This partially encrypted message, in Latin, was sent by Doge Michele Steno to the ambassadors Fantino Michiel in Rome, and Bartolomeo Nani in Poland, and was published in 1871 in Pasini's book, as the oldest encrypted message found in the Venetians Archives. Pasini added this remark:
At the foot of that document is the decryption * taken from the cited volume IV: we plan to rebuild the key that so far is missing.
No trace of this key was found among Pasini's papers left in the State Archives; either Pasini failed, or his solution was lost.
The original encrypted message on a large parchment is shown in figure 1 on the right; the plain text is found in the transcription on the secrets deeds of the Venetian Senate on pages 184v and 185r,* and is shown in figure 2. The one published by Pasini, figure 3, is only the last paragraph of the letter, with the clear text that it is not the reproduction of the acts of the Senate but a transcription by Pasini's hand; he completed some abbreviations, for instance the last suprascripta for sscripta.
Pasini's figure appears in the third edition (1947) of Luigi Sacco's Manual of Cryptography, see figure 4; but at the bottom there is also the key! In the text alongside Sacco writes:
The first complete cipher with arbitrary signs for each letter, with homophones for vowels, with many voids and with an added nomenclator, is the Venetian one dated 1411, cited by Pasini [105] and reproduced in fig. 17, on which we also give the so-called key, as resulted to us from the attached clear (not faithful) text.
The last statement leaves no doubt that it was Sacco himself who reconstructed the key by comparing the clear text transcribed by Pasini with the corresponding ciphertext * . A task that may appear "easy" being a monoalphabetic cipher, but the presence of many nulls and abbreviations and other small differences between the two texts complicates things somewhat. In any case, it was likely an "easy" task for a cryptanalyst with the experience of Luigi Sacco.
The cipher thus recovered is obviously not complete, the rare F H X letters are missing, which do not appear in the paragraph published by Pasini, K Y Z are also missing but these are Greek letters that in classical Latin did not exist. Furthermore, the dictionary of only four words is probably a subset of the original one, these are the four words used in the text.
Here are some lines deciphered via software, from top of page 185v, of the ciphertext *; the transcription, in the secret deeds of the Venetian Senate, is visible on the website of the 'Archivio di Stato, Progetto Divenire: . Select page 184v . Below the deciphered text written having recovered spaces and capitals.
Similiter sit in libertate nostra acipiendi licentiam a dicto dno Rege et repaeriandi et ita in bona gracia facere possit ds quia redimus nos certos quo eciam ambaxiator florentinos recedet in dicto casu
The key of the cipher was recovered (see Michele Steno's cipher page) comparing the ciphertext with the plaintext; it was possible to recover the complete alphabet, except maybe for homophones not used in this letter, and other words of the dictionary that remains certainly incomplete.
The tables on the side use an ad hoc font, which presently is visible only with the latest versions of browsers supporting the css3 style sheets; with other browsers Latin letters similar to the cipher signs are visible. Alternatively, you can choose the graphic mode, where the original signs are always visible. ( Work in progress )