'Citizenfour' NSA leaker Edward Snowden. It's not easy to be truly anonymous online. Sure, there are plenty of chat apps and secret-sharing sites that claim to offer you privacy, but it's tricky to know whether US intelligence agencies have a backdoor to access them.

The best way to stay anonymous online has been to use Tor, a special kind of web browser developed to help US government employees hide their tracks online.

But if you want to be properly anonymous, you need a combination of extra services and websites on top of Tor to avoid detection. Plenty of online guides offer advice on the subject, but it's always been hit and miss.

Now, leaked NSA documents provide a big clue on how to remain hidden. They show that the agency has trouble breaking certain methods of encryption.

Der Spiegel published a collection of documents that detail what systems the NSA has troubling decrypting. Previous documents have focused mainly on what the NSA is good at, not what it finds difficult.

The documents reveal that the NSA ranks targets according to how difficult they are to decrypt. There are five internal levels: One to five. Level one is known as "trivial," meaning it's pretty easy for the NSA to track targets or decrypt messages. But level five is "catastrophic," which essentially means that the NSA can't break the encryption.

The NSA says that reading someone's Facebook message is a level two "minor" task. And monitoring people using Tor is tricky, with the NSA classing that as a "major" level four problem.

So any level of anonymity classed as level five, known as "catastrophic," means that the NSA will find it nearly impossible to break.

The NSA identifies one anonymity method that it warns is virtually impossible to break. Here's the method outlined in the leaked documents:

Tor

VPN

CSpace

ZRTP

Let's break that down.

Tor is the special kind of web browser that helps people stay anonymous online by encrypting their web traffic.

A VPN is a service that makes an internet connection more secure, using proxy servers to hide their real-world location.

CSpace is a kind of anonymous internet chat service that uses heavy encryption to protect any files sent over its network.

ZRTP, the last part of the method, is a kind of encryption for voice calls and text chats.

Combine the above stack of services together, using multiple kind of encryption, a special web browser, and a service to hide your location, and the NSA says in its internal documents that it probably won't be able to read your messages. The leaked NSA document says that the encryption method results in a "near-total loss/lack of insight to target communications, presence."



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