What better way to celebrate the holidays than a fiendishly difficult puzzle. Well, that’s what one of the UK’s intelligence agencies, GCHQ, reckons—which is why the Christmas card it’s been sending out features this infuriating puzzle. Can you solve it?


This grid-shading puzzle is, however, just the start. “By solving this first puzzle players will create an image that leads to a series of increasingly complex challenges,” the agency explains.

Well, better get started then. The full grid you need to shade is below, or you can access a high-res version here. GCHQ explains what you need to do:


“In this type of grid-shading puzzle, 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.”

Solutions to the full set of challenges can be submitted to GCHQ up until the end of January 2016. The email address you’ll need to send your answers seems to be revealed during the course of the puzzle-solving. Good luck!