Consider calls to single-purpose hotlines: NSA collection of our metadata means the government knows when we’ve called a rape hotline, a domestic violence hotline, an addiction hotline, or a support line for gay teens. Hotlines for whistleblowers in every agency are fair game, as are police hotlines for “anonymous” reports of crimes. Charities that make it possible to text a donation to a particular cause (say, Planned Parenthood) or political candidate or super PAC could reveal an enormous amount about our political activities. And calling patterns can reveal our religious beliefs (no calls on Sabbath? Heaps of calls on Christmas?) or new medical conditions. If, for instance, the government knows that, within an hour, we called an HIV testing service, then our doctor, and then our health insurance company, they may not “know” what was discussed, but anyone with common sense—even a government official—could probably figure it out.

But there’s more, says Felten: By analyzing our metadata over time, the government can separate the signal from the noise and use it to identify behavioral patterns. The government can determine whether someone is making lots of late-night calls to someone who isn’t his spouse, for example. When those calls cease, the government might reasonably conclude that the affair has ended. Metadata may reveal whether and how often someone calls her bookie or the American Civil Liberties Union or a defense attorney. And by analyzing the metadata of every American across a span of years, the NSA could learn almost as much about our health, our habits, our politics, and our relationships as it could by eavesdropping on our calls. It’s not the same thing, but the more data the government collects, the more the distinction between metadata and actual content disappears.

Seventy of the world's leading human rights organisations have written to David Cameron to warn that the government's reaction to the mass surveillance revealed by Edward Snowden is leading to an erosion of fundamental rights and freedoms in the UK.

The coalition, which includes organisations from 40 countries, said it had become increasingly alarmed at the way the UK government had applied pressure on media groups covering the leaks and its use of national security concerns to close down important public interest debates.

"We have joined together as an international coalition because we believe that the United Kingdom government's response to the revelations of mass surveillance of digital communications is eroding fundamental human rights in the country," the letter states. "The government's response has been to condemn, rather than celebrate investigative journalism, which plays a crucial role in a healthy democratic society."

The intervention comes five months after the Guardian, and major media organisations in other countries, including the New York Times and the Washington Post, began disclosing details of the extent and reach of secret surveillance programmes run by Britain's eavesdropping centre, GCHQ, and its US counterpart, the National Security Agency. The revelations – now appearing in European media outlets – have sparked a huge debate on the scale and oversight of surveillance by the US and UK intelligence agencies.