In April 1542 a letter left Rome for the court of Charles V in Spain. On its first page the Italian stops in the middle of a line and digits begin:

Figure 1. The opening of the cipher, f. 70r. Archivio Apostolico Vaticano (AAV), Segr. Stato, Spagna 1A, photograph supplied through DECODE record 92 . Detail enlarged from the photograph.

Read with the key recovered below, the first ten digits group like this:

73 4 57 4 9 03 5 d o p o · l a

Dopo la , "after the". The 9 stands for nothing: it is a null. The difficulty is deciding where each code ends. 73 is d , but 7 3 is also two letters, n m . Even with the key, a run of digits can be read more than one way.

Two problems follow: finding the key without knowing where one code ends and the next begins, and then, once a key exists, telling the writer's words from a plausible guess.

The letter

The sender was Cardinal Alessandro Farnese, grandson of Paul III and, at twenty-one, head of the papal secretariat.…