West Wind Clear: Cryptology and the Winds Message Controversy--A Documentary History

2.1. The Cryptography of the J-19 System

As mentioned in the previous chapter, like many countries, Japan relied on a mix of ciphers and codes to secure its diplomatic traffic. Also, Japan was one of the earliest countries to rely on cipher machines to encrypt some of its more sensitive or important communications. However, these machines were expensive to make, maintain, and protect. In the summer of 1940, only about ten diplomatic missions held Red and Purple cipher machines, though more were scheduled to receive the machines.^[] Also, these devices could be available only at missions whose physical facilities were considered secure from physical compromise to the intelligence services of host countries. The physical security status of Japan's ...