A 1542 papal cipher cracked with simulated annealing
a day ago
- In April 1542, a letter from Cardinal Alessandro Farnese to Bishop Giovanni Poggio switched from Italian prose to a code of digits, initiating a cryptographic puzzle.
- The letter's cipher key, recovered through computational search, maps letters to one- or two-digit codes, with a null mark meaning 'nothing' and context distinguishing numbers.
- Using constrained optimization and Italian-language modeling, the key was identified as a simple substitution with 18 letters, verified by correctly deciphering both halves of the letter independently.
- The recovered key, stable under repeated searches, revealed the plaintext: 'Dopo la partita del Montepulciano' (After Montepulciano's departure), detailing diplomatic matters between the Pope and Emperor.
- Decryption uncovered historical context, including Andrea Doria's advice on arming against the Turkish fleet, the Emperor's request for a Hungarian subsidy, and a proposed castle name, with edits confirming readings like 'S.M.tà [h]a voluto'.
- The transcribed cipher contained errors—five doubled groups, fourteen omitted digits, and two swapped pairs—fixed by comparing against the original manuscript, refining the decipherment.
- The Vatican archive dated the letter as April 1542, matching the dateline, and the edition's gaps (e.g., '[che potrà]') were corrected by language-model-driven readers who proposed 'verà'.
- The solution is corroborated by external sources like Cardauns's Nuntiaturberichte, and the process demonstrates that breaking this cipher hinged on finding boundaries between codes and resolving ambiguous digits.