A 1542 papal cipher cracked with simulated annealing

In April 1542, a letter was dispatched from Rome to the Spanish court of Charles V. On the first page, the Italian text begins mid-line, with digits following. These numbers, such as "Dopo la" (after the), use a null symbol, represented by the digit 9. Deciphering the code and determining where each code ends poses challenges. The sender was Cardinal Alessandro Farnese, the grandson of Pope Paul III and head of the papal secretariat, while the recipient was Giovanni Poggio, bishop of Tropea and the Pope's ambassador at the Emperor's court.

The letter opens with ordinary Italian prose, informing Poggio about the peace between the Emperor and France. Cardinal Farnese, the sender, did not repeat this information. Montepulciano, who had returned from Spain in February with the Emperor's terms, departed for Spain again in late March. Montepulciano's name appears on a cipher key, which Aloys Meister dated between 1538 and 1542.

The letter spans eight pages, with the cipher occupying the middle portion of pages 70v to 72r. The cipher concludes on page 73r, leaving the final page clear and dated "Da Roma alli X[?] di Aprile 1542," with the day's numeral partially obscured. The cleartext frames the cipher, containing church business, news from Ancona, and a date. The instructions for deciphering are encrypted within the letter.