MAGIC Codebreaking

The American decryption of Japanese diplomatic ciphers before and during World War II

Overview

MAGIC was the codename for American decryption of Japanese diplomatic communications, achieved by Army and Navy cryptanalysts against the Purple cipher machine. Tokyo's traffic was read throughout the war, yielding negotiating positions and strategic signals. In late 1941 MAGIC produced fragments pointing to a break in relations but nothing pinpointing the Hawaii attack, a failure of interpretation that fed the postwar investigations. The experience directly influenced the creation of centralized American intelligence in 1947. Purple machine replicas built by American cryptanalysts replicated the encryption without ever capturing the device.