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.