15th-Century Spanish Diplomat's Encrypted Letter Cracked After 166 Years

A letter sent by Spanish diplomat Pedro de Ayala, rediscovered in 1860 and resistant to analysis ever since, has finally been decoded. Ayala used a hybrid scheme that combined symbol substitution with a partial codebook — replacing some whole words with single glyphs — and assigned multiple symbols to the same letter to flatten frequency statistics. The result was a system sophisticated enough for its era to obscure both letter-level patterns and the subject indicators that typically give cryptanalysts a foothold.

The decoding fits a small but growing pattern of historical cipher breaks, including a Charles V letter to an HRE diplomat cracked four years ago. The technical lesson is that homophonic substitution layered with codebook elements remains surprisingly durable: even by modern standards, ambiguity introduced through one-to-many mappings and semantic redaction can stall analysts when corpus and context are thin. The Voynich manuscript remains the conspicuous holdout.