When Christmas approaches, the spies of the Five Eyes intelligence services can look forward to a break from the arduous daily work of spying. In addition to their usual job -- attempting to crack encryption all around the world -- they play a game called the "Kryptos Kristmas Kwiz," which involves solving challenging numerical and alphabetical puzzles. The proud winners of the competition are awarded "Kryptos" mugs.

Encryption -- the use of mathematics to protect communications from spying -- is used for electronic transactions of all types, by governments, firms and private users alike. But a look into the archive of whistleblower Edward Snowden shows that not all encryption technologies live up to what they promise.

One example is the encryption featured in Skype, a program used by some 300 million users to conduct Internet video chat that is touted as secure. It isn't really. "Sustained Skype collection began in Feb 2011," reads a National Security Agency (NSA) training document from the archive of whistleblower Edward Snowden. Less than half a year later, in the fall, the code crackers declared their mission accomplished. Since then, data from Skype has been accessible to the NSA's snoops. Software giant Microsoft, which acquired Skype in 2011, said in a statement: "We will not provide governments with direct or unfettered access to customer data or encryption keys." The NSA had been monitoring Skype even before that, but since February 2011, the service has been under order from the secret US Foreign Intelligence Surveillance Court (FISC), to not only supply information to the NSA but also to make itself accessible as a source of data for the agency.