See if you can solve these 6 fiendish brain-teasers written by NSA employees

Sure, between the spying on millions of Americans and the sitting on critical security vulnerabilities for years, the National Security Agency (NSA) has a pretty bad reputation.

But have you heard about its brain-teasers?

The US spy agency employs some of America's best and brightest as spooks and code-breakers, so it's no surprise that its employees might have an interest in riddles and puzzles.

Every month, the NSA publishes on its website a brain-teaser written by an employee that members of the public can try their hand at.

One month it's a maths challenge created by an applied research mathematician; the next it's a logic puzzle by a systems engineer. They're all published in what the NSA calls its "Puzzle Periodical."

"Intelligence. It's the ability to think abstractly. Challenge the unknown. Solve the impossible. NSA employees work on some of the world's most demanding and exhilarating high-tech engineering challenges. Applying complex algorithms and expressing difficult cryptographic problems in terms of mathematics is part of the work NSA employees do every day," the NSA says on its website.

We've rounded up a six of the most interesting brain-teasers below. So take a read, and see if you can out-smart the NSA's most fiendish riddlers!