Geospatial Constraint Satisfaction in the Zodiac Z32 Cipher

The Z32 cipher, a 32-character homophonic substitution cipher sent by the Zodiac killer on June 26, 1970, has resisted cryptanalysis for over 55 years. With only 29 unique symbols in 32 positions, Z32 falls below the unicity distance required for linguistic frequency analysis—the method that solved the Zodiac's longer ciphers Z408 and Z340. This paper reframes Z32 as a Geospatial Constraint Satisfaction Problem (GeoCSP), exploiting the boundary conditions the Zodiac himself provided: a Phillips 66 road map centered on Mount Diablo, a crosshair aligned to magnetic north with clock-position numbers, and the explicit hint "radians and # inches along the radians". We enumerate 2,044,224 candidate phrases across 12 structural templates and filter by three independent constraints: character length, homophonic lock consistency, and geographic projection within the map bounds. Only 54 candidates survive (99.9974% rejection rate). Of these, a single candidate—IN THREE AND THREE EIGHTHS RADIANS TEN—uniquely converges with a 100-foot equilateral triangular crop mark located 254 meters from the decoded coordinates. The solution is independently corroborated by its position near the geometric centroid of the Zodiac's operational triangle, by the alignment of all Vallejo crime scenes within the cipher's decoded clock-hour sector, and by the morphological correspondence between the ciphertext's triangle symbol and the ground feature. An internal structural property of the cipher further constrains the result: 47 of 54 survivors (87.0%) decode to clock hours 8 or 10—the two vectors corresponding to the Zodiac's crime zones—representing a 5.2x enrichment over random expectation. These independent lines of evidence render coincidence statistically untenable. Code and data: github.com/dstampher/zodiac-z32-cipher