When Russia initiated its full-scale invasion of Ukraine, a second, less visible battle in cyberspace got under way. The BBC’s cyber correspondent Joe Tidy travelled to Ukraine to speak to those fighting the cyber war, and found the conflict has blurred the lines between those working for the military and the unofficial activist hackers.

When I went to visit Oleksandr in his one-bedroom flat in central Ukraine, I found a typically spartan set-up common to many hackers.

No furniture or home comforts – not even a TV – just a powerful computer in one corner of his bedroom and a powerful music system in the other.

From here, Oleksandr has helped temporarily disable hundreds of Russian websites, disrupted services at dozens of banks and defaced websites with pro-Ukraine messages.

He is one of the most prominent hackers in the vigilante group, the IT Army of Ukraine – a volunteer hacking network with a Telegram group nearly 200,000-strong.

For more than a year, he has devoted himself to causing as much chaos in Russia as possible.

Even during our visit he was running complex software attempting to take his latest target – a Russian banking website – offline.

Ironically though, he admits the idea for his favourite hack actually began with a tip from an anonymous Russian, who told them about an organisation called Chestny Znak – Russia’s only product authentication system.

He was told all goods produced in Russia – including fresh food – have to be scanned for a unique number and a barcode supplied by the company from the moment of their creation at a factory, up till the moment of being sold.

Oleksandr smiles as he describes how he and his team found a way to take the service offline, using a hacking tool that floods a computer system with internet traffic – known as a targeted DDoS (Distributed Denial-of-Service) attack.

“The economic losses were pretty high, I think. It was mind-blowing,” says Oleksandr.

In reality, it’s hard to gauge the disruption prompted by the hack, but for four days last April Chestny Znak posted regular updates about the DDoS…