What better way to celebrate the holidays than to agonize over a really difficult brainteaser? Britain’s spy agency, better known as GCHQ, has stuck a puzzle in its Christmas card this year.

The Christmas card contains a grid-shading puzzle with instructions on how players should complete it. Once players solve the first puzzle, they will see an image that leads them to a series of increasingly more complex challenges. GCHQ explains how to get started:

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.

The spy agency will go through all the entries and announce the winner at a later date. If you’re interested in taking part, you can download the puzzle here. Those who think they’ve cracked the code are asked to submit their answers to GCHQ by Jan. 31, 2016.