Vigenère

Encrypts, decrypts and solves Vigenère, Beaufort, Autokey, Gronsfeld and Porta: periodic keys are recovered by index of coincidence and chi-squared, autokey primers letter by letter, and most variants also take a dictionary attack. An alphabet keyword keys the Vigenère tableau (Quagmire III, as in Kryptos parts 1–2). Bellaso introduced the keyword in 1553; Vigenère's Traicté des chiffres (1586) credits “un certain Belasio” and adds the autokey.

• Paolo Bonavoglia, “Trithemius, Bellaso, Vigenère: Origins of the Polyalphabetic Ciphers”, Proceedings of HistoCrypt 2020, pp. 46–51
• David Kahn, The Codebreakers: The Story of Secret Writing (New York: Macmillan, 1967)