The Codebreaker’s Christmas Puzzle

I do not like everything done by the Government Communications Headquarters (GCHQ), Britain’s electronic comms spymasters, but I have to give them credit for the artful way they approach their task. Many large organizations send out greetings cards at this time of year, but only GCHQ would turn them into a clever way to recruit codebreakers. This year GCHQ’s cards contain an intriguing brainteaser, which leads to yet more complex puzzles. Presumably the prize for anyone who correctly answers them all is a job whose existence cannot be confirmed because it is covered by the Official Secrets Act.

If you fancy that job, or just want to see if you are the equal of Britain’s electronic spies, it does not matter if your name was not on GCHQ’s Christmas card list. The puzzle is also available from GCHQ’s website, or you could just copy the image above. The puzzle consists of a 25 by 25 grid, and instructions for which squares should be colored black. The numbers listed tell you the ordered sequence of consecutive blocks of black squares in each row and column. For example “2 1 6” means the row or column contains a block of two consecutive black squares, then a single black square, and then a block of six black squares, with at least one white square between each block. To get you started, a few black squares have already been filled in. Can you complete the rest, and work out the meaning of their pattern?

Wannabe codebreakers have until January 31st to solve all the puzzles and submit their answer to a particular GCHQ email address. Good luck if you enter, though I suppose we will never know if you won…

Eric Priezkalns
Eric Priezkalns
Eric is a recognized expert on communications risk and assurance. He was Director of Risk Management for Qatar Telecom and has worked with Cable & Wireless, T‑Mobile, Sky, Worldcom and others.

Eric was lead author of Revenue Assurance: Expert Opinions for Communications Providers, published by CRC Press. He was a founding member of Qatar's National Committee for Internet Safety and the first leader of the TM Forum's Enterprise Risk Management team. Eric currently sits on the committee of the Risk & Assurance Group, and is an editorial advisor to Black Swan. He is a qualified chartered accountant, with degrees in information systems, and in mathematics and philosophy.

Commsrisk is edited by Eric. Look here for more about Eric's history as editor.