A brain-teasing puzzle sent out in a Christmas card has hundreds of thousands of people stumped.
No one has been able to complete the puzzle put out as part of the UK's Government Communications Headquarters (GCHQ) traditional Christmas card.
GCHQ is a security and intelligence organisation tasked to protect the UK from threats and works closely with MI5 and MI6.
According to news.com.au the first stage of the challenge is a grid-shading puzzle and has been online since December.
The puzzle consists of a grid of 25 x 25 squares, each of which must be black or white.