To his neighbours James Ellis seemed a man of unremarkable anonymity, leaving his home in Cheltenham's leafy Leckhampton Hill each morning in the timeworn manner of countless office workers.
"None of our friends or neighbours had any idea what he did," says his widow, Brenda. "To be honest, I had no idea what he did until years after his death." In fact Ellis was one of Britain's great unsung heroes - a latter-day Alan Turing, whose work led to ground-breaking techniques for keeping our country safe from today's threat of devastating digital attacks.
Despite his pioneering work as a brilliant cryptographer and mathematician at GCHQ, the Government's secret listening centre, paving the way for some of the methods used to tackle cyber terrorism and crime today, his role has been largely forgotten.
Now, his former bosses in the usually ultra-secretive world of spying have published a number of documents written by Ellis in 1970, to demonstrate the key role he played - not just in the UK and USA's national security, but in the online security used by ordinary individuals on a daily basis, from the sending of confidential documents to internet shopping.
His achievements have also been hailed in a speech by Robert Hannigan, the director of GCHQ, to an audience at the prestigious MIT university, in Massachusetts, America.
Mr Hannigan said last week: "The sheer boldness of Ellis's concept, and of [fellow GCHQ cryptographers] Malcolm Williams and Clifford Cocks' subsequent work is still staggering, reversing centuries of assumptions about how communications could be protected. Encryption went from being a tool of strategic advantage between super-power blocs, to a key enabler of individual freedom and safety."
Ellis's genius lay in finding a solution to the age-old problem of sending secret messages from one person to another without the need for a cumbersome system of codes. In 1969, in the words of science writer Simon Singh, he "turned the problem on its head", by suggesting the receiver of the message, and not the sender, should play the key role in encryption.
Now known as Public-Key Cryptography, it is one of the greatest breakthroughs in modern encryption, but for decades it was top secret.
Instead, American researchers at Stanford University and MIT came up with a similar solution to the problem shortly afterwards. As academics, they were not bound by secrecy rules and were able to publish their work, claiming the glory. Now, Ellis's story can finally be told in full.
After Ellis almost died during birth, doctors declared the trauma would mean he would grow up an "idiot".
Defying expectations, he quickly showed a gift for mathematics and physics and after grammar school in Leyton, east London, graduated from Cambridge. At the outbreak of the Second World War, Ellis enlisted in Army intelligence at the Admiralty Research Laboratory, in Teddington, south west London. Later, he worked at the Post Office Research Station and, in 1952, joined GCHQ at its then base in Eastcote, west London.
He had married Brenda, an artist and designer, three years earlier and they had four children. But she never knew anything of his work.
Even when GCHQ transferred to Cheltenham in 1965, and Ellis moved the family, she continued to believe he was a simple "researcher".
"I never knew what he did or who his friends and colleague were. In all those years he never explained anything to me. He was very introspective, sitting alone thinking all the time. He lived in his own world," she told The Telegraph.
Ellis died in 1997, following a battle with cancer - taking his secrets to the grave, as did so many GCHQ workers.
The first Mrs Ellis knew of her husband's work was two years after his death, when she was contacted by a friend who had read a brief mention of Ellis in Singh's 1999 book on cryptography, The Code Book.
In 2010 Mrs Ellis was invited with her children and grandchildren inside GCHQ, to unveil a posthumous plaque recognising her husband's work.
Mrs Ellis, now 88, says he would have been quietly proud of his part in transforming global cyber security.
"James never had any glory for his ideas and for all that brain work of his. The Americans took it instead and that must have been very frustrating for him," she said. "I'm sure he realised how important his work was for Britain and our security.
"But of course he couldn't say so to anyone at the time."