This season, Britain’s secretive spy agency the Government Communications Headquarters, or GCHQ, has sent out some rather peculiar Christmas cards. Instead of greetings full of merriment and joy, they presented a challenging puzzle for anyone bold enough to attempt to solve it. 

The puzzle consists of a grid with select blocks shaded in and series of numbers along each axis. Once code-breakers solve the initial problem, they’ll be left with an image that leads to "a series of increasingly complex challenges." The agency explains how to tackle the puzzle:

“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 labeled 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.”

If you weren’t lucky enough to make it onto GCHQ’s mailing this year, you can take a crack at the puzzle below or download a high resolution version by going to their website. The agency’s director is asking for anyone who completes the challenge to send in their work by the end of January 2016 so they can select a winner. Apparently the email address you need to direct your answers to is revealed in the puzzle, so you’ll know when you’ve qualified. The GCHQ also asks that anyone participating make a donation to the National Society for the Prevention of Cruelty to Children this holiday season.