CESG has existed in one form or another since World War I. Poor radio communications security during the war had resulted in severe security compromises by British forces, so the British government decided to set up a special organisation both to study the methods of cipher communications of foreign powers, and to advise on the security of British codes and ciphers.
1919 to early 1950s
The Government Code and Cipher School (GC&CS) was established In 1919, following a Cabinet decision. The GC&CS was originally set up under the civil administration of the Admiralty, but with the decline in military signal traffic after the end of World War I, it was then placed under the administrative responsibility of the Foreign Office in 1922.
The communications security part of the GC&CS could only advise on the use of the various cipher systems, but had no authority to mandate government departments or the services to use those ciphers and procedures which were really secure. This weakness was recognised during World War II, and in 1944 clearer lines of authority were established, with the communications security section of the GC&CS becoming responsible to a Cipher Policy Board. The GC&CS was officially renamed the Government Communications Headquarters (GCHQ) on 1 April 1946.
1950s to 1970s
In the early 1950s, a review of the Cipher Policy Board's organisation and terms of reference led to the creation of a new agency, the London Communications Security Agency (LCSA). The LCSA had its own Director, but still remained administratively under GCHQ.
In 1965, the LCSA became the Communications-Electronics Security Department (CESD), still based primarily in London, although parts were now co-located with GCHQ in Cheltenham, Gloucestershire. In 1969 CESD formally merged organisationally with GCHQ and was renamed the Communications-Electronics Security Group (CESG). In 1978 the last London elements of CESG moved to Cheltenham, where it has remained to the present day.
1970s to today
One of the more significant changes to CESG since the late 70s has been the move to operating on a cost-recovery basis, rather than being funded directly by central government. Since 1997, CESG now charges for most of the information security / Information Assurance (IA) services it offers.
Today Government turns to CESG as its National Technical Authority for advice and services to protect its voice and data networks. In this network-enabled environment, Communications-Electronics Security no longer adequately describes the full extent of the organisations work, so in 2002 it was decided to drop the expanded name. The title CESG was retained as it has an excellent reputation as an independent technical organisation with a critical role in keeping the Government's IT systems safe.
In October 2003 CESG moved into the new GCHQ / CESG accommodation at Benhall, Cheltenham. The distinctive circular new building, known locally as the 'Doughnut', co-located both organisations under one roof for the first time in their history.