Over half a million tried but Artificial Intelligence and Mathematics graduate David McBryan is one of only three people who came close to solving the brain teaser.
GCHQ is a security and intelligence organisation tasked by Government to protect the UK from threats, including cyber attacks. In December Director Robert Hannigan issued a traditional Christmas card with a difference - it contained a grid-shading puzzle which, when solved, led to a series of increasingly complex challenges.
Players were invited to make a donation to the National Society for the Prevention of Cruelty to Children if they enjoyed the puzzle.
Dave studied here between 1992-6 and has since gone on to make a living out of quizzes.
Speaking in 2011 as a finalist in BBC Competition, The Code, he said:
“I'm originally from Dublin but came to university in Edinburgh 19 years ago, and loved the city so much that I never left. Although my degree was in Maths and Artificial Intelligence, ever since graduation I've made a living as a writer and presenter of pub quizzes.”
He went on to win the Channel 4 TV game show Fifteen to One and now writes questions for the programme.