Using Script Against Undesirable Readers: the Coded Messages of Mihovil and Antun Vrančić

U svojoj korespondenciji braća Antun i Mihovil Vrančić u nekoliko su se prilika poslužili kodiranim porukama: Antun u jednom pismu s diplomatske misije iz Pariza 1546, a Mihovil dvanaest godina poslije u četirima pismima iz Šibenika. Tijekom istraživanja provedenog za potrebe ovoga rada ustanovljeno je da jedno od pisama dosada nije bilo poznato. Mihovilov sustav znakova uspjeli smo dešifrirati usporedbom učestalosti znakova u kodiranim odlomcima s učestalošću slova u latinskim dijelovima pisama. Većina je slova preuzeta iz latinice, samo im je promijenjena vrijednost, pri čemu je imala utjecaja, čini se, knjiga Polygraphia onodobnoga njemačkog kriptografa Ioannesa Trithemiusa. Usto je dodano nekoliko arapskih znamenki i znakova iz drugih pisama. Glavnina skrivena sadržaja odnosi se na ulaganja u nekretnine i unosne poslove, te je svrha tajnog pisma očigledno bila onemogućiti konkurenciju da preduhitri Antuna i Mihovila, u trenutku kad je Antun bio nagrađen od cara Ferdinanda nakon četverogodišnjeg poslanstva u Turskoj. U dvjema pak porukama Mihovil upozorava brata na opasnost od dvojice braće s vrlo lošom moralnom reputacijom. Antunov odlomak ostao je neriješen, jer se poslužio drugačijim sustavom, a sam tekst je prekratak, tako da izračun frekvencije ne pomaže.Spending the whole of his working career in Hungary, Antun Vrančić (1504¬1573) climbed to the very top of the ecclesiastical and political ladder. For the sake of his office, but still more so in consequence of his humanist education and beliefs, as well as family affections, he was constantly in touch by letter with a wide circle of people. Today about eight hundred of his letters are known. The whole of this time, the person and associate closest to him was his younger brother Mihovil (1507-1571), also a humanist and writer. The epistolary communication between the two spanned more than thirty years. Writing to his brother from Paris, where in 1546 he was envoy of the Queen of Transylvania, Isabella, Antun inserted not quite two lines of coded writing, in a paragraph where he is talking about being well received at the court of France, and hoping to complete his assignment soon. What this success was to consist of, he did not write in Latin script, but in a mixture of characters. Information concerning this appeared before the scholarly public some ten years ago, in an article about the unpublished Vrančić family papers kept in the National and University Library in Zagreb. Mihovil also coded several fragments in four of his letters sent at the end of 1558 and the beginning of 1559 from Šibenik to his brother in Hungary. Information about three of them is found in three editorial notes (in Hungarian) alongside the letters of Mihovil published in the mid-19th century in the collected works of Antun Vrančić (Verancsics Antal Ősszes munkái) as part of the series Monumenta Hungariae Historica of the Hungarian Academy of Sciences. In the actual text of the letters, lacunae were left in the places of the coded messages. The fourth letter was unknown to the editors. That is what we realised when we obtained from Budapest photographs of the autographs, on which it too was to be found. When it appeared that the secret script was not a simple replacement of the Latin alphabet with Bosančica, or Croatian Cyrillic, it was necessary to approach the decoding more systematically. We first selected Mihovil’s messages, as being incomparably longer and providing more to analyse. Three factors were essential. First character frequency scales were made: in the coded text, in the Latin part of the letters, and in an extant prayer of Antun Vrančić in Croatian. It turned out that the coded and Latin parts were close to each other, and at the same time distant from the Croatian text. It was fairly certain that Latin was hidden in the secret messages. The final confirmation came when we became aware of the fact that in the Latin there was a (semi-)obligatory combination of the characters q and u, where the first regularly appeared with the second, but the second also appeared on its own. We discovered that in the coded part of the text a similar (semi-)obligation existed. The last factor that led to the code being broken was the comparison of the frequency of geminates in the coded and in the Latin text. On the basis of the assumed probable letter values, we were able to read the first word – possessio. It contains two identical geminates, which also made the identification easy. The rest went relatively easily and quickly. The basic stock of characters is taken from Latin script, only shifted to the left or right more or fewer places. Some Latin symbols are replaced with Arabic figures, and a negligible number probably come from the Greek and Bosančica scripts. The biggest influence there might be is visible in the fact that the Latin letter values of almost a third of Mihovil’s signs coincide with the values in the book Libri polygraphiae VI of the German cryptographer Ioannes Trithemius. Most of the concealed content relates to investment in real estate and profitable business deals. Mihovil is on the whole informing his brother about good deals to be had on the land and housing market in Šibenik and surroundings, and of his negotiations with the vendors. The objective of the secret script was clearlyto stop the competition forestalling the entrepreneurial brothers, at the time when Antun had been rewarded by Emperor Ferdinand with the office of bishop of Eger after his four-year period as envoy in Turkey, after which his income must have risen considerably. In two messages Mihovil warned his brother of the danger of two other brothers with very poor reputations for morality. To our surprise, Mihovil’s system did not assist us in deciphering those two lines in code of Antun’s. Clearly the system had changed in the meantime, or each one of brothers had his own. The text itself is too short for the calculation of frequency to help. Accordingly, this mysterious fragment about the expected success at the French court remains unsolved