The story won lots of journalistic awards, including a Pulitzer prize. A story for the digital age, it inspired several plays in London and New York, an Oscar-winning documentary and a Hollywood film released last year.
What helped make Snowden a perfect source is that he is self-effacing, motivated neither by money nor fame. It made it difficult for the US and British governments to demonise him. Sources that are shifty, politically motivated, looking for money or disgruntled at being passed over for promotion are easier to discredit.
What rounded out the perfect source scenario is that the outcome for him turned out to be a lot happier than he had anticipated in Hong Kong. He enjoys relative freedom in exile in Moscow; not a perfect existence but preferable to idling away the decades in a US supermax prison.
So all good? Not quite. He did not feel like a perfect source at the time. Crammed into his room in the Mira Hotel with him were filmmaker and journalist Laura Poitras, then Guardian columnist Glenn Greenwald and myself, a Guardian reporter. There were a lot of uncertainties. There is no template for dealing with sources. Each one is different. And Snowden was very different, the story outside anything I had experienced before. There are some things that with hindsight I did not handle well.
Read any textbook on journalism or guidelines about the relationship between journalists and their sources and two key points are always made. The first is that journalists have an obligation to protect source anonymity. The second is that they also have to protect confidential information or data provided by a source. But the reality, as we found in the Mira Hotel, is often much more complex, throwing up many more issues than just these two.
On first meeting Snowden, the priority was to establish that he was who he said he was. Normally, a few discreet inquiries should help establish an identity. But we could not do that. We had to rely on interviewing him. He sounded plausible, trustworthy and the documents looked real. A lot of it came down to instinct. In the end though, I only knew for sure when the White House, just hours before publication, effectively confirmed the first of the documents was real.
The biggest and most awkward issue when dealing with sources is usually anonymity. A source might be a friendly press officer offering up more information than they are authorised to do, or an employee deep within an organisation who has spotted wrongdoing. In both cases, they could lose their jobs if identified. There are other stories that are riskier for the source, with the prospect of jail or even loss of life.
There are other, less principled motives for leaking; perhaps a personal grudge or for political advantage, and that can be awkward. I was offered a negative story about an opposing candidate by one of the campaign teams during the 2008 Obama-Clinton fight for the Democrat presidential nomination. Anonymity was demanded. I turned it down, partly because the story did not feel that strong and partly because I felt queasy being used in this way.
I did a similar story about a decade earlier as part of a Guardian team that brought down a UK Cabinet minister. That too was politically motivated. The difference is that the UK story seemed definitely to be in the public interest. It is a fine distinction.
The question of anonymity with Snowden barely arose. We discussed it with him but he said from the outset that he would identify himself at some point. Even if he had wanted to remain anonymous, it would not have been practically possible. He had left a clear trail to Hong Kong that would not have taken long to find when the first stories appeared. What we needed in the first week was security. And that meant an instant immersion for me into the world of digital security and encryption. If Snowden has a lasting legacy beyond the surveillance v privacy debate, it is that there is much more awareness now among the public, but especially among journalists, about security of communications. More and more journalists are shifting to encrypted communication.