GCHQ Puzzle For Alan Turing £50 Note
Written by Sue Gee   
Thursday, 25 March 2021

The final design of the banknote featuring Alan Turing has been unveiled by the Bank of England. GCHQ, the UK's intelligence and security organization, which has a tradition off setting puzzles, has come up with a Turing Challenge based on aspects of Turing's work featured on the £50 note.


Being featured on a bank note is one of the highest accolade's a person can have. Better still when it is done with the involvement of the public. When it was announced that the new £50 note would commemorate the field of science, the Bank of England received a total of 227,299 nominations, covering 989 eligible characters. This was reduced to a shortlist of 12 options, from which the then Governor of the Bank of England, Mark Carney made the final selection, stating in 2019:

"Alan Turing was an outstanding mathematician whose work has had an enormous impact on how we live today. As the father of computer science and artificial intelligence, as well as war hero, Alan Turing's contributions were far-ranging and path-breaking. Turing is a giant on whose shoulders so many now stand.” 

By the time he made his announcement a design for the note, incorporating many different aspects of his pioneering work, had already already been drawn up, see my report Alan Turing On £50 Note.

Today, 90 days before its release, we can see the final banknote. It will come into circulation on June 23rd, 2021 - the date of course being the anniversary of Alan Turing's birth in 1912.

The new note is polymer and has several security features that are detailed by the Bank of England but it is the Turing related features that are of interest.

Alan Turing could be considered a brave choice for the £50, given that as well as being a brilliant mathematician and polymath he also contravened the societal norms of his time but the Bank of England's potted biography doesn't try to avoid the issue:

Alan Turing provided the theoretical underpinnings for the modern computer. While best known for his work devising code-breaking machines during WWII, Turing played a pivotal role in the development of early computers first at the National Physical Laboratory and later at the University of Manchester. He set the foundations for work on artificial intelligence by considering the question of whether machines could think. Turing was homosexual and was posthumously pardoned by the Queen having been convicted of gross indecency for his relationship with a man. His legacy continues to have an impact on both science and society today.

Personally I think it is fitting that the note has both his portrait - based on a photo which the Bank of England tells us is based on a photo by Elliott & Fry which is part of the Photographs Collection at the National Portrait Gallery, and that of Queen Elizabeth II who came to the throne during his lifetime and who personally signed the warrant to pardon him under the Royal Prerogative of Mercy.

The Bank of England also itemizes the following features of the artwork which celebrate Turing's pioneering work with computers: 

  • A mathematical table and formulae from Turing’s seminal 1936 paper “On Computable Numbers, with an application to the Entscheidungsproblem” Proceedings of the London Mathematical Society. This paper is widely recognised as being foundational for computer science.
  • The Automatic Computing Engine (ACE) Pilot Machine which was developed at the National Physical Laboratory as the trial model of Turing’s pioneering ACE design. The ACE was one of the first electronic stored-program digital computers.
  • Ticker tape depicting Alan Turing’s birth date (23 June 1912) in binary code. 
  • Technical drawings for the British Bombe, the machine specified by Turing and one of the primary tools used to break Enigma-enciphered messages during WWII. 
  • The flower-shaped red foil patch on the back of the note is based on the image of a sunflower head linked to Turing’s morphogenetic (study of patterns in nature) work in later life.
  • A series of background images, depicting technical drawings from The ACE Progress Report.


All these features and more are the basis of GCHQ's Turing Challenge. Billed as the GCHQ's:

"toughest puzzle to celebrate Alan Turing featuring on the £50 banknote"

the challenge requires you to solve a set of puzzles, all of which relate in some way to features on the bank note and which increase in difficulty. The first is a simple word game with very straightforward clues, the second requires a knowledge of Morse code. We I reached the third I realized I would need help in working out what I was required to do - and the site does say:

Keep an eye on our Twitter and Instagram channels throughout the day for tips and hints.

As the entire set of puzzles requires some seven or eight hours to complete, I had the choice of doing them or sharing news about them so that those who like brain teasers can join in the fun. Suffice it to say that the solution to each of the first 11 puzzles is a single word or name and only when you have all of thse can you move to the final step - the Enigma, described as a meta-puzzle in which you are instructed to:

Cash in your answers in increasing order and take careful note of your change because they form an enciphered message.

To decipher this message you are directed to a virtual Enigma machine that uses CyberChef - an open source tool downloadable from GitHub that we first met in 2019. The Enigma Machine has already been set up so all you have to do here is to type in the message as the input and see the reveal as the output!    


More Information

Bank of England £50 Note

Turing Challenge

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Last Updated ( Friday, 26 March 2021 )