Can you solve Britain's spies' puzzle?
Britain's spies have given the public an early Christmas gift - a puzzle, and it's mind-bendingly hard.
Instructions are included on how the brain teaser should be completed.
"By solving this first puzzle players will create an image that leads to a series of increasingly complex challenges," the GCHQ says.
"Once all stages have been unlocked and completed successfully, players are invited to submit their answer via a given GCHQ email address by 31 January 2016.
"The winner will then be drawn from all the successful entries and notified soon after.
"Players are invited to make a donation to the National Society for the Prevention of Cruelty to Children, if they have enjoyed the puzzle."
In the grid-shading brain teaser, each square is either black or white. Some of the black squares have already been filled in, the GCHQ says.
"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".
Do you think you can complete the puzzle? Try it below or click here for a link to a bigger version.