Clothoff, one of the most popular sites using artificial intelligence to generate fake nude photos of real people, uses what are called redirect sites to trick online payment services, according to a researcher.

Millions visit the more than 100 "nudify" sites online each month, according to Graphika, a company that analyzes social networks. And it's not clear who's running Clothoff, according to Kolina Koltai, a senior researcher at the international investigative group Bellingcat.

"There is a really inherent shadiness that's happening," she said. "They're not being transparent about who owns it. They're obviously trying to mask their payments."