By cracking the code of a PoW's wartime diary, a mathematician gave a woman a glimpse into her husband's torment and passion. Karen Gold reports

In the dining room of the Savoy Hotel, still standing among the blitzed and burnt-out debris of London in 1946, RAF officer Donald Hill stood amazed at his sumptuous wedding breakfast. His new wife, Pamela, recently reunited with him after seven years of wartime separation, caught a glimpse of how scarce food must have been in the Japanese prisoner-of-war camp.

"Was it very hard in the camp?" she asked him. "I kept a diary for some of the time," he told her. "It's in code. I'll show it to you one day." Yet within months of their honeymoon, the cracks in Donald's fragile mental recovery from long imprisonment began to show. As Pamela recounted many years later to Andro Linklater, who described the Hills' story in the book The Code of Love, first Donald had a breakdown, then she herself began a gradual descent into alcoholism.

They had children, they fought, they were estranged and divorced. Just before Donald died in 1985, they were reconciled and he told her that he had always loved her. But he never did reveal what had happened to him during the war or what secrets were in his diary. And they would have remained secrets for ever, had it not been for a senior mathematics lecturer at Surrey University.

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Early in 1996, the maths department at Surrey received a telephone inquiry. Colonel Ian Quayle of the Soldiers', Sailors' and Airmen's Families' Association was calling on behalf of a widow, Pamela Hill, who lived in Tunbridge Wells. She had an old exercise book full of mathematical tables, which she claimed was her dead husband's coded wartime diary. Philip Aston just happened to be the mathematician who took the call. His field was chaos and bifurcation theory; he had never worked with codes. But he was intrigued by the request and agreed to see what he could do.

In a few days, 12 photocopied pages of the diary arrived in the post. Each page was covered in a pencil-drawn grid of boxes, and each box held four digits. Each grid had 33 rows across and 34 columns down. At the top they were headed "Russel's Mathematical Tables", which, Mrs Hill said, was Donald's way of disguising his diary-keeping, an activity forbidden to soldiers in wartime. There was nothing that looked like a key.

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Aston's first move was to get books about codes from the university library. The diary was, after all, written by a former accountant with a pencil and paper in a prison camp. "Today, with computers, these things are very high-tech," Aston says. "But this was 50 years ago, and as far as I knew he was doing it as a kind of hobby. I couldn't believe it would be very difficult to crack."

The books gave him the basic principles. Most numerical codes use individual numbers to replace letters: from the simplest version of 1 for "a", 2 for "b" to the Nazis' constantly evolving Enigma code. All such codes are made easier to crack by quirks of language. In English, the letter "t" is frequently followed by the letter "h", and the letter "e" appears more frequently than any other. Computerised searches should help the code-breaker quickly to pinpoint likely candidates for "a", "e", "I" and "th", and therefore to reveal the coding system.

Using these principles, and a few free Friday afternoons, evenings and weekends, Aston translated Hill's columns of figures into letters. But the most frequent letters were not, as expected, the "e"s and "I"s.

Aston tried another translation, assuming that Hill had used some kind of two, three or even four-stage coding process that replaced one letter with another before converting them to numbers. That would mean that Hill had used key words or phrases to construct his code - making the possible permutations of letters virtually incalculable.

For almost six months, Aston worked on the two pages of the diary that he had transcribed to his computer. The work dominated his evenings and weekends. "In a sense it is a similar process to doing research," he says. "It is unknown, you work away at it, and in the end you work it out. That's my job: to beaver away at things."

Nevertheless Aston was aware that, with the autumn term approaching, he would be unable to justify much more time on the apparently futile task. His wife was beginning to resent what was becoming an obsession and he was on the verge of admitting defeat when he experienced a classic "eureka" moment.

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He woke with an image in his mind of the inside cover of Hill's exercise book. At the bottom of that page were printed two names in Donald's handwriting: DONALD SAMUEL HILL and PAMELA SEELY KIRRAGE. Counting the letters, he found 34 - the same as the number of letters in each of the diary's incomprehensible lines.

It was still early in the morning when Aston dashed to his computer and worked out a program using the order of the letters in the lovers' names as the key to the code. If his theory was right, then as one letter replaced another, the gobbledegook of apparently random letters should turn into English words. They did not. Then it occurred to him that, imprisoned as he was by the Japanese, Hill might have composed his columns to be read from top to bottom instead of left to right. He retyped all the letters from a page of the diary into his computer, this time working in vertical columns instead of horizontal lines.

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Then he set the computer to translate using the entwined keyword names. It worked. Words such as "pilot" and "war" began to appear on the screen. After a day's teaching, he continued on the diary in the evening. "I had to go through it, putting in all the spaces between the words," he says. "It was quite slow: I could only type in two pages in an evening, so I read it in instalments. It was quite gripping."

The diary covers the fall of Hong Kong, where Donald was posted, in December 1941, followed by his flight, capture by the Japanese and transfer to a PoW camp. "He was writing some of it while he was running," Aston says. "He just got out of his bed when a shell exploded on top of it. He crashed a car trying to escape, he was forced to drive a truck for the Japanese." The diary tails off in late March in the PoW camp, though not before it becomes clear that the search for food has become desperate.

Aston felt he never really knew the whole story until Mrs Hill visited the university for the official hand over of Donald's translated diary. It was only then that he understood why the diary had meant so much to her. "They had had this golden summer before the war when they were so in love, and then been separated for all those years. When he came back they got married quickly, but although they couldn't live apart, they couldn't live together either. He was a troubled man. She felt that the war years were a closed chapter of his life; she wanted to know what he had been through because he would never talk about it."

The story interested more than just Mrs Hill. Newspapers picked it up; the book was written; there may be a film. Aston, who receives a small royalty for his translation, still seems bemused by it all: "No one bats an eyelid at my research. I stick it in the journals, it gathers dust on the library shelves. Yet I do a little numerical puzzle like this and everyone takes notice."

Other would-be decoders have since contacted him: one with an indecipherable gravestone, another with telegrams believed to have been sent by Queen Victoria. In both cases, however, there was too little material to make code-breaking feasible, he says.

If he did it again it would be for more than intellectual interest. Pamela Hill, who has since died, was desperate to know what lay behind her husband's sadness. "Apparently once she read the diary she felt a sense of peace at knowing what was in it, says Aston. She understood why he had been so moody and why they had had difficulties in their marriage. I was very pleased that some technical work had brought some comfort to her in that way: time could easily have slipped by without her ever managing to get the diary decoded. I found that very moving."

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The Code of Love by Andro Linklater is published by Weidenfeld and Nicolson, ?16.99.