Stasi-Zentral. Getty images. All rights reserved.Four decades of domination over almost all aspects of life in East Germany came to an abrupt halt 25 years ago on 31 March 1990. One of the most intrusive surveillance organisations in human history, the Ministry for State Security, more infamously known as the Stasi, was dissolved. This is a poignant milestone as a global debate about privacy rages in the wake of revelations of massive US surveillance of internet communications.

Two months ago I was at the old Stasi headquarters, today a museum in Berlin, for an open day commemorating the storming of the building by East Germans a few weeks after the Berlin Wall fell. There were film screenings, discussions, information stands and a tour through the Stasi’s enormous archive that at one point contained files on an estimated six million people. Some say a file was kept for one in three citizens.

It took me an hour to wander through the archive. Thousands of Germans visiting the site appeared shell-shocked amid the labyrinth of corridors and ceiling-high filing cabinets that had documented and controlled their lives – or the lives of others – for years. The very building, with its grey concrete Communist-era architecture, was a symbol of fear – the place you were taken for interrogation or intimidation.

While the Stasi archive is overwhelming, today’s spies can gather far more information with a fraction of the effort.

The Snowden revelations suggest the NSA can collect 5 billion records of mobile phone location a day and 42 billion internet records – including email and browsing history – a month.

German organisation OpenDataCity estimates that while the Stasi archives would fill 48,000 filing cabinets, just one US government server could store so much data that, if printed out, the reams of paper would fill 42 trillion filing cabinets.

We know very little about what the NSA does with all this data. But, leaving historical parallels aside, the Stasi archive is a timely warning of the potential consequences of unchecked surveillance. It shows how quickly a system for identifying threats evolves into a desire to know everything about everyone.

Knowledge is power, so is personal data

If knowledge is power, then by extension so is our personal data. The Stasi took surveillance to unprecedented, intrusive levels, to gather deep knowledge about what people did and said which they used to manipulate and control the population.

Like the USA and the UK, who today intercept our emails and internet records, the Stasi sought to infiltrate personal life to collect intimate information about peoples’ lives to identify those they considered a threat. In the Stasi museum today you can see the personal and seemingly inane material kept on file, including photographs of bedrooms and record collections.

The Stasi’s surveillance network spiralled out into every aspect of daily life. Among an estimated 274,000 employees were at least 174,000 informants, which would have been about 2.5% of the working population.

Informants snooped in every office, cultural and sporting society, and apartment building. They recorded people in their own homes and in the homes of their friends.

Modern mass surveillance achieves this omnipresence with a fraction of the manpower. Spies can scoop up massive quantities of electronic communications directly from the cables that deliver them and the servers that store them. Cold War snoops have been pushed aside by computers and algorithms.

Tools of the spy trade

Visit the Stasi museum and you will see the sinister spying tools and techniques of the past: machines for steaming open letters, disguises (fake moustaches) and training for infiltrating “subversive” groups and the cameras hidden in ties, cigarette packs or simply in apartment walls.

The Stasi steamed open letters, copied them, filed them and sent them on. They went into homes when people were out and bugged them. They tapped into the phone infrastructure of buildings.

By contrast, today’s spies sit behind a desk and rely on dozens of electronic programmes for spying on our life, from Prism to Tempora, to do the dirty work for them.

The NSA uses computer programmes like Optic Nerve to access web chats, or The Three Smurfs to turn on your mobile phone’s microphone to listen in and track your location. For the Stasi, that technology would have been “a dream come true” in the words of one former officer.

What is in your file?

The most emotionally-charged discovery for many visitors to the Stasi museum is finding out whether the secret police had a file on them.

Since the fall of the Berlin Wall, 2.75 million people have asked to see their files. Germans and many foreigners who visited East Germany have been shocked to discover that Stasi spies took an interest in where they went and what they said, but also who spied on them – sometimes friends and family had been recruited or blackmailed into the informant network.

No wonder Germans are more convinced than their European neighbours about the importance of the human right to privacy. A whopping 69% of them are opposed to government mass surveillance, according to a recent Amnesty International poll conducted in 13 countries around the world.

Germans most opposed to surveillance according to a recent Amnesty poll. Credit: Amnesty InternationalWe don’t yet know what impact mass surveillance of our internet use will have on today’s digital societies. But by its very existence, the Stasi museum shows the chilling effect surveillance can have on free expression. As a result, few Germans take their right to privacy lightly.

There are important lessons for us to learn from this. The question is, in 25 years will millions be asking the NSA or other intelligence agencies for our files, to see if and when they snooped on our private lives?

An original version of this article was published in The European magazine on April 1, 2015.