Japanese Code-Breaking: Intercepted Messages and Questions Over British, US and Japanese Code-Breaking

A file of correspondence and memoranda concerning Japanese code-breaking, and the possibility that the Japanese were decrypting United States and British secret communications. Documents in this file include a memorandum on the strength of U.S. codes and the potential ability of the Japanese to break them; letters discussing the possibility of the Japanese breaking British codes; a letter giving details of the British code-security arrangements, including the intervals at which they were changed; a translation of an intercepted Japanese report containing decrypted British Foreign Office messages; a letter reporting a conversation with U.S. Under Secretary of State Sumner Welles, in which he gave his opinion that the Japanese had broken the British codes; and letters on the engagement of Sir George Sansom as a consultant on Japanese translation.