This work contains a reprint of an 1872 book by Luigi Pasini, an archivist who worked at the State Archives of the Republic of Venice for some three decades, until his death in 1885. Pasini discovered many encrypted documents that he was able to decode. In his book, Pasini gave some details on the methods he followed to decode the documents, with crucial omissions. Thanks to documents found in the Archives at Venice, the editor has been able to understand how Pasini deciphered at least part of the messages. He proved that Pasini was in possession of fragments containing both encrypted and decrypted messages that enabled him to decrypt the messages of a Venetian ambassador in France, Giacomo Soranzo (1518–1599).