A 1542 papal letter from Cardinal Alessandro Farnese to nuncio Giovanni Poggio, containing a cipher section of 6,577 digits across multiple pages, was cracked using simulated annealing after remaining unsolved for decades on cipher puzzle platforms.
In April 1542, Cardinal Alessandro Farnese sent an enciphered letter to Giovanni Poggio, the papal nuncio at Emperor Charles V's court, regarding peace negotiations between the Emperor and France. The letter uses a digit-based cipher with a key connected to Giovanni Ricci da Montepulciano and remains largely unsolved despite being published as a puzzle online in 2019.
Researchers used simulated annealing to decrypt a 500-year-old cipher from a 1542 letter sent by Cardinal Alessandro Farnese to the papal nuncio Giovanni Poggio. The letter, stored in the Vatican Archives, contained a polyalphabetic substitution cipher that had remained unsolved until computational methods were applied to recover the key and plaintext.