New documents released by whistle-blower Edward Snowden and reported in German newspaper Der Spiegel show some forms of encryption were still causing trouble for the NSA as recently as two years ago.
Encryption is a secure way of protecting sensitive information. Several services claim to be encrypted, but that has not always halted NSA surveillance of communications technologies such as Skype, according to the documents cited by Der Spiegel.
The newspaper reports the NSA has "major" problems de-coding the email service Zoho, the browser Tor, and Truecrypt, a programme to encrypt files on computers.
They also have trouble with ZRTP, a system used to encrypt phone calls, and CSpace, a peer-to-peer online messaging service.