• Secret files show scale of Silicon Valley co-operation on Prism
• Outlook.com encryption unlocked even before official launch
• Skype worked to enable Prism collection of video calls
• Company says it is legally compelled to comply
Microsoft has collaborated closely with US intelligence services to allow users’ communications to be intercepted, including helping the National Security Agency to circumvent the company’s own encryption, according to top-secret documents obtained by the Guardian.