A secret intelligence pact between the UK and US began during the Second World War, focused on intercepting communications and breaking codes. Expanding to what is now known as Five Eyes, with the addition of Australia, New Zealand and Canada, the secrecy around the alliance continued for decades. It wasn’t until 2013, when the ex-NSA contractor Edward Snowden leaked classified documents, that the extent of its operations came into the public spotlight. Gordon Corera, the BBC’s security correspondent, talks to Claire Graham about the origins of this spy network and how relevant it is in a modern digital world.