GCHQ Christmas card features 'cryptographic challenge'
GCHQ has released a head-scratching Christmas card to keep puzzle fans busy over the holidays.
Instead of the traditional Santa and reindeer, Britain's intelligence and security agency has designed a festive card featuring a cryptographic challenge.
Participants have to fill in a grid-shading puzzle to unveil a picture, which is the first in a series of increasingly complex challenges.
The competition will also raise funds for the NSPCC, with the winner to be announced after January 31 2016.
GCHQ said: "In his Christmas card this year, director GCHQ Robert Hannigan has included a cryptographic challenge that should exercise the grey matter over the festive season.
"Participants who enjoy the challenge are invited to make a donation to the NSPCC, which is the director's chosen charity this year."