By Richard Aldrich
http://www.economist.com/node/16537028

WHAT is the essence of Britain’s “special relationship” with America? Shared history, perhaps, or a common language, a military alliance? No less significant is the extensive exchange of intelligence between the two countries’ spies. The cold-war games of Britain’s MI6 and America’s CIA are reasonably familiar. But the British outfit that is most tightly bound to America is GCHQ, the Government Communications Headquarters, whose job is to break codes and process electronic-signals intelligence, known in the trade as “sigint”. It is also the biggest and most secretive of British agencies, so Richard Aldrich’s book is a welcome account of the body that inhabits a mysterious doughnut-shaped building outside Cheltenham.

The web of secret treaties in the 1940s which laid the foundation of intelligence co-operation between America and Britain—and by extension with Canada, Australia and New Zealand—had at its heart the pooling of sigint on the Soviet Union. Early in the second world war, Britain took its time to let America into the secrets of Ultra, the decryption of German secret communications by code-breakers at Bletchley Park. By the end of the war, however, sigint—particularly Venona, the system for breaking Soviet codes—had become an American-led effort. Venona revealed the vast extent of Soviet espionage in the West. Indeed, America and Britain were so deeply penetrated that the Russians soon found out about Venona, changing their codes and cipher machines on October 29th 1948—“Black Friday” for code-breakers.

Proud of their privileged partnership with America’s National Security Agency (NSA), the men and women at GCHQ are torn between worry that America will tire of the liaison and bouts of anxiety at being too dependent. For America, the payoff has been access to listening posts in Britain’s residual imperial outposts and the chance to share the labour- and computer-intensive job of reading the messages of enemy and friend alike—particularly after American spy satellites started beaming back a wealth of intercepts.

Intelligence is not so much shared as traded, even among close allies. Both Britain and America have withheld information from the other. And the relationship has not always been smooth. William Odom, the NSA’s abrasive director from 1985 to 1988, wrote in his notes that Peter Marychurch, his British counterpart, was his least favourite of European sigint chiefs. The then-chief of GCHQ, wrote General Odom, saw his main task as staying “fully entangled with the US system” and “to stand between us and the other Europeans”. The Brits, he reckoned, were “a pain in the ass”.

Mr Aldrich skilfully weaves together the personal, political, military and technological dimensions of electronic espionage. He recognises the limits of even the most sophisticated sigint: GCHQ can track the movements of armed forces but cannot always fathom what those forces are up to. In the internet age, the agency faces two challenges: how to monitor the rivers of digitised information that flow around the world; and how to maintain political legitimacy for governments to gather and store large quantities of personal data so that the information can be searched for patterns of terrorist and criminal activity. Its work is harder than ever.