Nearly 5 years ago, whistleblower Edward Snowden leaked documents that exposed how the National Security Agency (NSA) conducts widespread global surveillance on the masses.

The classified documents Snowden leaked also reveal that the NSA took a particular interest in Bitcoin users, according to a new report by The Intercept:

It turns out the conspiracy theorists were onto something. Classified documents provided by whistleblower Edward Snowden show that the National Security Agency indeed worked urgently to target Bitcoin users around the world — and wielded at least one mysterious source of information to “help track down senders and receivers of Bitcoins,” according to a top-secret passage in an internal NSA report dating to March 2013. The data source appears to have leveraged the NSA’s ability to harvest and analyze raw, global internet traffic while also exploiting an unnamed software program that purported to offer anonymity to users, according to other documents.

Bitcoin wasn’t the only cryptocurrency the agency was interested in surveilling, but it was a “#1 priority”, according to a March 15, 2013 internal NSA report.

“Tracking down” Bitcoin users went far beyond closely examining Bitcoin’s public transaction ledger (known as the Blockchain, where users are typically anonymous), The Intercept reports:

…the tracking may also have involved gathering intimate details of these users’ computers. The NSA collected some Bitcoin users’ password information, internet activity, and a type of unique device identification number known as a MAC address, a March 29, 2013 NSA memo suggested. In the same document, analysts also discussed tracking internet users’ internet addresses, network ports, and timestamps to identify “BITCOIN Targets.”

The March 29 memo also revealed the NSA wanted even more data. The memo raised the question of whether the data source validated its users. It suggested that the agency retain Bitcoin information in a file named “Provider user full.csv.”

It also suggested powerful search capabilities against Bitcoin targets, hinting that the NSA may have been using its XKeyScore searching system, where the Bitcoin information and wide range of other NSA data was cataloged, to enhance its information on Bitcoin users. An NSA reference document indicated that the data source provided “user data such as billing information and Internet Protocol addresses.” With this sort of information in hand, putting a name to a given Bitcoin user would be easy.

It appears that the NSA’s Bitcoin spying operation was enabled by the agency’s ability to siphon traffic from physical internet cable connections. As of 2013, the agency used a program called OAKSTAR – “a collection of covert corporate partnerships enabling the agency to monitor communications, including by harvesting internet data as it traveled along fiber optic cables that undergird the internet” – to track Bitcoin, according to The Intercept report.

The NSA targeted Bitcoin through a sub-program of OAKSTAR called MONKEYROCKET. According to classified descriptions, MONKEYROCKET tapped network equipment to gather data from the Middle East, EuropeSouth America, and Asia.

Matthew Green, assistant professor at the Johns Hopkins University Information Security Institute, told The Intercept that he believes the “browsing product” component of MONKEYROCKET sounds a lot like a virtual private network, or VPN:

VPNs encrypt and reroute your internet traffic to mask what you’re doing on the internet. But there’s a catch: You have to trust the company that provides you a VPN, because they provide both software and an ongoing networking service that potentially allows them to see where you’re going online and even intercept some of your traffic. An unscrupulous VPN would have complete access to everything you do online.

Green added that the NSA’s interest in cryptocurrency is “bad news for privacy, because it means that in addition to the really hard problem of making the actual transactions private … you also have to make sure all the network connections [are secure].” He said he is “pretty skeptical” that using Tor, the popular anonymizing browser, could thwart the NSA in the long term.

In other words, even if you trust Bitcoin’s underlying tech (or that of another coin), you’ll still need to be able to trust your connection to the internet — and if you’re being targeted by the NSA, that’s going to be a problem.

In 2017, Snowden called the privacy-oriented cryptocurrency zcash the “most interesting alternative” to Bitcoin on Twitter:

It should come as no surprised that the government is searching for ways to regulate (and possibly, ban) the use of cryptocurrencies. Earlier today, we reported on the case of long-time Bitcoin enthusiast and CEO of Bitcoin, Inc., Morgan Rockcoons, who is facing money laundering charges relating to a Bitcoin transaction. And, earlier this month, the Securities and Exchange Commission (SEC) publishedwarning on “potentially unlawful” online platforms used to trade digital assets. The federal government needs to have a say in how the trading of digital assets operates, because without an official hand in the game, it risks obsolescence.

To read the entire report from The Intercept and view the related leaked documents, please see The NSA Worked to “Track Down” Bitcoin Users, Snowden Documents Reveal.