This is the grid shading puzzle that was printed on Christmas cards sent out for the 2015 season by Britain's intelligence agency, GCHQ.

I've created this site as a way to play the puzzle without pen and paper.

For explanation and rules, check out the original puzzle press release at GHCQ's home page .

Version specific instructions:

• Tap a grid cell to toggle its shading.
• Use a long-tap to toggle a cell's lock. This allows you to lock in cells that you're certain about, so that you don't accidentally change them.
• When you've met the requirements for a row or column, the label will turn green. When all rows and columns are green, you've won!