodes a full name it must fit within 13 characters. If it uses the same cipher system as the Z408 or Z340, known elements of those systems might constrain the possibilities. None of these approaches has produced a consensus solution.
Proposed Solutions
[THEORY] Numerous proposed solutions to the Z13 have been published over the decades. Among the most discussed:
Some researchers have proposed that the Z13 encodes the name “Lawrence Kane” — one of the named Zodiac suspects — using a specific cipher key derived from elements of the Z408. This decoding is mathematically consistent with one proposed reading of the cipher but requires assumptions about the cipher system that have not been independently verified. The Lawrence Kane theory as a whole has not been accepted by mainstream Zodiac researchers or law enforcement.
A significant body of researcher opinion holds that the Z13 was never intended to be solved — that the Zodiac created a short, ambiguous cipher specifically because 13 characters cannot be decoded reliably, ensuring that whatever name he used could never be confirmed. Under this reading the Z13 is a tease rather than a genuine disclosure — the Zodiac offering the appearance of revelation while guaranteeing its impossibility. This interpretation is consistent with the psychological profile of someone who wrote letters for attention while carefully maintaining anonymity.
Some researchers have proposed that even if the Z13 could be decoded, it would not contain the Zodiac’s real name — that he encoded a false name, a pseudonym, or a red herring specifically to mislead investigators who managed to break it. This is consistent with the Zodiac’s documented pattern of deliberate misdirection in his letters. [SPECULATION] There is no way to verify this possibility without first decoding the cipher and cross-referencing the result against other evidence.
The Z32 — The Map Cipher
The Z32 is less famous than the Z13 but potentially equally significant — if its accompanying letter is to be believed.
The Letter and Map
[FACT] On June 26, 1970, the Chronicle received a letter containing a map of the Mount Diablo area in Contra Costa County, with a crossed-circle symbol marking a specific location. The accompanying letter stated: “I am crackproof” and implied the 32-character cipher contained information needed to locate a bomb the Zodiac claimed to have planted near the marked location.
[FACT] The map and cipher were taken seriously enough by law enforcement to prompt searches of the marked area. Nothing was found. Whether the bomb was a real threat, had already been removed, had never existed, or required the decoded cipher to locate precisely — is unknown.
The Cryptographic Challenge
[FACT] Like the Z13, the Z32’s brevity is its greatest obstacle. Thirty-two characters provides more material than 13 but is still far below the threshold at which standard cryptographic techniques become reliable for an unknown cipher system. The Z32’s character set includes some symbols used in the Z408 and Z340 — suggesting possible cipher system overlap — but the specific mapping has not been established.
[THEORY] Some researchers believe the Z32 uses the same basic cipher system as the Z408 but with a different key — meaning the same symbols represent different letters. If correct, the cipher might be breakable using known elements of the Z408 system as a starting point. No proposed solution using this approach has achieved consensus acceptance.
[THEORY] Others have proposed that the Z32 was intentionally constructed to be unsolvable — that the Zodiac, aware of his ciphers attracting cryptographic attention, created short and ambiguous codes specifically to frustrate analysis while maintaining the appearance of meaningful content. The bomb threat that requires the cipher to locate would ensure law enforcement took the cipher seriously while the cipher’s insolubility guaranteed the threat could never be precisely located or confirmed.
Modern Approaches — What Has Been Tried
[FACT] The success of the Z340 decryption in 2020 — using computational search methods — has renewed hope that the Z13 and Z32 might be approachable through similar techniques. David Oranchak and his collaborators have publicly stated their intention to apply the methods that cracked the Z340 to the remaining unsolved ciphers.
[FACT] AI-assisted cryptanalysis — applying large language models and machine learning to cipher analysis — has been proposed as a potential tool for the remaining Zodiac ciphers. The fundamental challenge remains the same: with only 13 or 32 characters, even AI systems lack the statistical leverage to reliably distinguish correct from incorrect decodings.
[FACT] Crowdsourced analysis — communities of amateur researchers working collaboratively — has been applied to the Zodiac ciphers continuously since the internet made such collaboration possible. The community-maintained website zodiackillersite.com maintains comprehensive documentation of all proposed solutions and the evidence for and against each.
[FACT] The FBI’s Cryptanalysis and Racketeering Records Unit — which has access to the original cipher documents rather than reproductions — has not publicly disclosed any progress on the Z13 or Z32. Whether the Bureau has made progress that has not been shared publicly is unknown.
What a Solution Would Mean
The stakes of solving the remaining Zodiac ciphers — particularly the Z13 — are significant.
[ANALYSIS] If the Z13 encodes a real name, solving it could identify a suspect who might still be living, might have living family members, or might be identifiable through DNA databases. The Zodiac case remains officially open. A confirmed name, even from a cipher, could provide the starting point for a DNA comparison or other evidence-based identification.
[ANALYSIS] However, the evidentiary weight of a cipher solution alone would be limited. A decoded name would need to be corroborated — by DNA, by physical evidence, by consistent witness descriptions, by documentary evidence placing the individual at the crime scenes. A cipher solution would be a starting point for investigation, not a conclusion.
[FACT] The Zodiac case remains officially open with the San Francisco Police Department and the FBI. Any new evidence — including a confirmed cipher solution — would be relevant to the ongoing investigation. The San Francisco Police Department has publicly stated it would welcome any credible new leads.
Conclusion
The Z13 and Z32 have resisted every attack for over 50 years. They may encode the Zodiac’s name and the location of a bomb he claimed to have planted. They may encode deliberate misdirection. They may have been constructed specifically to be unsolvable. They may hold the answer to the most persistent question in the case — who was the Zodiac Killer?
The Z408 fell in a week. The Z340 fell after 51 years. The techniques that cracked the Z340 are being applied to the remaining ciphers. AI-assisted cryptanalysis is improving rapidly. The community working on these problems is larger and more capable than at any previous point.
If the Z13 contains a real name — and if that name can one day be confirmed against physical evidence — the Zodiac case may not be permanently unsolvable. The answer may already be encoded in 13 characters on a letter sent to a newspaper in 1970.
We just cannot read it yet.
Written and reviewed by the MysteryVerse editorial team. Facts sourced from FBI official case documentation, David Oranchak’s published Z340 solution paper, the zodiackillersite.com research archive maintained by Michael Butterfield, and verified reporting from the San Francisco Chronicle and associated archives.
For the most comprehensive and up-to-date Zodiac cipher research, the community at zodiackillersite.com and David Oranchak’s zodiackillerciphers.com maintain detailed analysis of all four ciphers.
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