Privacy Pass is an extension for Chrome and Firefox that reduces the number of CAPTCHAs users are presented with while browsing.
Privacy Pass interacts with supporting websites to introduce an anonymous user-authentication mechanism. In particular, Privacy Pass is suitable for cases where a user is required to complete some proof-of-work (e.g. solving an internet challenge) to authenticate to a service. In short, the extension receives blindly signed ‘passes’ for each authentication and these passes can be used to bypass future challenge solutions using an anonymous redemption procedure. For example, Privacy Pass is supported by Cloudflare to enable users to redeem passes instead of having to solve CAPTCHAs to visit Cloudflare-protected websites.
The blind signing procedure ensures that passes that are redeemed in the future are not feasibly linkable to those that are signed. We use a privacy-preserving cryptographic protocol based on ‘Verifiable, Oblivious Pseudorandom Functions’ (VOPRFs) built from elliptic curves to enforce unlinkability. The protocol is exceptionally fast and guarantees privacy for the user. As such, Privacy Pass is safe to use for those with strict anonymity restrictions.
The developers wrote a 2018 paper describing in detail how the protocol works to preserve user privacy while not compromising the security of sites that rely on CAPTCHAs to limit brute force and DDOS attacks.