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"Black Friday" - a major change occurs in Russian cypher procedures

Weisband

William Weisband was a Russian-speaking member of the U.S. Army Security Agency, one of the American service cryptanalytical arms. His family had emigrated from the Soviet Union to America in the inter-war period and he had served as an American codebreaker in the Middle East  during World War II.  However, he had been recruited as a Soviet agent as early as 1934 and revealed many of America's codebreaking triumphs, including "Venona", the patient reconstruction of wartime KGB traffic. As a result, the Soviets changed their codes siginficantly on Friday 29 October 1948 and many mid-level military systems were lost. This event was known by the codebreakers as "Black Friday".  Weisband was never charged with espionage for fear that he would reveal more information about codebreaking. However, he was removed from the NSA and served a year for contempt of a grand jury. By 1949, other Soviet agents were privy to "Venona" material, including Kim Philby, the SIS liaison officer in Washington.