It’s the not knowing that’s the hardest thing, Laura Poitras tells me. “Not knowing whether I’m in a private place or not.” Not knowing if someone’s watching or not. Though she’s under surveillance, she knows that. It makes working as a journalist “hard but not impossible”. It’s on a personal level that it’s harder to process. “I try not to let it get inside my head, but… I still am not sure that my home is private. And if I really want to make sure I’m having a private conversation or something, I’ll go outside.”

Poitras’s documentary about Edward Snowden, Citizenfour, has just been released in cinemas. She was, for a time, the only person in the world who was in contact with Snowden, the only one who knew of his existence. Before she got Glenn Greenwald and the Guardian on board, it was just her – talking, electronically, to the man she knew only as “Citizenfour”. Even months on, when I ask her if the memory of that time lives with her still, she hesitates and takes a deep breath: “It was really very scary for a number of months. I was very aware that the risks were really high and that something bad could happen. I had this kind of responsibility to not fuck up, in terms of source protection, communication, security and all those things, I really had to be super careful in all sorts of ways.”

Bad, not just for Snowden, I say? “Not just for him,” she agrees. We’re having this conversation in Berlin, her adopted city, where she’d moved to make a film about surveillance before she’d ever even made contact with Snowden. Because, in 2006, after making two films about the US war on terror, she found herself on a “watch list”. Every time she entered the US – “and I travel a lot” – she would be questioned. “It got to the point where my plane would land and they would do what’s called a hard stand, where they dispatch agents to the plane and make everyone show their passport and then I would be escorted to a room where they would question me and oftentimes take all my electronics, my notes, my credit cards, my computer, my camera, all that stuff.” She needed somewhere else to go, somewhere she hoped would be a safe haven. And that somewhere was Berlin.

What’s remarkable is that my conversation with Poitras will be the first of a whole series of conversations I have with people in Berlin who either are under surveillance, or have been under surveillance, or who campaign against it, or are part of the German government’s inquiry into it, or who work to create technology to counter it. Poitras’s experience of understanding the sensation of what it’s like to know you’re being watched, or not to know but feel a prickle on the back of your neck and suspect you might be, is far from unique, it turns out. But then, perhaps more than any other city on earth, Berlin has a radar for surveillance and the dark places it can lead to.

“There is just a very real historical awareness of how information can be used against people in really dangerous ways here,” Poitras says. “There is a sensitivity to it which just doesn’t exist elsewhere. And not just because of the Stasi, the former East German secret police, but also the Nazi era. There’s a book Jake Appelbaum talks a lot about that’s called IBM and the Holocaust and it details how the Nazis used punch-cards to systemise the death camps. We’re not talking about that happening with the NSA [the US National Security Agency], but it shows how this information can be used against populations and how it poses such a danger.”

“Jake” – Jacob Appelbaum – is an American who helped develop the anonymous Tor network, and went on to work with WikiLeaks. He’s also in Berlin, having discovered that he was the subject of a secret US grand jury investigation, and it was he who advised Poitras to come here. “I’d been filming him doing this extraordinary work training activists in anti-surveillance techniques in the Middle East and I asked him where I should go, because I just didn’t think I could keep my footage safe in the US. And he said Germany because of its privacy laws. And Berlin because of all the groups doing anti-surveillance work here.”

Facebook Twitter Pinterest Edward Snowden in Poitras’s film Citzenfour. Photograph: PR

People’s reactions in Germany to the Snowden revelations differed to those in Britain or America. There was full-on national outrage when it was revealed that even chancellor Angela Merkel’s phone had been bugged. I know this already, vaguely, in theory, but it’s a different matter to actually come to Berlin and hear person after person talk about it. I start out with three names, three high-profile “digital exiles” who have all taken refuge in the city: Poitras, Appelbaum and Sarah Harrison, another WikiLeaker who was with Snowden during his time in transit in Sheremetyevo airport near Moscow and helped him apply for political asylum in 21 countries. But I end up with reams of others. And, I can’t help thinking that Berlin, the city that found itself at the frontline of so much of the 20th century’s history, has found itself, once again, on the fracture point between two opposing world orders. And I wonder if the people I meet are the start of the internet fightback; if Berlin really is becoming a hub for a global digital resistance movement.

Is that too fanciful a word, I ask Martin Kaul, the social movements editor of Berlin’s most radical newspaper, Die Tageszeitung, or “Taz” as it’s known – and if anyone is in a position to know, it’s him (he is the only social movements editor he’s ever come across, he tells me). Is it a movement? Kaul ums and ahs a bit at first, especially about the idea of the city as a harbour for “digital exiles”, a concept I’d first heard in a talk Julian Assange gave at the South by Southwest festival in Austin, Texas, earlier this year.

“They are very high profile, the exiles,” he says, “but I don’t think there are hundreds of them here, or even dozens. I’d be interested to know if they are growing. But, what is true is that there were already many very influential groups here. Hacker culture is especially strong in Germany. There were a lot of people already working on these issues. And then the exiles arrived. They are like an international avant garde at the cutting edge of it.”

“It” is the ideological fault line that has opened up between a free and open web, and a web where everything is logged, catalogued. “It is a movement,” says Kaul. “But it’s not out on the streets. It’s more like Berlin is a laboratory, an experimental space, where practices of subversion, of hacktivism, of cyber-resistance are taking place. Because if it’s not working in Germany … where is it going to work?”

That is the question that troubles almost everyone I meet. Because there are so many angles to the subject in Germany, and even more in Berlin, where history seems so recent, so present. I do a double-take when I pass a bookshop and see copies of Das Kapital piled high in the window – it takes me a moment to realise it’s Thomas Piketty’s, not Marx’s version – and many of the people I interview seem to unconsciously pick places of historic significance to meet me in. I meet Diani Barreto, a Cuban-American activist who’s been in the city since 1990, in Unter den Linden’s most historic cafe, the Einstein, and she tells me how it’s the artists who created the fertile ground that brought in the later wave of technologists and campaigners, groups she brings together in a monthly salon. And Markus Hesselmann, the editor of Tagesspiegel’s website, who talks to me about the city’s deeply rooted suspicion of authority, selects a museum cafe in the formerly Jewish area of the city. It’s no coincidence that you can barely use a credit card to buy things in Berlin, he says. “People think – why should anyone know what I spend my money on?”

And when I meet Martin Kaul, it’s in a cafe in Prenzlauer Berg, the former East Berlin suburb that’s now the city’s centre for artisan coffee and over-specced baby buggies, and afterwards he drops me in his camper van by the underground station next to the bridge where the first crossings from East to West Berlin took place 25 years ago today. Or, as it seems to me, as someone who first came to Berlin in the very early 1990s – a time when Prenzlauer Berg was still just shabby and not yet chic – the blink of an eye.

Facebook Twitter Pinterest The Berlin wall, which fell in November 1989. Photograph: Getty Images/Moment Open

But then, it is the blink of an eye. It’s 25 years since the wall came down. And, in a strange historical collision, 25 years since the world wide web was invented. When I first came to Berlin, the internet didn’t exist and I was still some years away from sending my first email. In a historical time frame, the evolution of digital technology, its capabilities, the never-going-back cultural cataclysm that it’s precipitated, has all happened while most of us, a single generation, were working out what to have for dinner, or who to marry, or how to earn a living; a microscopic sliver of time that has changed not just the world at our fingertips but, we’ve discovered since Snowden, the secret world beyond our fingertips. What is known about us. Who we are. What our records say.

Because there are records. That’s what we also know since Snowden, and especially in Britain: everyone in Berlin takes a horrified delight in telling me that we have what Poitras calls “the worst of the worst”. It’s notable that she travelled back to the US last month for the premiere of Citizenfour but she wouldn’t come to Britain. “It’s what I was advised by my lawyers.” We don’t just have GCHQ, which goes far beyond even what the NSA is doing – according to Snowden it harvests “everything” – but we also have no constitutional protections, no amendments that guard the freedom of the press, no nothing. Just a historical perspective that gives us one, possibly distorted, view of how our intelligence services work.

Annie Machon, a whistleblower from another time, makes this point to me. She and her then partner, David Shayler, were MI5 agents who went to the press back in 1997. “In relative terms, that was a golden time for MI5. It was after it had finally publicly acknowledged its existence in 1989 but before the war on terror, and yet, we were still horrified by what we saw happening. There were no limits on its power. And there were so many things it was doing: illegal wiretapping of journalists, state-sponsored terrorism, files being held on government ministers, withholding of evidence, the imprisonment of innocent people… ”

She’s now an activist on behalf of whistleblowers, who she calls “the regulators of last resort”. It’s why she has left Britain and relocated part-time in Berlin, having become aware that she was, again, under surveillance. Our problem, in her view, is that for most of us James Bond is our main point of reference when it comes to our intelligence services. “We think they’re the good guys.” Whereas we actually don’t have any way of knowing if they are or not. We have no legitimate means of knowing anything about what they’re doing.

In Germany, they don’t know either, but no one assumes they’re the good guys. Everyone cites the Stasi when talking about NSA surveillance, and I wonder how meaningful that comparison is. Hubertus Knabe, a historian who’s the head of the Berlin-Hohenschönhausen Memorial, a former Stasi prison, tells me how he wrote to the public prosecutor last year. “Because I was not satisfied that he had decided to investigate only the case of who bugged the German chancellor, not the cases of ordinary people. He said that it is because in this case it is clear there is a victim. Whereas you can’t investigate a case against everyone.”

So they’re saying because it’s a crime against everyone, it’s a crime against no one?

“Exactly! It makes no sense to me.”

Germany has some of the strongest laws in the world when it comes to surveillance and privacy. It is illegal for the foreign security service, the BND, to spy on its own citizens. But, the NSA has had bases in Germany since 1945 and there are no laws that govern its behaviour. A parliamentary inquiry is now under way, to try and establish what the BND knew – the only one of its kind in the world, post-Snowden – but when I visit Hans-Christian Ströbele, the veteran Green MP who is leading the inquiry, in his office in the Bundestag he tells me: “We think we will find good information about what the BND has been doing.” And the NSA? GCHQ? He shakes his head. “Isn’t that a bit depressing?” I say. “That we’re sitting here in the parliament of one of the greatest democracies on earth, with a constitution that had to be rebuilt from the ground up, and there is nothing, legislatively that you can do?”

“It is,” he says.

But then Hubertus Knabe tells me: “The minister of the Stasi always said, ‘We have to answer the question, who is who?’ Those were his words. That means, who thinks what? It used to be an obvious fundamental difference between a democratic state and a dictatorial one that you don’t investigate someone until they did a criminal act. Innocent people are not surveiled. And in this, the difference between how a democratic state acts and how a totalitarian one acts has diminished. And this is very, I don’t know the English word. Besorgniserregend? Hold on, I will look it up,” and he taps into his phone. “Alarming! This is very alarming to me.”

I’m about to leave when he tells me about a conference he held recently at the museum. “And this man, a former prisoner, kept saying this very strange thing. It was very annoying at first. He kept saying, ‘I am your future’. ‘I already experienced what will be your future.’ But he was very serious. He had emigrated to Paris. He really meant it.”

Facebook Twitter Pinterest Snowden with Glenn Greenwald in Citzenfour. Photograph: PR

The German premiere of Citizenfour is at the Leipzig film festival. It’s a town in the former East Germany that’s famous for its role in starting what the Germans call “the peaceful revolution”, the acts of civil disobedience that led, seemingly out of the blue, to the fall of the Wall on 9 November 1989. And before the screening, an introduction from Edward Snowden to the people of Leipzig is broadcast. “Your history is an inspiration to me,” he says. “It is critical to remember the lessons of history.” Of how a regime was changed “by ordinary people in the streets”.

Having now met Poitras, it’s no surprise that Citizenfour is such a quietly humane film. It shows Snowden’s courage and conviction but also his vulnerability, his youth; the terrible self-awareness he has of everything he’s giving up. Poitras is the softly spoken, self-effacing counterpart to Glenn Greenwald’s more strident style of media engagement. It was Snowden who first got in touch with her, and it was her familiarity and facility with encryption techniques and security measures that made the entire story possible. It’s not just Snowden who comes across as brave and principled.

I speak to dozens of people after the event, of all ages, and the more people I talk to, the more depressing it becomes; the more poignant Snowden’s appeal seems; the more unlikely and far-fetched this idea, of a groundswell of public opinion effecting political change, appears. From Jürgen Kleinig, a 44-year-old maker of investigative films from Berlin, who tells me “there have been no political consequences. None. It’s such a massive threat, to democracy, to everything, but nothing has changed.” To Ulrike Böhnisch, a 28-year-old documentary maker from Leipzig, who tells me how scary she finds it in theory. “But then I think who is going to be interested in my silly little love notes to my boyfriend? For ordinary small people with simple ordinary lives, I think it is not so much of an issue.”

But what if they are? What if somebody is interested? What if Ulrike decides, in 20 years’ time, to stand for parliament? What if Germany’s government changes? What if someone does read her silly little love notes? What if they don’t seem so silly – or so innocent – at some unknown point in the future?

It could happen because it has happened. Anne Roth, a political scientist who’s now a researcher on the German NSA inquiry, tells me perhaps the most chilling story. How she and her husband and their two children – then aged two and four – were caught in a “data mesh”. How an algorithm identified her husband, an academic sociologist who specialises in issues such as gentrification, as a terrorist suspect on the basis of seven words he’d used in various academic papers.

Seven words? “Identification was one. Framework was another. Marxist-Leninist was another, but you know he’s a sociologist… ” It was enough for them to be placed under surveillance for a year. And then, at dawn, one day in 2007, armed police burst into their Berlin home and arrested him on suspicion of carrying out terrorist attacks.

But what was the evidence, I say? And Roth tells me. “It was his metadata. It was who he called. It was the fact that he was a political activist. That he used encryption techniques – this was seen as highly suspicious. That sometimes he would go out and not take his cellphone with him… ”

He was freed three weeks later after an international outcry, but the episode has left its marks. “Even in the bathroom, I’d be wondering: is there a camera in here?”

Knabe tells me how the modern, digital system “is more abstract. It’s not so violating of your personal emotions”. He speaks as one who discovered in his Stasi file that he had been betrayed by a friend. But the difference is perhaps not so clear cut. Mathilde Bonnefoy, the French-American editor of Citizenfour, says the same thing, initially. “It’s signals intelligence, not personal intelligence. It’s mostly a theoretical threat. It’s not like you know there’s people standing on the street corner looking at you.”

Bonnefoy doesn’t know. Can’t know. And since she lives in Berlin and is in a relationship with Dirk Wilutzky, the film’s producer, they found themselves conducting that relationship under some sort of unknown, unknowable scrutiny. Are they still under surveillance? Wilutzky pushes his mobile phone toward me. “I think you are probably talking directly to them.”

They chose to ignore it. It’s what dissidents in East Germany did too, Knabe tells me, a political and philosophical act of resistance. Though talking to Bonnefoy, you wonder what the other options were. “There was a moment, I remember, when it had become very clear to us that we were being listened to and we started speaking in hushed words and elliptical phrases at home when we were talking to each other,” she says. “And I remember Dirk said, ‘We have to stop now. We can’t let this change our lives this much.’ Though, even now, there are still things that we don’t talk to each other about.”

Facebook Twitter Pinterest Ulrich Mühe as Stasi captain Gerd Wiesler in the Lives of Others. Photograph: PR

The comparison to the film The Lives of Others is unavoidable. Poitras tells me that someone in the intelligence community told her that it’s probable “that Glenn and I had our own psychologist assigned to us. That there was someone who is paying attention to your friends, to what you might do next. It’s very creepy.”

And, even without that, it’s unclear if this modern version of what’s called “signals intelligence” is less intrusive. Roth’s husband’s metadata is an example of that, and even your Google search terms are practically a psychogram of your thoughts. “I’m so careful about that,” says Poitras. “I use different computers for different uses.” And all over the city, there are people working on ways to fight the technology with technology; who’ve devised the crypto equivalent of what, in the former German Democratic Republic, was done by turning on the radio or running the tap.

There’s Claudio Agosti of GlobaLeaks, a platform he describes as “like WikiLeaks but open source” and Stephanie Hankey, a Brit who’s director of Tactical Tech, an anti-surveillance NGO which moved to Berlin a couple of years ago. And Christian Mihr, the German director of Reporter Ohne Grenzen (Reporters Without Borders), whose office specialises in cases of international digital repression and who helps journalists from oppressive regimes around the world find safe harbour in Berlin. Though it’s not until I finally track down Andy Müller-Maguhn of the Chaos Computer Club (CCC) that I start to really understand why. Everywhere I go, people tell me about the CCC, that it’s one of the most influential digital organisations anywhere, the centre of German digital culture, hacker culture, hackitivism, and the intersection of any discussion of democratic and digital rights. It holds an annual congress which started in Berlin in 1990 and is attended by more than 10,000 people.

But then so much started in 1990 in Berlin. “Half of the people were coming from the east and others, like me, from the west, and at that time, it was pretty easy to break some rules somewhere,” he says. “It was so cheap and the infrastructure was a bit shit but we came together during this period when Germany was in the process of revealing what the East German intelligence did.

“There was this incredible transparency. It was one of the best documented intelligence agencies ever. We had access to all these manuals: ‘how to destroy social relationships’, ‘how to organise distrust’, ‘how to destroy political movements’ and all these things we discussed in the club. We were very aware of how the intelligence services could do these things… and this was part of our creation from the very beginning.”

What’s so interesting about this is that the CCC has helped define important parts of what is now considered internet culture. “The power we had,” says Müller-Maguhn, “was the power of definition. We helped explain to people how technology was part of society.” It’s why hacker culture is so much stronger in Germany than almost anywhere else in the world, but certainly Europe, and why it’s largely seen as a force for good. “Unlike in the US and Britain, we were able to promote our ideas in a positive way.”

And suspicion of authority is encoded into that DNA. It considers digital rights no different from the rest of our fundamental human rights and there’s an intellectual thread that leads from the CCC to one of the most affecting scenes in Laura Poitras’s film, in which Edward Snowden talks about the excitement he felt as a child about the internet, “the greatest invention the world had ever seen”. And his determination to try to defend that vision.

There are so many impassioned voices in Berlin telling the same story in different ways. Diani Barreto describes the city as having an entre-deux-guerres feel, how there’s a touch of Weimar, a hint of Christopher Isherwood, to the way the international community has discovered the city, not least the freedom it offers from the constraints of Piketty’s Das Kapital (I visit a friend whose teenage daughter bursts into the room to say she’s found a one-bedroom flat to rent for “€300, warm, ie including heating and hot water”). Wilutzky describes the experience of coming to West Berlin in the 1980s: “There was this terrible feeling of oppression as you drove through the east, and then suddenly this amazing sense of freedom! It felt like the freest place on Earth. You could do anything here.”

Facebook Twitter Pinterest Activist Sarah Harrison, who spent four months assisting whistleblower Edward Snowden, has moved to Berlin. Photograph: Sunshinepress/Getty Images

Berlin was for a long time this strange geopolitical anomaly, a shadow theatre for the great powers, the capital of nazism, the frontline of the cold war, and the alternating experiences of stifling oppression and mind-blowing liberation are the twin strands of its 20th-century history. The most compelling voice of all that I encounter belongs to a woman called Anke Domscheit-Berg, who has known both. She’s a 46-year-old feminist and activist who used to work as a lobbyist for Microsoft (and whose name is possibly familiar because her husband, Daniel, was a spokesman for WikiLeaks until he fell out with Assange). She was born and grew up in the east and was 21 when the wall fell, an event she describes as “the most emotional day of my life”.

She was an art student and she tells the story of how the Stasi tried to recruit her as an informant. “People say of the NSA, ‘I have nothing to hide.’ But it doesn’t matter. There is no such thing as innocent information. I had things I needed to hide from the East German authorities but that wasn’t what they blackmailed me with. They blackmailed me with my father’s job. He was a doctor, employed by the state. They said: ‘Don’t you care about what happens to your family if he loses his job?’

“All information can be used against you in some way. And we have an entire generation, the first one ever, about whom everything will be known. Their entire youth is being monitored. And we don’t know what that might mean. How that might be used against them. I look at my father who is 80 and he has only known democracy for the shortest portion of his life. And that is why we have to act now. We have the power to change things. I remember how hopeless it seemed, 25 years ago, that it would ever change. But it did. And we did that. We, the people. And that is why it’s up to us Germans to tell this to the world.”

She is such a powerful, clear, impassioned voice. And it’s obvious that for her, this is personal. “I feel responsible. I feel like I look into one of those glass balls, where others see fog, I see a clear picture and I feel obliged to tell people. These are the tools of a totalitarian system. And just as you cannot be a little bit pregnant so you cannot be a little bit totalitarian without corrupting democracy. And we … in this city … we know where that ends up. We have seen the darkest times, right here.”

Poitras tells me how she has come to censor herself. “It’s not whether or not they’re watching, but the fact that you don’t know if they’re watching. You’ve internalised in some way this authority of the state.” At the end of the interview, I tell her how Snowden spoke at the Observer Festival of Ideas and how afterwards I and my colleague John Naughton asked him questions via Google Hangouts from my laptop. “Am I on the grid?” I ask her.

She guffaws. “You are so on the grid.” It’s only semi-serious but still. “As soon as you start to censor yourself,” Domscheit-Berg tells me, “then you leave the path of free speech. So many people now do this in Berlin. They avoid certain expressions. When we have meetings they leave their phones in different rooms. You have already lost your freedom.”

Have I already lost mine? Has it affected my online behaviour? Possibly. My thoughts have always flowed seamlessly from my brain to my fingers to Google’s all-knowing rectangular white box. And now? There’s the briefest pause. A hesitancy. It’s not exactly an iron curtain but it’s not nothing either. I’m being watched. But then, you are too. And, if you think it doesn’t matter, go to Berlin. Go to the Stasi museum. See how it all panned out last time around.

Citizenfour is on general release now