5-UCO
ENCRYPTION SYSTEM MADE BY THE UK DURING THE SECOND WORLD WAR
BID 30; BID/30; 5UCO; 5 UCO
The 5-UCO (5-Unit Controlled)Ralph Erskine, "The 1944 Naval BRUSA Agreement and its Aftermath", Cryptologia 30(1), January 2006 pp14–15 was an on-line one-time tape Vernam cipher encryption system developed by the UK during World War II for use on teleprinter circuits. During the 1950s, it was used by the UK and US for liaison on cryptanalysis.