Once all stages have been unlocked and completed successfully, players are invited to submit their answer via a given GCHQ email address by 31 January 2016. The winner will then be drawn from all the successful entries and notified soon after. Players are invited to make a donation to the National Society for the Prevention of Cruelty to Children, if they have enjoyed the puzzle.
In this type of grid-shading puzzle, 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.
This app won't tell you when the puzzle is complete, that is for you to work out yourself!