Photo: Brian Jackson/Alamy

Spies, of course, have always spied, but the era we live in sees the agencies at their most powerful.

When Britain’s GCHQ was established after the First World War private documents were stored in filing cabinets under lock and key and international phone calls were a thing of the future. Now, private documents are kept in unknown data centres around the world, international communications are conducted daily, and our lives are lived online.

The potential of these technological changes has not gone unnoticed. A leaked US national Security agency strategy document reads:

‘Digital information created since 2006 grew tenfold [by 2011]… a trend projected to continue; ubiquitous computing is fundamentally changing how people interact as individuals… and the traces individuals leave when they interact with the global network will define the capacity to locate, characterize and understand entities.’

Operating in the shadows and misleading the public, the spying agencies boast in secret how they ‘have adapted in innovative and creative ways’ that have led some to describe the current day as ‘the golden age of SIGInT’– or ‘signals intelligence’.

Remember that the next time intelligence agencies call for greater powers or when you read about law enforcement ‘going dark’ and losing the access it once had.

There are things citizens can do, though. We can encrypt our email using free open source tools like GPG, our phone calls using apps like Signal and our text messages with TextSecure from Whisper Systems, making mass passive surveillance that much harder. but for long-lasting meaningful change, the policies and practices of the agencies must change too.

Mass, indiscriminate surveillance violates the essence of the fundamental human right to privacy. Thanks to edward Snowden we now are able to fight back. Litigation brought by Privacy International and others has forced the British government to reveal its secret policies, for the first time allowing us to challenge its practices in court.

These are just the first steps in what must be a global push-back. Intelligence agencies have existed shielded from the scrutiny of the public for too long. They must be brought under the rule of law.

Eric King is deputy director of Privacy International