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Metadata is back in the news, following The Guardian's extraordinary revelation on Wednesday revealing that the National Security Agency has been secretly scooping up the phone records of millions of Americans. But intelligence officials – echoed by President Obama today, who characterized access to metadata a "modest encroachment" on privacy – are implying that the information they're collecting is relatively innocuous, since they don't listen in on the actual phone conversations.

In an , Ben Wizner and I explain why government access to metadata – which reveals whom you talked to, from where, and for how long – is a gross privacy invasion:

A Massachusetts Institute of Technology study a few years back that reviewing people's social networking contacts alone was sufficient to . Consider, metadata from was sufficient to of then-CIA Director David Petraeus and then drive him out of office.

The "who," "when" and "how frequently" of communications are often more revealing than what is said or written. Calls between a reporter and a government whistleblower, for example, may reveal a relationship that can be incriminating all on its own.

Repeated calls to Alcoholics Anonymous, hotlines for gay teens, abortion clinics or a gambling bookie may tell you all you need to know about a person's problems. If a politician were revealed to have repeatedly called a phone sex hotline after 2:00 a.m., no one would need to know what was said on the call before drawing conclusions. In addition sophisticated data-mining technologies have compounded the privacy implications by allowing the government to analyze terabytes of metadata and reveal far more details about a person's life than ever before.

Read the piece in full . And check out this blog post for more on why "metadata is king."

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