The 1943 BRUSA Agreement (Britain–United States of America agreement) was an agreement between the British and US governments to facilitate co-operation between the US War Department and the British Government Code and Cypher School (GC&CS). It followed the Holden Agreement of October 1942.
Colonel Alfred McCormack of the Special Branch of Military Intelligence Service, Colonel Telford Taylor of Military Intelligence, and Lieutenant Colonel William Friedman came to Bletchley Park, the Government Code and Cypher School Headquarters in England in April 1943. The American trio worked with Commander Edward Travis (RN), the head of the British communications intelligence (COMINT) facility.
The British decided to inform them of "Ultra", their successful decoding of German signals. Likewise, the Americans shared their solution to the Japanese Purple machine.
This led to the signing of the 1943 BRUSA Agreement on 17 May, which was a formal agreement to share intelligence information. It covered:
The security regulations, procedures and protocols for co-operation formed the basis for all signals intelligence (SIGINT) activities of both the US National Security Agency and the British GCHQ.
The agreement was formalized by the UKUSA Agreement in 1946. This document was signed on 5 March 1946 by Colonel Patrick Marr-Johnson (who had headed the Wireless Experimental Centre in Delhi during the war) for the U.K.'s London Signals Intelligence Board and Lieutenant General Hoyt Vandenberg for the U.S. State–Army–Navy Communication Intelligence Board.