A Florentine ‘polyalphabetic’ cipher in the 15th century

The 15th century in Italy was a period of revolution in cryptography. Leon Battista Alberti developed the first western polyalphabetic cipher, while the monoalphabetic system spread throughout the peninsula. The aim of this study is to present a never before published late medieval 15th-century Florentine polyalphabetic cipher, explain its functioning, and shed light on a system—specifically the polyalphabetic cipher—that, although seemingly unused during the 15th century, was in fact employed in Florentine diplomacy.