We may joke about the government watching our every move online, but for some U.K. residents, it's reportedly a reality.

According to The Intercept, which cited documents obtained by former NSA contractor Edward Snowden, the U.K.'s version of the NSA—the Government Communications Headquarters (GCHQ)—has been collecting a massive amount of data on Web users in the country.

Starting about seven years ago, GCHQ quietly began collecting data about "ordinary people's online activities," The Intercept said, from what news sites they visited to the porn they watched.

The effort, dubbed Karma Police, scraped the Web for information stored in a database known as Black Hole. By 2012, that database was amassing about 50 billion metadata records per day, and Karma Police "would soon be the biggest government surveillance system anywhere in the world," The Intercept said.

As the report notes, Karma Police wanted to develop "a Web browsing profile for every visible user on the Internet or...a user profile for every visible website on the Internet."

One aspect of Karma Police was identifying Internet radio stations broadcasting readings from the Quran. GCHQ then drilled down to figure out who the stations' listeners were, based on IP addresses, and tried to find them on Skype or social networks.

How? Analysts dropped IP addresses into another system known as Mutant Broth, which perused Black Hole for Internet cookies that matched up with those IP addresses.

"If the agency wants to track down a person's IP address, it can enter the person's email address or username into Mutant Broth to attempt to find it, scanning through the cookies that come up linking those identifiers to an IP address," The Intercept said. "Likewise, if the agency already has the IP address and wants to track down the person behind it, it can use Mutant Broth to find email addresses, usernames, and even passwords associated with the IP."

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Mutant Broth was one of the tools GCHQ (and the NSA) used to hack SIM card maker Gemalto and telecom firm Belgacom, the site said.

Earlier this year, U.K. authorities called for a crackdown on illegal spying, but as The Intercept noted, GCHQ "operates a bewildering array of other eavesdropping systems," so Karma Police, Black Hole, and Mutant Broth are just one part of the puzzle.

Karma Police is the name of a well-known Radiohead song, which includes lines like "This is what you'll get / When you mess with us" and "For a minute there / I lost myself," but it's not clear if the effort was named after the song.

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