Researchers Say Bitcoin Data Could Help Spot Sex Traffickers

Bitcoin is often described as an anonymous way to pay online, but a group of researchers says that data revealed through the cryptocurrency might help track down sex traffickers.

In a paper presented last month at the Association for Computing Machinery’s annual conference on knowledge discovery and data mining, researchers analyzed sex-related ads placed on the controversial classified listings site Backpage. They sought to locate groups of ads placed on the site by the same person, which they say could help investigators spot behavior that points to human trafficking and sex slavery.

“Such a tool would allow officers to confidently use timing and location information to distinguish between ads posted by women voluntarily in this industry versus those by women and children forcibly tracked,” they write. “For example, groups of ads—posted by the same owner—that advertise multiple different women across multiple different states at a high ad output rate, is a strong indicator of trafficking.”

One technique, using statistical analysis to compare the actual texts of different ads, proved to have a high success rate at finding postings apparently by the same group, says coauthor Rebecca S. Portnoff, a graduate student in computer science at the University of California, Berkeley. To validate whether their algorithms were, in fact, finding ads written by the same people, they looked for postings featuring common email addresses or phone numbers. That’s a technique often used by law enforcement and other investigators looking for sex or labor trafficking rings. Portnoff says it has limitations, since it’s easy to register new phone numbers and email accounts, but for now, it’s the most reliable method available.

“The best ground truth that exists in the space is the hard identifier, the phone numbers, and email addresses,” says Portnoff, who is also a research and data scientist at Thorn, a nonprofit cofounded by actor Ashton Kutcher to fight child sex exploitation.

The researchers also looked to the public, shared ledger of bitcoin transactions to find payments for multiple ads that seemed to stretch back to the same bitcoin wallet, essentially the cryptocurrency’s equivalent of a bank account.

Backpage has come under fire over the past few years for hosting sex-related ads. While the site looks and functions almost identically to Craigslist, the company has often been reluctant to stamp out erotic listings, and the postings in many cities where Backpage operates are dominated by ads for sexual services. That’s led Visa and Mastercard to cut ties with the company. (While Backpage eliminated some adult-themed classified sections early this year, sex ads have generally migrated to other sections, especially personals sections nominally focused on traditional dating.) The lack of credit card support has left the sex-ad posters paying for prominent placement with limited options, generally only bitcoin or personal checks, according to the researchers.

And when the ad posters choose bitcoin, they need to send the virtual currency to Backpage’s bitcoin payment processor, which means broadcasting the transaction to the bitcoin network for recording on the currency’s ledger, or blockchain. The researchers logged bitcoin transactions initiated just before ads popped up by the site (since Backpage usually posts listings quickly after payment is sent) and used data from blockchain analytics service Chainalysis to identify the ones that seemed to be headed to the payment processor.

Then they looked for apparent Backpage transactions associated with the same wallets, which suggest they might come from the same source, though they didn’t attempt to figure out real-world identities associated with the wallets. “We don’t take that final step to try to de-anonymize the wallet itself,” Portnoff says.

Police looking to do so would usually need to do some additional detective work, either tracing bitcoin transactions back to a digital exchange with police-accessible records or working from the ads themselves, she says.

“If you’re not that savvy, and your ads reveal some kind of [personally identifiable information] about you, that’s one way that law enforcement activities might be able to work backwards to the owner of the wallet,” she says.

Using automated systems to spot links between ads can help save law enforcement time and money and protect officers “from some of the psychological repercussions” of reviewing the sexually explicit ads, the researchers write.

But some advocates for sex workers worry such systems could drive their industry further underground or make it easy for dangerous clients or police to harass workers, putting voluntary participants in the sex industry at greater risk. The Sex Workers Outreach Project has for years applauded Backpage for providing a relatively safe and anonymous way for voluntary sex workers, including those operating both legally and illegally, to reach and screen potential clients.

According to Norma Jean Almodovar, founder and president of the International Sex Worker Foundation for Art, Culture and Education, overly broad crackdowns on trafficking have contributed to police harassment of voluntary sex workers and made it more difficult for them to safely ply their often illegal but nearly universal trade.

“They’ve taken away the places where sex workers felt safe and felt they had the opportunity to screen their clients,” she says. “First they used Craigslist, then they used Backpage, now they can’t even do that.”

Worries about unintended consequences from trafficking crackdowns haven’t been limited to the sex trade: Major internet companies including Google, Facebook, and Airbnb have recently warned that efforts to make sites like Backpage liable for sex trafficking posts could have unintended consequences for free speech on the rest of the Web.

In the U.S., sites have generally been exempt from liability for what users post, and a tech industry trade group called the Internet Association warned last month that changing that could lead to unforeseen consequences, even for sites that are taking action to stem illegal activities on their servers.

“While not the intention of the bill, it would create a new wave of frivolous and unpredictable actions against legitimate companies rather than addressing underlying criminal behavior,” said Michael Beckerman, CEO of the Internet Association, in a statement. “Furthermore, it will impose new, substantial liability risks for companies that take proactive measures to prevent trafficking online, hampering the ability of websites to fight illegal activity.”

Portnoff says she’s not too concerned about the group’s research being used to further endanger sex workers. And the researchers point out that the tool could even help police better distinguish between voluntary and involuntary transactions.

They do, however, acknowledge there might be ways for clever traffickers to make it harder for law enforcement to identify related purchases, like paying for Backpage credit in advance, using unrelated bitcoin wallets to pay for different ads, or even finding ways to pay via paper check. Backpage itself could also delay or randomize how long it takes for ads to appear, to thwart efforts to tie ads to bitcoin transactions, though Portnoff predicts that might annoy or confuse the site’s ad buyers.

The group plans additional research in the area, including looking into ways to analyze other data included in the ads, she says.

“One of the things we’re interested in is the new information that the images on the ads might be able to provide,” she says.