Researchers using Claude solved the century-old Chinese gold bar cipher mystery by proving the 1933 Shanghai bars were never encrypted at all—the 263 letters were deliberately arranged with perfectly uniform frequency distribution, which no actual cipher could produce. The case, unsolved for 30 years on cryptography lists, demonstrates statistical analysis can identify non-ciphers.
A transcription of an undeciphered letter from Le Tellier to Marquis de Castelnau dated 12 May 1657 concerning royal instructions for troop movements in Flanders, containing a cipher substitution that remains unsolved despite its potential regularity in number assignments.