In the book "Founders at work", there is a chapter about Paypal's founder and half his interview is about how banks told them they'd be crushed by fraud, then how fraud was probably eating up 10-30% of their transfers, then how they built such an incredible alrogithm against fraud that they had to build a room around the server to protect their trade secret.

Oh wow. That's crazy how it seems never to be mentioned in the history of the "PayPal Mafia," or at least not the ones that I've read. Cool info, thanks.