Z32 Research Dossier:
Evidentiary Audit of the Zodiac Killer’s Mt. Diablo Cipher
Disclosure: This research was conducted interactively with ChatGPT as an analysis tool. I directed the hypotheses and tests; the dossier documents both positive and negative results. This is not being presented as a solution to Z32.
Status: Unsolved
Research conclusion: The surviving evidence is compatible with Z32 encoding a Mt. Diablo coordinate, but does not uniquely determine its plaintext, numerical coordinate, destination, or key.
⸻
1. Purpose
This dossier records an adversarial investigation of the Zodiac Killer’s 32-symbol Mt. Diablo cipher, Z32.
The objective was not to produce another plausible-looking solution. It was to determine which conclusions survive when candidate solutions are subjected to independent tests, null models, alternative interpretations, and attempts at falsification.
The investigation deliberately separated:
authenticated evidence;
cryptographic constraints;
contextual clues;
interpretive assumptions;
candidate solutions;
retrospective coincidences.
A candidate was not treated as a decryption merely because it produced grammatical English or an interesting geographic endpoint.
⸻
2. Hard Evidence
Zodiac supplied a Phillips 66 map centered on the San Francisco Bay Area and marked Mt. Diablo with his crosshair symbol.
The crosshair was numbered:
0 — 3 — 6 — 9
Zodiac instructed that 0 was to be set to magnetic north.
He stated that:
“The map coupled with this code will tell you where the bomb is set.”
In the subsequent July 1970 communication, he supplied the additional clue that the Mt. Diablo code concerned:
“Radians & # inches along the radians.”
These statements establish with relatively high confidence:
Origin: Mt. Diablo.
Orientation: magnetic north.
Angular component: radians.
Radial/distance component: a number of inches.
Function: map and code jointly specify a location.
They do not independently specify the angular value, number of inches, plaintext wording, or destination.
⸻
3. Z32’s Internal Cryptographic Constraints
Z32 contains 32 positions but 29 distinct symbol classes.
Only three symbols repeat.
Consequently, any simple substitution plaintext must satisfy:
[
P_1=P_{26}
]
[
P_2=P_{32}
]
[
P_6=P_{14}
]
These are the principal plaintext equality constraints supplied internally by Z32.
They are genuine constraints, but extremely weak compared with the redundancy available in Z408 or Z340.
⸻
4. Repeat-Geometry Analysis
The repeated-symbol pairs occur at:
[
(1,26),\quad(2,32),\quad(6,14)
]
Their linear separations are:
[
25,\quad30,\quad8
]
Their shortest circular separations are:
[
7,\quad2,\quad8
]
The six repeat endpoints are:
[
1,\ 2,\ 6,\ 14,\ 26,\ 32
]
with circular gaps:
[
1,\ 4,\ 8,\ 12,\ 6,\ 1
]
Results
No nontrivial rotational symmetry was found.
No reflection symmetry was found.
No unified modulo-3, modulo-6, or modulo-9 system explains all three pairs.
The repeat geometry therefore does not independently reproduce the Mt. Diablo 0–3–6–9 dial.
⸻
5. Boundary-Topology Test
Z32 appears visually unusual because two repeated pairs couple its beginning and ending regions:
[
1\leftrightarrow26
]
[
2\leftrightarrow32
]
while:
[
6\leftrightarrow14
]
is internal.
An initial topology statistic made this arrangement appear relatively rare.
After correcting for the freedom involved in selecting boundary widths and short-chord thresholds, however, the apparent significance weakened substantially.
A stronger control then compared the phenomenon with 32-character windows drawn from authentic solved Zodiac ciphertext.
The result did not establish Z32’s boundary topology as exceptional.
Conclusion
The boundary structure is interesting descriptively, but there is insufficient evidence that it represents an intentional secondary encoding.
⸻
6. Cross-Cipher Key Transfer
Z32 shares much of its symbol inventory with Zodiac’s other ciphers.
However, comparison of Z408 and Z340 demonstrates that Zodiac reused cipher symbols without consistently preserving their plaintext assignments.
Therefore:
[
\text{same cipher symbol}\not\Rightarrow\text{same plaintext letter across ciphers}
]
Directly imposing the Z408 or Z340 substitution keys on Z32 does not produce a defensible unique plaintext.
Conclusion
Simple cross-cipher key reuse fails.
⸻
7. Plaintext Underdetermination
Because Z32 imposes only three equality constraints, many possible English strings can satisfy its topology.
One investigated candidate was:
AT SIX SIX INCHES FROM MOUNT DIABLO PIT
Removing spaces gives:
ATSIXSIXINCHESFROMMOUNTDIABLOPIT
Positions 1–32 are:
A T S I X S I X I N C H E S F R O M M O U N T D I A B L O P I T
Relevant repeated letters are:
A: positions 1, 26
T: positions 2, 23, 32, including required equality 2=32
S: positions 3, 6, 14, including required equality 6=14
Thus the candidate satisfies all three Z32 equality constraints.
That establishes compatibility, not decryption.
The same principle applies to other grammatically persuasive candidates.
⸻
8. Coordinate-Language Grammar Tests
A deliberately conservative grammar was constructed using only concepts justified by Zodiac’s explicit clues:
radians;
numbered inches;
ordinary coordinate/instruction language;
optional Mt. Diablo wording;
ordinary number words.
No target names, π, PIT, Neary, La Honda, Ingleside, or preferred numerical solution was privileged.
Under the narrowest grammar, no exact 32-letter construction satisfied all three Z32 equalities.
When the permitted vocabulary was expanded, compatible constructions began appearing.
Further expansion produced multiple families of solutions rather than one uniquely selected sentence.
Conclusion
Z32’s equality structure constrains possible language, but does not uniquely recover ordinary English coordinate instructions.
⸻
9. Semantic Coordinate Test
Candidate sentences were reduced conceptually from wording to their underlying payload:
[
(\text{angular value},\text{distance})
]
The ambiguity did not disappear.
Multiple coordinate payloads remained compatible with the cipher constraints and contextual grammar.
Therefore the problem is not merely uncertainty about how Zodiac phrased a known coordinate.
The coordinate itself remains underdetermined.
⸻
10. π Radians + 6 Inches
The hypothesis:
[
\pi\text{ radians}+6\text{ inches}
]
received considerably more scrutiny than an ordinary plaintext guess because separate structural work has produced results compatible with a trigonometric interpretation and a salient six.
Nevertheless, the bridge from those structural observations to exactly:
[
\pi+6
]
contains interpretive choices.
The original correspondence does not independently select dial position six as the angular value.
Therefore:
π radians + 6 inches remains compatible with some evidence but is not cryptographically forced.
⸻
11. Neary Quarry
A nominal-scale interpretation of π radians plus six inches produces an intriguing geographic relationship with the historical Neary excavation/quarry vicinity.
However, Zodiac’s wording says inches along the radians, naturally suggesting measurement on the physical map.
When six is treated as six literal map inches, the resulting endpoint does not reproduce the Neary result.
Therefore reaching Neary requires an additional scale interpretation not explicitly supplied by Zodiac.
Conclusion
Neary is an interesting geographic hypothesis, not a validated Z32 destination.
⸻
12. Other Geographic Candidates
Alternative interpretations can produce apparently meaningful locations, including areas associated with Ingleside and La Honda.
These exercises exposed a substantial look-elsewhere problem:
Given enough reasonable angular values, distances, scales, magnetic corrections, map reconstructions, and nearby landmarks, retrospectively meaningful geographic hits become comparatively easy to obtain.
Therefore geographic correspondence should provide strong evidence only when the coordinate is derived independently before the target is inspected.
⸻
13. Phillips 66
The conspicuous 66 in the Phillips 66 branding was investigated as a possible source for repeated sixes.
Period road maps branded by gasoline companies were commonplace.
No authenticated Zodiac instruction establishes the brand name as part of the cipher mechanism.
Consequently:
Phillips 66 should not be counted as independent evidence for 6/6.
⸻
14. Artifact Audit
The physical map/cipher presentation was examined conceptually for additional information channels.
The strongest deliberate feature remains the Mt. Diablo crosshair:
numbered like a dial;
centered on Mt. Diablo;
oriented to magnetic north.
This strongly supports treating it as an angular measuring device.
No comparably strong physical evidence was found for:
a hidden ruler value;
symbol-rotation encoding;
a second spacing cipher;
a unique 3/6/9 traversal;
π;
six inches;
four radians;
five inches;
PIT;
a particular destination.
Conclusion
The map diagram appears to define the coordinate system, not the missing coordinate values.
⸻
15. Other Zodiac Correspondence
Zodiac frequently used:
crosshairs;
quadrants;
numbers;
spatial arrangements;
unusual diagrams;
symbolic layouts.
However, no independent authenticated communication examined in this investigation establishes a reusable procedure equivalent to:
origin → magnetic orientation → radian angle → measured inches
Thus no external private convention has yet supplied the missing Z32 key.
⸻
16. Information-Theory Result
Z32 is not inherently too short to encode a unique location.
A coordinate within a roughly 50-mile-radius region at approximately one-mile resolution requires only about:
[
13\text{ bits}
]
Even hundreds of angular and radial possibilities require only on the order of:
[
15\text{ bits}
]
A 32-character message has ample theoretical capacity.
The difficulty is therefore not message capacity.
It is recoverability.
Z32 contains 29 symbol classes and only three repetitions. Without the substitution or encoding key, insufficient redundancy survives to reconstruct the intended mapping uniquely.
Thus:
[
\boxed{\text{capacity is sufficient; recoverable constraint is not}}
]
⸻
17. Historical Wording
Zodiac wrote that:
“The map coupled with this code will tell you where the bomb is set.”
That is subtly different from explicitly asserting that Z32 could be solved as an ordinary cryptogram.
His later radians-and-inches clue supplied additional operational information.
This is compatible with—but does not prove—the possibility that Z32 was a keyed coordinate encoding whose intended values were definite to Zodiac while its key was not recoverable from the transmitted ciphertext alone.
⸻
18. Current Status of Major Hypotheses
π radians + 6 inches: Compatible, not uniquely derived.
Neary Quarry: Interesting geographic correspondence under one measurement interpretation; fails as a literal six-inch result.
PIT AT SIX SIX…: Satisfies Z32 equality constraints but is not uniquely decrypted.
FOUR RADIANS / FIVE INCHES family: Contextually plausible but not uniquely selected.
La Honda: Reproducible only under particular constructions; not a demonstrated target.
Phillips 66 as numerical key: Unsupported.
Z408/Z340 key reuse: Fails.
0–3–6–9 repeat geometry: Not independently recovered from Z32.
Boundary topology as hidden encoding: Not established against authentic Zodiac controls.
⸻
19. What Would Constitute a Serious Future Solution?
A strong Z32 solution should ideally satisfy several independent requirements.
It should derive its mechanism before selecting a geographic target.
It should explain all three repeated-symbol constraints naturally.
Its key or procedure should arise from authenticated Zodiac material rather than solver-added assumptions.
It should explain why Zodiac supplied the Mt. Diablo dial and subsequent radians/inches clue.
It should produce a coordinate without adjustable scale, orientation, transcription, or target-selection parameters.
It should make at least one successful prediction not used to construct the solution.
Most importantly, it should outperform plausible competing solutions under the same rules.
A readable sentence and interesting destination are not sufficient.
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20. Final Evidentiary Conclusion
The investigation does not establish a solution to Z32.
What survives is narrower but stronger:
[
\boxed{\text{Z32 is strongly contextualized as a Mt. Diablo coordinate puzzle.}}
]
However:
[
\boxed{\text{the surviving ciphertext and explicit clues do not uniquely recover its coordinate.}}
]
The evidence currently supports the possibility that Zodiac encoded a definite location using an unrecoverable or undisclosed key/convention.
Accordingly, the responsible status is:
Z32 remains unsolved
Future work should prioritize genuinely independent keys, newly authenticated source material, high-resolution forensic evidence, or mechanisms capable of making prospective predictions.
It should not treat compatibility with three repeated-symbol constraints or a retrospectively interesting geographic endpoint as sufficient evidence of decryption.
⸻
Research principle going forward
Derive first. Predict second. Look at the target last.
