GCHQ Christmas card puzzle winners announced
Three people have been declared winners of GCHQ's Christmas puzzle challenge.
More than 600,000 people tried to solve the puzzle series, with 30,000 making it to the final round of the challenge.
David MacBryan, 41, from Edinburgh, US-born Kelley Kirklin, 54, from London, and Wim Hulpia, 40, from Lovendegem in Belgium, came closest to completing the series of puzzles.
They win a GCHQ paperweight, a pen and a signed copy of Alan Turing Decoded, by Turing's nephew Dermot Turing.
Mr MacBryan, who is originally from Dublin and describes himself as a professional quiz master, told the BBC: "The more puzzles you do, the better you get at doing puzzles - and I have done a lot of puzzles. I am a bit of an addict."
The first stage of the puzzle series appeared on the Christmas card sent out by GCHQ director Robert Hannigan.
Filling in squares produced a Quick Response, or QR code, which when scanned took people to a website with the subsequent stages.
The puzzles were developed by a team from GCHQ - working in their spare time out of the office, they stress - over a period of months.
"I don't think there's any coincidence that you find a bunch of people setting these sorts of problems who are also working on the kind of problems GCHQ works on," one of the team - who requested anonymity - told the BBC.
"We are faced with problems where there is incomplete information, there is ambiguous information, you have to decide whether you have pushed an angle far enough… that's the sort of thing we expect people to do with these sort of questions."
Another of the quiz-setters explained how one of the puzzles was solved - it involved not just interpreting a series of letters as Roman numerals to solve an equation, but also recognising that the Xs could be multiplication symbols rather than just representing the number 10.
Another puzzle involved arranging squares to get the first word of the title of films consisting of two words.
Arrow and dice surrounding the word point you to one of the letters from the second word of the film title.
When you have all of these letters they reveal the answer. The full set of answers will be published on GCHQ's website.