The NSA’s headquarters in Fort Meade, Maryland: The US foreign intelligence agency views all encrypted communications as a “threat” to its operations.

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 […]