Hash Check Service: innovating in the fight against online child exploitation

Offlimits' mission

"Offlimits is a foundation that works to stop online abuse, particularly the online sexual abuse of children," explains Yvette Velzeboer, the Hash Check Service Project Leader. "We began with the Reporting Hotline for Internet Child Pornography, which came into being at a time when the internet was becoming more accessible for consumers. We quickly saw that additional support was needed, so we added Helpwanted, a help service for the victims of abusive image sharing online, and Stop It Now, a help line for people worried about their own behaviour and/or feelings towards minors."

Offlimits therefore tackles online sexual exploitation in a variety of ways: reporting and removal of illegal content, support for victims, and prevention. "Our field of activity is now wider than it used to be. As well as fighting child abuse, we also work to combat other forms of internet-based abuse, such as cyberbullying and doxing," says Yvette. "However, protecting children against online exploitation remains central to what we do."

What is the Hash Check Service?

The Hash Check Service is a free technological solution that prevents child sexual exploitation images being reposted online. Yvette explains: "Once an image has been identified as illegal, it's given a unique digital fingerprint known as a 'hash'. The hash is then added to a database and used for vetting subsequent uploads. If anyone tries to upload the same image again, it's immediately blocked. Affiliated actors, such as hosting service providers, can voluntarily check any image uploaded to their servers to see whether its hash is in the Hash Check Service database."

The service therefore helps to prevent the 'revictimisation' of abused children. "The repeated posting of abusive material means that victims keep reliving the abuse. By using hashes, we can proactively prevent the reposting of abusive images, thus breaking that cycle of suffering."

"Another benefit of the Hash Check Service is that our analysts don't have to review as many images, because platform operators take down abusive material before it's reported to us. So the system eases the pressure on our team."

Constant innovation needed

Although the Hash Check Service has been in use for several years, ongoing technological refinement remains important. "At the moment, we're using SHA1 and MD5, technologies that depend on exact matches," says Yvette. "However, the people involved in sharing abusive images often modify them by, for example, cropping them or superimposing logos. And a modified image can evade detection by the existing systems."

Offlimits is therefore turning its attention to new hashing methods such as PDQ and PhotoDNA. "Methods like that enable the detection of slightly modified images. In the course of this year, we therefore plan to see which option is most effective, and how we can implement it," adds Yvette.

Support from SIDN Fund

Financial support from SIDN Fund means that Offlimits can push ahead with its innovation plans. "SIDN Fund's help is enabling us to make the Hash Check Service more resilient and to refine the technology," affirms Yvette. "We want to ensure that the tool can handle large volumes of data, and we want to make it even faster and more accurate."

As well as making technological upgrades, Offlimits is aiming to extend its network. "We'd like to build up the number of users by getting more hosting service providers of all sizes affiliated. At the moment, our focus is on approaching potential customers and finding out what technical criteria the service needs to meet for it to be convenient for them to use."

SIDN Fund's Elise van Schaik: "Offlimits is doing extremely important and valuable work. The Hash Check Service that they've developed is an innovative tool that prevents child sexual exploitation images being reposted online. The project we're funding is aimed at further technical development of the tool and expansion of the Offlimits network. It'll help to clean up the internet and make it more secure. More importantly still, it'll mean that children are better protected against online sexual violence. All in all, it's a very important project, which we're pleased to support."

Future plans

Ultimately, Offlimits hopes to get the Hash Check Service used all over the world. "We want to establish the tool as a standard weapon in the fight against the online sexual exploitation of children," Yvette emphasises. "By collaborating with hosting service providers and colleagues in other countries, we can establish a global network dedicated to stopping material of this kind ever getting onto the internet again."

Offlimits will therefore go on striving for technological innovation and increased collaboration. "We have to keep innovating in order to stay one step ahead of the abusers. SIDN Fund's support is enabling us to make important progress in that regard. Together, we can make the internet safer for children, and prevent revictimisation."

Want to know more about the Hash Check Service? Got any questions? Drop a line to Yvette at info@hashcheckservice.nl.