GCHQ has created its toughest ever puzzle in honour of Alan Turing, with puzzlers warned it will take seven hours to complete.
The challenge appears on a new £50 note revealed by the Bank of England, featuring the scientist and mathematician who pioneered modern computing.
But for those who haven't yet got hold of one yet, GCHQ has put the puzzle online too.
Turing was instrumental in breaking the German Naval Enigma cipher in 1942, at Bletchley Park (GCHQ’s wartime home) but was persecuted for his sexuality and died at just 41-years-old.
It will be the first time a banknote has ever featured a gay man.
Jeremy Fleming described Turing as "one of the most iconic LGBT+ figures in the world," adding: "his appearance on the £50 note is a landmark moment in our history."
"Turing was embraced for his brilliance and persecuted for being gay.
"His legacy is a reminder of the value of embracing all aspects of diversity, but also the work we still need to do to become truly inclusive."
The puzzles are based on the unique design elements of the new banknote, such as the technical drawings for the British Bombe - the machine designed by Turing to break Enigma-enciphered messages.
GCHQ officials said the full challenge could take an experienced puzzler seven hours to complete.
Colin, GCHQ's chief puzzler, said: "Alan Turing has inspired many recruits over the years to join GCHQ, eager to use their own problem-solving skills to help to keep the country safe.
"So it seemed only fitting to gather a mix of minds from across our missions to devise a seriously tough puzzle to honour his commemoration on the new £50 note.
"It might even have left him scratching his head – although we very much doubt it."
Turing joined the Government Code & Cypher School, GCHQ’s wartime name, in 1938 to help with the code-breaking effort during the Second World War, working alongside Gordon Welchman.
In January 1940, he had a meeting in Paris with Polish counterparts, who gave him the insights he needed to design the Bombe.
The combination of the Bombe and the brilliant minds and perseverance of those working at Bletchley Park led to the breaking of Enigma.
In January 1952, Mr Turing was prosecuted for "indecency" over his relationship with another man in Manchester.
He was given a choice between imprisonment and probation on condition of undergoing "chemical castration," he chose the latter.
In 1954, Turing was found dead at just 41-years-old. It is believed he took his own life.
Nearly 60 years later in 2013, Turing was given a posthumous royal pardon.