Times readers who fancy themselves as amateur codebreakers are invited to decipher a 350-year-old message sent by French spies conniving to strike a killer blow against England’s allies in Europe. Only one man, a 17th-century master cryptographer, has ever been able to crack the message but his method remains a mystery because he refused to say how he did it.

John Wallis, whose breaking of the French codes was pivotal in thwarting England’s longstanding enemy, shared his techniques only with his son and grandson, who took the secret to their graves. He claimed he was obliged to guard his methods to prevent rival European powers from improving their codes but he also knew that his knowledge ensured that those in power would always need him.

Philip Beeley, editor of Wallis’s correspondence at Linacre College, Oxford University, has called upon Times readers to use their skills to break the code so that Wallis’s techniques may be revealed for the first time.

The letter, which survives in the archives of the British Library, consists of a series of two and three-digit numbers, each of which corresponds to a French word. For example, 269 is roi, the French word for king. Another common number, 121, is de, meaning of. Longer words may be represented by a series of numbers for each syllable. The French spies who sent the letter from Poland to France in 1689 encoded only sensitive parts of the letter. Unbeknown to the French, it was intercepted at a post office in Brandenburg, Prussia, where German agents unsealed it. European powers often employed spies in “black chambers” attached to post offices so that diplomatic correspondence could be decoded, but Brandenburg was ill-equipped for the task. The Germans copied the cipher and sent it to England in the hope that English cryptographers would be able to break it. They then resealed the letter and sent it on its way so that the French would not suspect any foul play.

John Wallis, who was considered the best codebreaker of his day, was assigned to the task, and he quickly discovered that the French were conspiring with Poland to attack Prussia. Louis XIV of France had promised the Poles 6,000 soldiers and 2,500 cavalry to attack Brandenburg, a move that would destabilise the fragile alliance on the Continent.

Dr Beeley said that Wallis’s brilliance allowed England and her allies to thwart the French and embarrass them. “We don’t know how he cracked it,” he said. “When you’re confronted by these numerical codes, you have no idea what you have got to do. The letter was incriminating for Louis XIV. The French ambassadors were thrown out in disgrace and that was the end of this plot.”

William of Orange, who ascended to the English throne a few months before the letter was intercepted, would have been exultant that he had foiled France’s ambitions to become the dominant Christian power in Europe. “Had Wallis failed, it would have created havoc in northern Europe, dangerously close to England,” Dr Beeley said.

Wallis, a mathematician and linguist who is believed to have been the first person to teach a deaf-born child to speak, was a wily operator who thrived despite working for opposing political masters. He was principal decipherer for Oliver Cromwell’s forces during the English Civil War but also worked for Charles II after the Restoration and for William of Orange.

He passed his methods on to his son, also called John Wallis, and then to his grandson, William Blencoe, but the expertise was lost when Blencoe had a fit of insanity and shot himself.

The challenge for Times readers is to crack the code and explain how they did it. “Wallis handed down copies of the ciphers he worked on and their solutions so it is clear what has to come out at the end,” Dr Beeley said. “The problem is knowing how he got there, so if Times readers could have a go then it could potentially be very useful for research.”

Part of the encrypted letter and the solutions in French and English, are printed below although readers will be asked to explain their working.

Amateur codebreakers who think they have mastered the code are invited to send an e-mail to: j.malvern@thetimes.co.uk

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