In a two-story building in the English university town of Cambridge, researchers at the U.K.'s Internet Watch Foundation pore over online images of sexually abused children in an effort to remove them from the Web. It is dispiriting work, and this year it grew more complicated when they found a new payment button next to the icons of Visa, MasterCard and PayPal: bitcoin.

The group's researchers in January started seeing the crypto-currency being accepted for child porn purchases ranging from as little as $1 to hundreds of dollars. Since then, the foundation has discovered almost 200 websites that accept bitcoin, and researchers in the U.S., Germany and several other countries are seeing the same. More than 30 sites accept only bitcoin, the IWF says.

"The emergence of bitcoins as payment for child sexual abuse represents the newest challenge in the fight" against child pornography, said Sarah Smith, a researcher at the IWF. "This is just the beginning."