A series of cryptic puzzles set by spy agency GCHQ on its Christmas card has yet to be cracked two days before the deadline.
About 30,000 people have reached the final stage of the challenge - roughly 5% of those who entered - but so far no one has successfully completed it.
GCHQ director Robert Hannigan, who set the string of teasers, offered a clue: "It's not as abstract as you think."
The deadline for entries is 31 January. Answers will be revealed in February.
The first puzzle, which was printed inside the agency's card of a traditional nativity scene and on its website, is a "nonogram" - a complex grid-shading puzzle.
When completed correctly, it creates a Quick Response, or QR, code which can be scanned to reveal a website link to the next challenge.
Nearly 600,000 people have successfully completed the nonogram stage of the puzzle so far.
Those who enjoyed the challenge are asked to make a donation to the National Society for the Prevention of Cruelty to Children (NSPCC).
Robert Hannigan, director of GCHQ, said he was delighted so many people had tried the puzzle and made a donation.
But he said: "With a few days to go no-one has cracked it all yet, so my one and only clue is: it's not as abstract as you think.
"What I hope the stages of the puzzle show is that to deliver our mission and keep Britain safe, we need people from all backgrounds, with all skills, who look at problems from every angle."
Aspiring codebreakers can have a go on the GCHQ website, and have until midnight on Sunday to complete the challenge.
How to play
- Each square is either black or white. Some of the black squares have already been filled in for you.
- 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 "2 1 6" indicates sets of two, one and six black squares, each of which will have at least one white square separating them
- Complete the grid with a black pen