As Ruma* was taking part in online university classes, her mobile phone started vibrating constantly.
Ordinarily, she would ignore it, but the sheer number of messages coming through gave her an "ominous feeling".
WARNING: This story contains details that may distress some readers.
What she saw on her phone left her stunned.
Her face had been digitally imposed over sexually explicit images; something known as deepfake pornography.
The images were accompanied by further abuse.
"Your p**** is popular," one message read.
"You're a rag. You're a toilet," it continued.
The doctored images flooding Ruma's phone were coming via Telegram, an encrypted messaging service that has gained notoriety for its role in digital sex crimes and providing anonymity to its users.
Ruma did not respond to the messages, but took screenshots and rushed to the police station.
The lack of interaction had clearly annoyed the sender.
"If you reply, I will teach you how to catch me," the texter taunted, trying to elicit a response.
"I will give you hints."
Ruma had no idea how explosive her case would become.
When arrests were finally made some three years later, it sent shock waves across the country.
Dozens of victims have been identified, many of them current or former students at Seoul National University (SNU).
In the months that followed, South Korea faced a reckoning about the proliferation of deepfake pornography, with countless victims from universities, high schools, middle schools and even primary schools.
Politicians and law enforcement have vowed to tackle the problem, while girls and women have frantically deleted their social media photos over fears their images have been compromised.
"Any of us could become a victim of such digital sexual crimes," South Korean President Yoon Suk Yeol warned in late August.
"I urge that the relevant authorities conduct thorough investigations and take steps to eradicate these digital sexual crimes."
But three years ago, when the abuse began, Ruma felt alone and helpless.
Police showed little interest in trying to solve the case.
Officers simply instructed her to file a report and continue taking screenshots. There was no assurance they would try to find those responsible.
"The assault is happening in real time on your phone, and you can't do anything," she recalled.
"It really felt like a physical assault. I was in survival mode."
Ruma's friends also deepfaked
Like many victims, Ruma deleted her social media accounts soon after her abuse.
She moved abroad for postgraduate studies and tried to forget the ordeal.
During a return trip to Seoul in June last year, Ruma met with two other alumni for lunch.
What they told her made her stomach drop.
They too had seen images from an unknown user on Telegram, with their faces plastered over sexually explicit material.
"I was literally experiencing the trauma all over again," Ruma said.
"As soon as I heard a few sentences, I knew it was the same person."
The other victims had also filed complaints to police but, like Ruma, they initially proved to be of little value.
The trio was certain the perpetrator must be another student, but had no way to prove their suspicions.
The breakthrough came when a friend suggested they speak with independent journalist Won Eun Ji.
Eun Ji had experience uncovering digital sex crimes in South Korea.
When she was at university, Eun Ji and a fellow journalism student infiltrated a Telegram chat group where images were shared of women who had been coerced into sexually depraved acts.
Eun Ji's work has been praised widely.
It led to a series of media investigations that eventually exposed the so-called Nth Rooms. Two masterminds were sentenced to over 30 years in jail.
After being contacted by Ruma and her friends, Eun Ji prepared to infiltrate and expose another sex-crime group.
"The sad reality is that for the police to investigate actively, victims need to provide more evidence," she said.
"So, I thought about how I could help."
Eun Ji gained access to a Telegram group through a male friend, who had also infiltrated the group.
Inside, the ringleader boasted about his deepfake porn collection and said he wanted more men to join to revel in his depravities.
A trap is set
Eun Ji's first goal was to build rapport with the perpetrator.
She pretended to be a man whose wife worked at SNU.
"They would send nude photos of the victims or photos manipulated to look like the victims were engaging in sexual acts," she said.
"He would say unspeakable things, like suggesting we rape together online or talking about gang-raping someone online."
It took her more than a year to build trust with the ringleader.
"I told him that I couldn't send photos [of women], but I could send [photos of] underwear," she said.
"He seemed to have fantasies about underwear."
Eun Ji provided the promised images, and the man was hooked.
"He became increasingly obsessed with the underwear photos," she recalled.
"He was sexually fixated on the fact that it was underwear actually worn by a woman."
So, Eun Ji set a trap.
"I asked if he wanted to receive real underwear," she said.
"He said he really wanted to. That's when I thought, 'We can catch him now.'"
Two years talking with a pervert
Teaming up with detectives, Eun Ji organised for underwear to be dropped at a campus toilet for the perpetrator to collect.
CCTV cameras would film whoever went to retrieve them.
The trap worked.
A suspect was captured walking into the toilets and out again.
But, to be absolutely sure, police asked Eun Ji to organise another two drops.
On the third, officers swooped.
"After nearly two years of conversation, it was the only moment I saw hope," Eun Ji recalled.
"I thought, 'This crime will end now. The victims can finally rest easy.'"
Two men, both former students at SNU, were arrested in May. At least 61 victims have been identified, including 12 current and former students at SNU.
Ruma remembers a few fleeting interactions with one of the alleged perpetrators, but he was about 10 years older than her.
"I felt relief," she said.
"Now I don't have to suspect all of the guys in the same department."
The ordeal has left her deeply traumatised, and she still seeks professional support.
"It's destroyed my sense of safety, and trust in my social network," she said.
"It was a very, very emotionally painful, taxing experience."
500 schools and universities caught up in deepfake scandal
The arrests sparked national headlines, with some media labelling the saga the "SNU Nth Room case".
Journalists in South Korea knew there had to be more cases.
In the following months, particularly in August, media reports identified more Telegram groups with hundreds if not thousands of members.
Some chat rooms allowed members to upload images to be converted into deepfake pornography.
Other groups would share deepfake pornography — even providing victims' information such as names and schools.
The biggest channel identified so far had more than 220,000 members.
Within a week, at least 500 schools and universities were identified as being affected.
South Korea's Advocacy Centre for Online Sexual Abuse Victims reported 781 deepfake victims had sought help from the public service this year — an 11-fold increase from the 69 victims reported in 2018.
The media storm led to most Telegram groups being deleted.
"This problem is being viewed almost as a national disaster in Korea," said Kim Myung-joo, an expert in information protection from Seoul Women's University.
It is not just the volume of material that is concerning authorities, but the age of the perpetrators.
Police data shows three-quarters of suspects involved in deepfake pornography are minors — male students targeting female students.
"Most of the perpetrators are … teenagers," the South Korean president said.
"Some may dismiss it as a mere prank, but it is clearly a criminal act that exploits technology under the shield of anonymity."
Creating deepfake pornography can attract a jail sentence of up to five years.
Society as much the problem as technology
South Korea's National Police Agency has announced it is investigating Telegram over allegations it abets the distribution of deepfake porn.
Telegram has responded by taking down some material and establishing an exclusive email for authorities to alert it about illicit content on the platform. The South Korean government has been pushing for a hotline.
The encrypted messaging service's founder was recently detained in France as authorities investigate child exploitation material, drug trafficking and fraud on the platform.
Mr Kim said it was crucial social media companies were made more legally responsible for content created or shared on their platform.
But considering many of the offenders were school students, he also argued education should prioritised.
"If youth are involved, it becomes a family issue and a social issue," he said.
"So now, almost all government departments are on high alert, and it's no exaggeration to say that the entire country is focused on this to come up with countermeasures."
South Korea may be dealing with a deepfake crisis now, but the rapid advancement of the technology that creates deepfake material has been alarming authorities worldwide.
"Korea often plays an early-adopter role in the digital world," Mr Kim said.
"Similar phenomena will likely spread to other countries."
What makes South Korea stand out, for now at least, is the sheer number of cases that have emerged, and its long history of digital sex crimes.
Before the current deepfake porn crisis, it was the Nth Room, and before that, the country was grappling with a plague of hidden cameras that men had set up to capture women in the bedroom and bathroom.
Ruma shares concerns about how artificial technology can be used for harm.
But, for her, there are underlying cultural problems that must be addressed.
"I'm less worried about the technologies, the advancement of technology, than with the persistent misogyny and rape culture in Korean society and also abroad."
*Ruma's name has been changed to protect her identity.
