Military Cryptanalysis - Part I: Monoalphabetic Substitution Systems is one of the most well-respected publications on cryptanalysis, as was written by William F. Friedman, one of the true experts in the field. Now declassified by the National Security Agency (NSA), this publication is key in helping individuals understand and solve cipher systems. Military Cryptanalysis - Part I is the first publication in a series of four and focuses on monoalphabetic substitution systems.