Wannabe code-breakers are being challenged by the UK's national intelligence and security agency in a new puzzle book backed by the Duchess of Cambridge.
GCHQ is releasing its first puzzle collection, which includes a foreword written by Kate, whose grandmother was a Bletchley Park code-breaker during the Second World War.
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 the book's foreword, the Duchess writes that she is "immensely" proud of her grandmother, Valerie Glassborow, and her twin sister, Mary, who were part of the Allied effort to break enemy codes.
She said: "They rarely talked about their wartime service, but we now know just how important the men and women of Bletchley Park were, as they tackled some of the hardest problems facing the country.
"In a new century, their successors at GCHQ continue this intellectual tradition. Like their Bletchley predecessors, they have become well known for valuing neurodiversity and understanding the importance of mental well-being."
All GCHQ proceeds from the book will be donated to Heads Together, the campaign spearheaded by the Duke and Duchess of Cambridge and Prince Harry, to tackle stigma, raise awareness and provide vital help for people with mental health challenges.
Kate added of the compendium: "I hope it will not only amuse and challenge readers, but help to promote an open discussion of mental health problems, which can affect anyone, regardless of age or background."
The brain-teaser will feature more than 140 pages of codes, puzzles and challenges created by GCHQ's expert code-breakers 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.
Writing in the book's introduction, Mr Hannigan said: "For nearly 100 years, the men and women of GCHQ, both civilian and military, have been solving problems.
"They have done so in pursuit of our mission to keep the United Kingdom safe.
"GCHQ has a proud history of valuing and supporting individuals who think differently; without them we would be of little value to the country. Not all are geniuses or brilliant mathematicians or famous names, but each is valued for his or her contribution to our mission."
::The GCHQ Puzzle Book, published by Penguin, will go on sale on Thursday October 20.