Cloudflare Research logo

Privacy Pass

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.

Research Areas

Cryptography, Privacy


pendingA Fast and Simple Partially Oblivious PRF, with Applications

Under submission, 2021
Nirvan Tyagi, Sofía Celi, Thomas Ristenpart, Nick Sullivan, Stefano Tessaro, Christopher A. Wood

2018Privacy Pass: Bypassing Internet Challenges Anonymously

Proceedings on Privacy Enhancing Technologies, no. 3 (2018), pp. 164-180. 2018.
Alex Davidson, Ian Goldberg, Nick Sullivan, George Tankersley, Filippo Valsorda

Blog posts