A decade ago, unless you were an intelligence buff, you would not have heard of the NSA. America is more grown-up than Britain about its spies - it has never tried to conceal the existence of the CIA, its whereabouts, and the names of the people who run it. But from the outset the NSA was different; it was America's unacknowledged eyes and ears in every cranny of the globe, collecting and decoding everybody's secrets.
'Gentlemen do not read each other's mail,' observed Henry Stimson, Secretary of War under Roosevelt and Truman. But governments are bounders, especially those with Washington's ability to steam open letters. During the Cold War, the NSA was the great unmentionable. Even GCHQ Cheltenham was but a waystation en route to Fort Meade.
President Truman created the NSA in October 1952 with an executive order whose text has never been published; indeed, the very code word stamped upon the order is classified. Seconded Defense Department personnel, the joke ran for years, worked for 'No Such Agency'. The identity of its director was a secret. Today the Soviet Union is no more, and even the NSA must nod towards glasnost. The agency, complete with phone number and name of a 'public affairs' officer, is now listed in government handbooks. In the museum, which opened its doors six months ago, hangs a portrait of Vice-Admiral John McConnell, NSA director since 1992.
But that is all you will glean. A few hundred yards away stands the NSA proper, a squat fortress of reflecting glass, set in a forest of outbuildings, satellite dishes and radio masts, sealed off by a wall of barbed wire and who knows what else. Some 20,000 people work at the complex. But the NSA's share of the estimated dollars 30bn ( pounds 20bn) annual US intelligence budget is a mystery (though sleuths have deduced it is the largest single component). Scrutiny reaches no further than the Pentagon, NSA's parent and protector from prying souls in Congress. The museum, focused on yesterday's foes, may be as unrevealing of the present as the real NSA alongside. But at least you can get inside. And it's fun.
The most fascinating exhibit is of Enigma, the German encrypting machine whose unlocking by the Poles, British and Americans shortened the Second World War. There are almost a dozen machines on display and you can encode your name on one.
An NSA brochure (each page marked 'unclassified') explains the standard Enigma, barely a foot square, 'generated the following number of coding positions: 5,172,165,503,971,832,752,302,775, 832,450,732,675 (and then 51 zeroes)'. But Enigma would be a mere morsel for modern NSA computers, the most advanced in existence, which crunch the raw data vacuum-cleaned by its satellites, sensors and other eavesdropping devices around the clock, from foe and friend alike.
Of course, when the adversary was a superpower, there were also setbacks. In a smaller section devoted to the Soviet Union, you can inspect the famous wooden replica of the Seal of the United States given by Soviet schoolchildren to the US embassy in Moscow in the late 1940s, and which briefly adorned the ambassador's office until it was found to be bugged. For NSA scriptwriters, the episode merely bears out Thomas Jefferson's dictum that 'the price of liberty is eternal vigilance'. But as so often in espionage, this is a red herring. These days, the eavesdropping is done by the NSA. Let others be vigilant - if they can.
In fact, the most relevant warning contained in this unlikely gallery of relics in suburban Maryland is a US government propaganda poster from the Second World War. 'The enemy is listening,' it warns. 'Keep it to yourself.' Half a century later, the first part might equally be the watchwords of Saddam Hussein, Kim Il Sung, or the generals in Port-au-Prince, or of Western economic competitors trying to steal a march on the US. As for the second, well, if you keep off the phone, and don't leave anything on the lawn for a satellite to see, you might have a chance. But probably not.
For some reason the museum has a visitors' book, which like most visitors' books asks for your address. Coming from the NSA that's some cheek, I thought. Defiantly, I left the space blank. As a futile gesture it takes some beating.