First discovered in a Hungarian library in 1838, the Rohonc Codex keeps privileged company with some of the most famous unsolved writing systems in the world, notably the Voynich manuscript, the Phaistos Disk, and Linear A. Written entirely in cipher, this 400-year-old, 450-page-long, richly illustrated manuscript initially gained considerable attention but was later dismissed as an apparent forgery. No serious scholar would study it again until...