Amateur code-breakers are being asked to earn their "licence to puzzle" in a competition to mark the launch of a new book from spy agency GCHQ.
The Government's listening post, based in Cheltenham, is releasing its first puzzle collection on Thursday, as well as a Sudoku and picture board conundrum for its competition.
The GCHQ Puzzle Book includes a foreword written by the Duchess of Cambridge, whose grandmother was a Bletchley Park code-breaker during the Second World War, and will raise money for Heads Together, the mental health awareness campaign spearheaded by Kate, her husband the Duke of Cambridge and his brother Prince Harry.
The brain-teaser will feature more than 140 pages of codes, puzzles and challenges created by GCHQ's experts in their spare time.
These will range from "easy" to "mind-bending" and include ciphers and substitution codes, tests of numeracy and literacy, picture and music challenges.
It follows the success of the agency's Christmas cryptography challenge, which saw some 600,000 people attempt to beat what was dubbed the "hardest puzzle in the world".
Nobody managed to entirely solve the series of challenges set by GCHQ director Robert Hannigan in his Christmas card last year, although three men who came closest were declared joint winners.
In announcing the latest contest, GCHQ dropped a hint, saying: "To start the competition, players are challenged to a form of Sudoku and a picture board of famous people - but in what shouldn't be a spoiler, this is a multi-stage competition.
"Solving these two grids unlocks a series of puzzles, challenges and instructions that will bend and twist the minds of players in their quest for their 'licence to puzzle'."
The closing date for the competition, which is only open to those who buy the book, is Tuesday February 28 2017.