Two Ciphers of Cosimo I de' Medici in his Letters to Agnolo Niccolini, 1537–1539

The family archive of the Niccolini di Camugliano in Florence keeps the letters that Duke Cosimo I de' Medici wrote to Agnolo Niccolini, his ambassador to Paul III, between December 1537 and August 1539. Twenty-one items from the bundle, twenty of them with cipher, were photographed for the DECODE database as records R4034 to R4054, and the key filed at the front of the bundle as R4033. The five letters written between 17 December 1537 and 4 March 1538 are in the cipher of that key. From October 1538 the letters use a second numerical cipher whose key is not in the bundle, and this paper recovers it from the ciphertext alone: the digit 0 stands by itself for e, the other letters are pairs of digits, most of them between 11 and 39, and a few pairs stand for words and titles. Five decipherments made at the time, found among the letters only after the key had been recovered, cover 551 letter codes, and 526 of them give the same letter as the key. A passage of Cosimo's draft printed in 1882 also agrees. The ciphered passages deal with the tithes that the Pope had laid on the Florentine clergy, a marriage proposal of 1538, the examination of Filippo Strozzi, and the claim, in 1539, that Cosimo's mother had tried to have Cardinal Cibo and Giulio de' Medici poisoned. A third cipher, in a letter of Maria Salviati, is still unread. Files: the paper (PDF), the transcription of the cipher passages, the two keys, and a machine decoding of every cipher passage. No images of the manuscripts are included; the letters are held by the Archivio Niccolini di Camugliano, Florence.