When they aren't busy riffling through your deleted emails for terrorism buzz phrases, the sleuths at GCHQ love nothing more than a good puzzle.
A really good puzzle. The sort of thing that makes the cryptic crossword look like a colour-by-numbers job.
To demonstrate their softer side, the agency has revealed its 2015 Christmas Card - whose festive message will only be revealed once you've deciphered a code.
"In this type of grid-shading puzzle, each square is either black or white," explains GCHQ. "Some of the black squares have already been filled in for you. Each row or column is labelled with a string of numbers. The numbers indicate the length of all consecutive runs of black squares, and are displayed in the order that the runs appear in that line. For example, a label '2 1 6' indicates sets of two, one and six black squares, each of which will have at least one white square separating them."
But that's just the start of the 'fun' - revealing the image "leads to a series of increasingly complex challenges".
Should you enjoy the puzzle, GCHQ is inviting you to pass on your Christmas joy by donating to the NSPCC.
Let us know how you get on...
(Images: Shutterstock, GCHQ)
[Via: Mental Floss]