Facebook encryption: Should governments be given keys to access our messages?

The UK and the US have just signed an historic agreement to give each other a much faster way of getting hold of private conversations - cutting down the process time from months or years, to weeks or days.

"A backdoor is rather like leaving a key under the mat - once someone knows it is there anyone can walk in," said Prof Alan Woodward, a security expert at the University of Surrey and a consultant to Europol.

"Proposals for a 'backdoor' have repeatedly been shown to be unworkable. There is no middle ground: if law enforcement is allowed to circumvent encryption, then anybody can," it said.