Mass surveillance not effective for finding terrorists

Prime minister David Cameron wants to reintroduce the so-called snoopers' charter – properly, the Communications Data Bill – which would compel telecoms companies to keep records of all internet, email and cellphone activity. He also wants to ban encrypted communications services.

Even if your magic terrorist-catching machine has a false positive rate of 1 in 1000 - and no security technology comes anywhere near this - every time you asked it for suspects in the UK it would flag 60,000 innocent people.