Venetian cipher - Suriano | Suriano Venetian cipher - Suriano
Correspondence of
Suriano

Venetian cipher

Early modern diplomatic despatches were filled with sensitive information that needed to be protected. Since postal routes were not always safe and secure, and letters were often intercepted by foreign powers, governments took specific measures to secure their communications. Chief amongst these was instructing their diplomats to use ciphers to encode the secret information. During the sixteenth-century, the ambassadors of the Republic of Venice were early adopters and all used ciphers.

The Venetian Republic had centralised the use of ciphers. During the sixteenth century, the Council of Ten created and oversaw a department of cryptology. The state employed several professional cryptanalysts to protect its state secrets and diplomatic correspondence. These deputies of ciphers (‘deputati alle cifre’) were responsible for the creation of new ciphers, breaking encoded foreign correspondence, and training chancery staff in using ciphers.

The working papers of this cryptographic organisation dating from the sixteenth to the eighteenth century are kept in the Venetian State Archive. These archival documents allow us to reconstruct which cipher Suriano used from 1616 to 1623.

He used a ‘Great cipher’(key “a” = z10; ASVe, Consiglio dei Dieci, Cifre, chiavi e scontri di cifra, busta 1, fol 3), one of the more simplified systems in use by Venetian embassies since the end of the sixteenth century. The key Z10 was a mixed code which combined a monoalphabetic cipher (one cipher sign for each letter), a syllabary (one cipher sign for each syllable) and a dictionary (one cipher sign for each word).

The nomenclature consisted of 295 ciphering signs: a letter followed by one or two numbers.

  • The letter “z” with numbers from 1 to 20 for monoalphabetic characters (for example z10 = a)
  • The letter “d” with numbers from 1 to 10 for numerals (for example d1 = 9)
  • The letters “m”, “g”, “b”, “n” with numbers from 1 a 70 ending in 1 to 5 for syllables, ordered according to the vowels (for example m1 = ba, m2 = be, m3 =bi, m4 = bo, m5 = bu);
  • The letters “m”, “g”, “b”, “n” with numbers from 1 to 70 ending in 6-0 in the dictionary, not always ordered alphabetically (es. n6 = Pontefice, n7 = qual).

In 1606 the Venetian cipher expert Pietro Partenio exposed the weaknesses of this Z10 cipher, which was relatively easy to break. Among the problems were:

    • An absence of homophones, meaning that the cipher does not provide for the use of different symbols to represent the same letter. This facilitates the decryption of the most common letters.
    • It lacks superfluous digits which have no meaning and could have confused cryptanalysts. These are useful to interrupt the predictability of sequences. To give an example, in the case of this cipher it could be z21 or any number over 70, because it is not included in the key.
    • The system allowed a certain degree of flexibility to its users, who did not always apply the rules strictly. Suriano, for instance, did not use the dictionary consistently and preferred the syllabary.
    • The system is most effective for the Italian language. Other languages use monoalphabetic signs more frequently than syllables, some very common syllables (f.i. in French: oi, au, eu, ou, in, etc.) are not included in this syllabary and are therefore represented by monoalphabetic signs. Cryptanalyst were always looking for patterns (frequency analysis) to crack the code and often relied on patterns in specific vernaculars.

    Despite these vulnerabilities and inconsistencies, it took more than a decade for the Council of Ten to act. In 1619, they created a committee to reform the cipher and two years later, they created a new cipher designed to address vulnerabilities identified by Partenio. It was composed of three numbers without spaces, with homophones and superfluous digits(chiave “a” = 156/174; ASVe, Cifre, chiavi, busta 2, fol. 15). This cipher was first used by Venetian diplomats in July 1623, exactly when Suriano left The Hague. Before the implementation of this new cipher, some ambassadors and agents tried to increase the security of their cipher. Suriano, for instance, took measures to increase security by eliminating spaces and doubles.

    Deciphering

    Most of the ciphered letters by Suriano have been decoded by a secretary on separate sheets of paper. These separate sheets have been kept together with the correspondence in the Venetian archives and have been integrated by the transcription team into the letters in italics (see also transcription rules).

    Federica D’Uonno decoded the remaining words and texts which had not been decrypted by a Venetian secretary. During the transcription of the ciphered text, she created a tool in Excel to automate the decryption and encryption of the ‘great cipher’. It employs two VBA (Visual Basic for Application) macros. One macro handles decryption, replacing encrypted characters with the corresponding unencrypted value, while the other carries out the reverse process. The system can handle both simple (1:1) and complex (1:n and n:m) substitutions as well as replace values even if they are concatenated in the text, thanks to the macros’ ability to recognise character sequences.

    For decryption, it is essential to transcribe the numbers accurately as well as recognise additional features (number endings in dictionary and gender of a word) of the system and separate the words correctly. During encryption, it is crucial to bear in mind that the result may differ from the original letters, writers varied in their use of dictionary and syllabary.

    Bibliography

    Ioanna Iordanou, Venice’s Secret Service. Organizing Intelligence in the Renaissance (Oxford University Press, 2019).

    Nadine Akkerman and Pete Langman, Spycraft. Tricks and Tools of the Dangerous Trade from Elizabeth I to the Restoration (Yale University Press, 2024).

    Paolo Bonavoglia, La crittografia della Repubblica di Venezia (Rome 2023)

    Paolo Bonavoglia, ‘Ottavian Medici and the decline of Venetian cryptography’, in Proceedings of the 6th International Conference on Historical Cryptology HistoCrypt 2023, ed. Carola Dahlke and Matthias Göggerle, 2023 https://doi.org/10.3384/ecp195.

    Luigi Pasini, Delle scritture in cifra usate dalla Repubblica di Venezia (1872), ed. Paolo Bonavoglia (Rome 2019).

    Co-written by Federica D’Uonno and Nina Lamal