It is the puzzle that has stumped would-be code-breakers the world over.
But, if you think you have the brains to battle the riddles that have defeated the minds of many others, you still have three days to put your wits to the test and solve it.
Back in December, Robert Hannigan, director of GCHQ – the UK government’s communications headquarters – included the challenge in his Christmas cards.
The puzzle was subsequently opened up to the public via GCHQ’s website, after the agency decided it deserved a wider audience, according to the Daily Telegraph.
The newspapers reports that since setting the quiz not one person, out of the 600,000 who have started it, has managed to crack each of its five stages successfully.
Once all five parts have been completed, participants can submit their answer to GCHQ via email.
The deadline is 31 January.
Details of Part 1 of the puzzle are given below.
In this grid-shading puzzle each square is either black or white. Some of the black squares have already been filled in. 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 of '2 1 6' indicates sets of two, one and six black squares, each of them will have at least one white square separating them. Complete the grid with a black pen.
Below is a taste of some of the puzzles in Part 5.
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