Have a go at solving this Christmas cryptographic problem

If you get bored over the festive period, you might want to have a go at cracking a challenge that has been set by the director of GCHQ. The web page starts:

"This year, along with his traditional Christmas cards, Director GCHQ Robert Hannigan is including a brain-teasing puzzle that seems certain to exercise the grey matter of participants over the holiday season.

The card, which features the 'Adoration of the Shepherds' by a pupil of Rembrandt, includes traditional Christmas greetings from Director on behalf of the department. However, unlike previous years ...."

Good luck.

[This post is part of a series of posts that I am publishing in the run up to Christmas 2015 (you can see my LinkedIn posts here). They fcous on Research and Knowledge Exchange that has a Christmas theme. If you have any examples, please let me know.]