Privacy Pass

Privacy Pass helps to make solving CAPTCHAs online less painful using zero-knowledge cryptography. This project was done in collaboration with researchers from Royal Holloway and the University of Waterloo. The core of Privacy Pass is a 1-RTT cryptographic protocol (based on an implementation of an oblivious pseudorandom function) that allows users to receive an amount of unlinkable tokens in exchange for solving a CAPTCHA challenge. Users can redeem tokens in the future for accessing to services without having to interact with a challenge, and the service cannot link the redeemed token to previously-issued tokens.

Launched in 2017, Privacy Pass is available in the form of a browser extension, and since then, it is in use by over a hundred thousand monthly active users in Chrome and Firefox.