So GCHQ have released the first puzzle in this years Christmas crytopgraphy challenge, it's a 25 x 25 picture logic puzzle commonly known as a Nonogram, the aim of the puzzle is to identify cells in a given grid which either need colouring in or left blank based on the numbers at the sides of the grid. When completed successfully a nonogram puzzle usually reveals a hidden message or picture.

In this puzzle type, the numbers are a form of discrete tomography that measures how many unbroken lines of filled-in squares there are in any given row or column. For example, a clue of "4 8 3" would mean there are sets of four, eight, and three filled squares, in that order, with at least one blank square between successive groups.

If you've never tried solving a nonogram puzzle before I recommend starting with a few 5 x 5 grids and working your way up from there, the Nintendo DS has an exceedingly good game called Picross DS which teaches the concepts and is perfect for beginners. You can also find plenty of nonogram puzzle websites and smart phone applications to get you started if Picross DS is not for you.

You can get more information including the puzzle here.