SEOUL — Over nine months starting in spring 2019, Cho Ju-bin lured his victims — whom he called "slaves" — with calculated precision.

From his home in Seoul’s suburbs, the 25-year-old orchestrated one of South Korea’s most infamous sex crimes. Under an online alias as the “Doctor,” he blackmailed at least 74 young women, including minors, into sharing sexually explicit videos of themselves, then sold the footage online through a chat group on the encrypted app Telegram.

On Thursday, a court convicted Cho of organizing a crime ring and violating child protection laws, and jailed him for 40 years.

The case fueled a national outcry in South Korea over what has emerged as a major societal problem: men secretly recording sexually explicit footage of women, or blackmailing their victims into doing so, and then selling the material online.

It’s a crisis fueled by a lack of respect for women in Korean society and a culture of impunity, exemplified by weak laws against digital sex crimes and often low penalties for sex offenders.

In sentencing Cho, the Seoul Central District Court said he needed to be “isolated from society for an extended period” given the number of victims, the damage he inflicted on them and the social repercussions of the crimes.

“The defendant lured and threatened a large number of victims into producing sexually degrading videos and raised a lot of money through distributing them to many people over an extended period,” the court said. “In particular, he inflicted irreparable damage by releasing the identities of many victims.”

Cho had lured women through social media, sometimes by posting fake modeling or employment advertisements, and then conspired with workers at local government offices to obtain their personal information so he could blackmail them. He then sold access to chatrooms for up to $1,300, paid in cryptocurrency.

The scale of the operation stunned the nation. Local media said that up to 260,000 people potentially viewed the content, though police say that number includes double-counting and nonpaying members. More than 2 million people signed a petition demanding the names of everyone who viewed the content to be made public.

“At the time, I was hardly concerned about human dignity and I just used people and sex as tool for crimes,” Cho told prosecutors, according to local media reports. “Now I declare an end to my life as a devil.”

Prosecutors had sought a life sentence for Cho. Both they and Cho have a week to appeal the verdict. Cho’s attorney could not be reached for comment.

The police have detained 124 suspects in relation to the crime ring.

In Thursday’s ruling, sentences of 15 years or less were handed down to some of Cho’s accomplices. A 24-year-old man, under the alias “Donald Putin,” was convicted of stealing victims’ personal information and supplying it to Cho to threaten the women. Others sentenced on Thursday included a 16-year-old and a public servant.

The outcry has provoked an official reaction.

In March, South Korea’s Justice Ministry established a task force to address online sexual crimes, and pledged to set up prevention measures and improve support for victims. In April, South Korea’s parliament passed a law increasing penalties for illegal sexual imagery, outlawing possession and viewing of such material.

Outside the Seoul court, women’s rights activists declared the verdict “not an end, but the beginning of rooting out sexual abuse on Telegram.”

A spokeswoman for the Joint Committee for Sex Abuse on Telegram, consisting of sexual violence hotlines and nongovernmental organizations, called for proper support measures for sex abuse victims who were “left neglected.”

The impact that Cho’s crimes had on the lives of young women was underlined this week in a public letter from an unnamed victim.

“You guys were confident about impunity and tried to scare me, devour me,” she wrote. “Seeing your faces getting revealed one by one, I only then realized that my life had been completely destroyed.”