"Much of the NSA’s defence is that the public should be unconcerned, summed up by the dictum: “If you have nothing to hide, you have nothing to fear.” But civil liberties groups such as the Electronic Frontier Foundation and the American Civil Liberties Union warn that surveillance goes well beyond what Congress intended and what the US constitution allows.

Cell phones, laptops, Facebook, Skype, chat-rooms: all allow the NSA to build what it calls ‘a pattern of life’, a detailed profile of a target and anyone associated with them.

And the number of people caught up in this dragnet can be huge.

You don't need to be talking to a terror suspect to have your communications data analysed by the NSA. The agency is allowed to travel "three hops" from its targets — who could be people who talk to people who talk to people who talk to you. Facebook, where the typical user has 190 friends, shows how three degrees of separation gets you to a network bigger than the population of Colorado. How many people are three "hops" from you?

Faced with growing public and political concern over the quantities of data it is collecting, the NSA has sought to reassure people, arguing that it collected only a tiny proportion of the world’s internet traffic, roughly equivalent to a “dime on a basketball court”. But in reality, that is still a huge amount of data. The Library of Congress, one of the biggest libraries in the world, gathers 5 terabytes a month. The NSA sucks up much, much more.

The NSA say it needs all this data to help prevent another terrorist attack like 9/11. In order to find the needle in the haystack, they argue, they need access to the whole haystack.

Snowden recognises the value of the NSA in counter-terrorism, but thinks the spy agency has dangerously over-reached itself. He is a fugitive from US law, in exile in Russia. But the debate he wanted to start when he decided to become a whistleblower is now happening."