Britain's spy agency GCHQ has sent a cryptic Christmas card

British spy agency GCHQ has revealed a cryptic Christmas card that includes a festive brainteaser.

The card, which was sent out by GCHQ director Robert Hannigan, is designed to “exercise the grey matter” of participants over the holiday season.

Players must complete a grid-shading puzzle to reveal an image that leads to a series of increasingly complex questions.

Here is the puzzle:

GCHQ grid shading puzzleGCHQThe GCHQ grid-shading puzzle leads to a festive greeting.

For those that don’t know where to begin, the GCHQ website states: “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.”

Upon finishing the puzzle, players are invited to send their answers to a GCHQ email address by January 31.

The card is only being sent to a select number of people on the spy chief’s contact list but others can download the puzzle here.

Players are invited to make a donation to the National Society for the Prevention of Cruelty to Children (NSPCC), if they enjoy the puzzle.

GCHQ is on the hunt for more staff after Prime Minister David Cameron announced the agency is hiring 1,900 more intelligence personnel to fight terrorism.

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