Credit: Deano/Ferrett/Splash News Online

Amber Portwood is a mom on a mission.

After nude photos of the 20-year-old Teen Mom star leaked online Thursday, the Indiana resident tells E! News she plans to sue the person who stole them.

"I've never given anything to anybody. I wouldn't do that," she says. "I know 100 percent who did it, and I'm going to go after them full force."

Though Portwood doesn't mention the alleged thief by name, she has already ruled out one suspect: on-again beau Gary Shirley, father to her two-year-old daughter, Leah.

"No, no, Gary didn't do it," Portwood says. "He wasn't around me because of the no contact order when I took those pictures, but there were a bunch of people who were and I'm sure I know who it was."

What's stopping the MTV reality star from outing the purported thief?

"I want to surprise them," Portwood explains. "Those pics were taken for personal reasons. I didn't send them to anyone. I didn't take them for sexual reasons. This is embarrassing enough. It's just weird. The person who did this is f***ed."