Government Communications Headquarters
The Government Communications Headquarters (GCHQ) is a British intelligence agency that provides signals intelligence (SIGINT) and information assurance to the UK government and armed forces, under the guidance of the Joint Intelligence Committee. CESG (originally Communications-Electronics Security Group) is the branch of GCHQ which works to secure the communications and information systems of the government and critical parts of UK national infrastructure.
GCHQ was started after the First World War as the Government Code and Cypher School (GCCS or GC&CS), by which name it was known until 1946. As part of ECHELON, it works with Intelligence agencies from other allied countries.