What is it like to keep top state secrets?
A secret world of people in the UK, many of them ordinary citizens, are living extraordinary double lives to help the government. But what is it like to live with the danger and loneliness involved and keep important state secrets, ask Peter Taylor and Richard Knight.
Throughout the many violent years that led up to the Northern Ireland peace process, Londonderry businessman Brendan Duddy and his family lived with an extraordinary secret.
Duddy was, for decades, the secret intermediary between MI6, MI5 and the IRA. Without him it's unlikely that Northern Ireland would be where it is today.
"It took somebody with a lot of brains," says Seanna Duddy, Brendan's daughter. "He had what it took to go into a room, be in danger and keep his cool."
The threat to Duddy's life came not just from some members of the IRA who suspected he might be working for MI5, but from the loyalist paramilitaries who wanted to kill off any negotiations with the IRA - and perhaps anyone associated with them.
So the family could not breathe a word about the meetings between British intelligence officers and the IRA leadership that took place in the "wee room" in their family home.
"People had absolutely no idea," says Larry Duddy, Brendan's son. "Really close friends of my father for 50 years didn't know what he was doing.
"There's part of you wants to let the world know what your father did and there's another part that doesn't want anyone to know. I was quite happy with no-one knowing because it was the end result which was important."
Duddy finally did get the end result he wanted - peace in Northern Ireland. But his children made personal sacrifices as co-inhabitants of their father's secret world.
"When you came home from school you couldn't bring your friends home," says Seanna. "If everybody was out playing in the gardens or the roads nearby and it was our turn [for] our mammy to make tea, that never happened."
Most individuals who operate in the secret world do not involve their families. Many, in fact, tell no-one about their hidden lives, not even those closest to them.
Ali, a pseudonym, is a Muslim who was recruited by MI5 shortly after 9/11. When he spoke to the BBC it was the first time he had discussed his work with anyone other than his handlers at MI5.
He said he'd been able to stop some terrorist attacks but did not want to get into "the specifics". The impulse to share his successes or failures must, he says, be ignored.
"If you want to be able to help out doing this kind of work then you just have to hold those feelings in, which could be challenging but you learn with time," he says.
Ali is proud of what he does and of what he believes he has achieved. But he knows some in his community would regard him as a traitor and that his life as a Covert Human Intelligence Source - CHIS - is risky.
"I think it's quite evident that if some people would find out what I'm doing there may be people that probably would ignore it, there would be people who would try to do something about stopping me from helping out as well," he says.
"Therefore I'm careful and my handlers are being careful as well. And I've got my own brain... so I just have to be vigilant."
Steve, also a pseudonym, is a former undercover Special Branch officer who infiltrated the hard left to counter subversion. Steve adopted a cover story - known as a "legend" - and lived it for four years. He says his work put tremendous strain on both him and his wife, who knew his secret role.
"You're a police officer and you know your role, and you're briefed to do a role, and then you are operating as a political activist. You're living two lives, but you have to remember which is which."
Despite the pressures Steve says his years as an undercover officer were the best of his service. Ali continues to face significant risks but he also says, without hesitation, he would do it all again. The Duddy family made astonishing sacrifices for peace. Now they are able to view the transformation of Northern Ireland with quiet satisfaction.
Whatever you may think about the morality and ethics of those who live in the secret world, many are remarkable men and women. They are prepared to live with secrets, danger and loneliness, for what they believe is a greater good.