Keen codebreakers can have a crack at a brainteaser this festive season in the form of a Christmas card challenge set by GCHQ’s director Robert Hannigan. The traditional grid-shading puzzle invites those who dare (or care) to fill in the squares black or leave them white depending on the pattern indicated by the numbers on each column and row. Each number indicates there should be a black square, with at least one white one left in between. Some have already been filled in to give you a head start. Once you’ve filled in all the squares, the image you’re left with leads…
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