Britain's most secretive organisation - GCHQ - has added a cryptic twist to Christmas card season by including a baffling brainteaser.
This year spy agency director Robert Hannigan is sending out a complex grid-shading puzzle inside his traditional Christmas cards of the nativity scene.
Successful codebreakers will uncover an image in the grid that leads to a series of tougher challenges.
Those not on the card list can have a go here or on the GCHQ website.
Mr Hannigan is asking players who complete all the stages to submit their answer to GCHQ by the end of January.
Those who enjoyed the challenge are asked to make a donation to the National Society for the Prevention of Cruelty to Children.
How to play
- Each square is either black or white. 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
- Complete the grid with a black pen