He returned to the US in 1920 and published an eighth monograph, "The Index of Coincidence and its Applications in Cryptography", considered by some to be the most important publication in modern cryptography to that time.
№ 22 в этой серии, «The Index of Coincidence and Its Applications in Cryptography» (1920), обыкновенно рассматривается самой важной научной публикацией в истории криптологии.