For nearly three years, the "Welcome To Video" website has been a secret world for sick people to exchange videos of child sexual abuse.

Welcome To Video contains nearly 8 terabytes of child abuse content and over 200,000 videos have been downloaded a total of more than one million times. On October 16, the Justice Department revealed how they tracked bitcoin transactions to find the one behind the world's largest child porn site: A 23-year-old Korean young man named Jong Woo Son.

Moreover, 337 suspects related to the site were arrested and prosecuted in the US, UK, South Korea, Germany, Saudi Arabia, UAE, Czech, Canada, Ireland, Spain, Brazil and Australia. 23 young victims of abuse were also rescued in the US, Spain and England.

The owner of Welcome To Video runs the website from South Korea and uses bitcoin to transact in order to hide his identity. Artwork: CNN.

Welcome To Video is formed on darknet, the deep web platform cannot be accessed by normal browsers. Therefore, website users are always confident that they can not be tracked. According to authorities, some customers pay for child sexual abuse images with bitcoin, the digital currency is spent without users revealing their true identities.

But the collapse of Welcome To Video shows that bitcoin is not as secretive as many cyber criminals think.

Welcome To Video started its operation in June 2015. Unlimited site creator account. Users can download pornography videos if they pay for it in bitcoin or redeem points earned by referring new customers or posting their own videos. According to the indictment, the download page of the Welcome To Video clips always featured a notice: "Do not post adult pornography."

Bitcoin is considered the perfect payment method for those who want to hide their identities and traces in cyberspace. The digital currency is decentralized, meaning no company or bank monitors transactions. Users store bitcoins on a virtual account called an electronic wallet without having to prove their true identities like depositing money at a traditional bank.

From June 2015 to March 2018, Welcome to Video earned at least 420 bitcoins, worth about $ 370,000, through 7,300 transactions with users from many countries like the US, UK or South Korea.

To access Welcome to Video, users must have special software. Because the website is built on the darknet, people cannot use popular browsers like Google Chrome, Safari or Firefox to access the site. Customers need to download software that helps hide their IP address, a unique serial number assigned to each device connected to the Internet.

But in September 2017, by simply right-clicking on the Welcome to Video homepage and selecting the "view page source" section, the authorities suddenly discovered an un-disguised IP address. By October, in the same way, they found another IP address. Both led to the same residence address in Korea, Son's house.

At the same time, US investigators conducted a secret operation. Once in September 2017 and twice in February 2018, an undercover agent sent bitcoin to the account provided by Welcome to Video. The money was then transferred to another bitcoin account called Son and registered with his phone number and email.

In March 2018, authorities ransacked Son's home and found the host of the Welcome to Video website. From here, authorities can trace other suspects.

"To solve the case requires the cooperation of a lot of people," said Urszula McCormack, a partner at the law firm King and Wood Mallesons in Hong Kong, specializing in blockchain, the technology behind bitcoin. "Usually, just a few weak links will bring down the system."

Data from the Welcome to Video server is shared among law enforcement officials around the world. They use it to track and prosecute site customers in 18 countries.

In March 2018, Son was arrested in South Korea on charges of producing and distributing child pornography. Last May, he was sentenced to 18 months in prison. However, Son's sentence may not stop there. Last August, Son was indicted on several counts in the United States, including advertising child pornography. US law sets the maximum sentence for this crime is 30 years in prison.

While bitcoin is well known in the community for its security, the reality is not entirely so. Each time bitcoin is transferred, transaction details are recorded on a public "permanent ledger," said Yihao Lim, senior analyst from cybersecurity firm FireEye. Therefore, one can fully see what an individual is doing even though they cannot know their true identity.

In the US, the law requires virtual currency exchanges, the platform where users buy and sell bitcoins to exchange for real money, must verify customers' identities in the real world. Currently, more and more developed countries are adopting this measure. That means bicoin is not really anonymous.

There are many ways for bitcoin users to hide traces, but in general, authorities are catching up, Lim stressed.

Over the past year, tools capable of analyzing bitcoin transactions have evolved to new heights, McCormack from law firm King and Wood Mallesons said. "Many people today are not aware of the sophistication of those tools and how much information authorities can gather from just one trace," she said.

According to Lim, the public has misunderstood that using bitcoin is safe. "Yes, they were successful anonymously at an early stage but law enforcement agencies caught up," he said.