Explosive new reporting by The Intercept published Thursday, based on documents obtained by NSA whistleblower Edward Snowden, reveals how the U.S. spy agency and their British counterpart, the GCHQ, worked together in order to hack into the computer systems of the world's largest manufacturer of cell phone SIM cards – giving government spies access to highly-guarded encryption codes and unparalleled abilities to monitor the global communications of those with phones using the cards.

Following its publication, journalist Glenn Greenwald called it "one of the biggest Snowden stories yet."

According to fellow journalists Jeremy Scahill and Josh Begley, who did the reporting on the top-secret documents and detail the implications of the program, the target of the government hacking operation was a company called Gemalto, based in the Netherlands, which makes SIM cards for some of the best known makers of cell phones and other portable electronic products, including AT&T, T-Mobile, Sprint, and hundreds of other global brands. The acronym SIM stands for "subscriber identity module" and is a small intergrated circuit within a phone that is used to authenticate users and relay key information to the network on which the phone is operating.

As Scahill and Begley report:

With these stolen encryption keys, intelligence agencies can monitor mobile communications without seeking or receiving approval from telecom companies and foreign governments. Possessing the keys also sidesteps the need to get a warrant or a wiretap, while leaving no trace on the wireless provider’s network that the communications were intercepted. Bulk key theft additionally enables the intelligence agencies to unlock any previously encrypted communications they had already intercepted, but did not yet have the ability to decrypt. As part of the covert operations against Gemalto, spies from GCHQ — with support from the NSA — mined the private communications of unwitting engineers and other company employees in multiple countries.

In a series of tweets, both Scahill and Greenwald offered context for the latest reporting:

NEW: One of the biggest Snowden stories yet: NSA/GCHQ hacked into company producing SIM cards for cellphones https://t.co/a4tajJ3WVn — Glenn Greenwald (@ggreenwald) February 19, 2015

The NSA & GCHQ covertly stole millions of encryption keys used to protect your mobile phone communications: http://t.co/dVjLuxl4k3 — jeremy scahill (@jeremyscahill) February 19, 2015

This is basically what the NSA & GCHQ are doing to cell phone "privacy" http://t.co/dVjLuxl4k3 pic.twitter.com/9ovQvJdzNs — jeremy scahill (@jeremyscahill) February 19, 2015

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Remember how Obama says NSA only monitors private comms of bad guys? Yeah, that's BS. They cyberstalk engineers http://t.co/dVjLuxl4k3 — jeremy scahill (@jeremyscahill) February 19, 2015

"People were specifically hunted & targeted by intel agencies, not b/c they did anything wrong, but b/c they could be used" -- @csoghoian — jeremy scahill (@jeremyscahill) February 19, 2015

This top secret document is so damn creepy. Look at how they spied on innocent people working for a SIM card company https://t.co/vtyWP9ed1o — jeremy scahill (@jeremyscahill) February 19, 2015

For its part, Gemalto told The Intercept it was totally unaware of the security breach or that the encryption keys to any of its cards had been compromised. In fact, after being reached for comment on the operation, Gemalto directed its own security team to investigate the situation, but told the journalists they could find no trace of the hack. However, according to the top-secret document detailing the program leaked by Snowden, an operative with the NSA boasted, "[We] believe we have their entire network."

Technology experts who spoke with Scahill and Begley said the theft of the encryption keys was highly troubling. Christopher Soghoian, the principal technologist for the American Civil Liberties Union, said the idea that the NSA has stolen these encryption keys "will send a shock wave through the security community.”

Told about the program, Gerard Schouw, a member of the Dutch Parliament, said the revelation was "unbelievable." And repeated: "Unbelievable."

According to The Intercept: