Balancing privacy and accountability is an age-old tradeoff. Give users anonymity, and it's difficult to tell if they're authorized to access a website, if they have sufficient funds to make a purchase, or to hold them accountable for what they do with either of those privileges.

What if we could make anonymous parties accountable to programmed rules they enforce themselves? Existing approaches, first developed in my work on private payments in Zcash, allow us to enforce a stateful system of rules using a combination of zero-knowledge proofs and, crucially,  techniques to ensure untrusted and anonymous clients cannot replay or fork their state. In this talk, I will give an overview of this approach and cover recent and ongoing work extending it to other privacy-preserving applications, including age verification, proof of humanity, and reputation.