Found via Dustbury, a recently invented technique to derive keys from physical objects. This reminded me of a Scientific American article, maybe 1985 to 1989, just a little sidebar story about a technique the US military-industrial complex developed to authenticate serial numbers on cruise missiles.

Spray a mixture of lacquer and microscopic ground glass particles over the serial number, and photograph it from several angles. The reflection patterns of each marking will be unique and reproducible (standardize the flash intensity and color, and the angle of incidence) but difficult or impossible to forge.

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