When the protests that followed Iran's presidential election in 2009 began to fizzle out, the state-controlled media put photos of the demonstrations online, with the faces of unidentified troublemakers highlighted. Viewers were asked to help identify them and even track them down. If the Iranian authorities are to be believed, arrests were made as a result. Today, as the web is being undermined by the rapid dominance of apps for smartphones and tablets, the Iranian police would probably, as the jargon has it, ‘go multiplatform’. That, at any rate, is what their colleagues in the Metropolitan Police have just done: unveiling, ahead of the Olympics, a new app called Facewatch ID.