TechPrufer
Get started
Cloudflare
Cloudflare logoCloudflare · Feature trail

Privacy Pass Protocol

9 posts · Nov 2017 – Aug 2025

What this trail covers

Cloudflare's Privacy Pass protocol has evolved with v2.0 of the browser extension, introducing TypeScript, a module bundler, and a refactored API for cryptographic primitives. This enhances maintainability and modularity, making it easier to integrate new service providers. The protocol is also progressing towards standardization at the IETF, with ongoing work on public verifiability and the inclusion of metadata in tokens to address hoarding attacks. Future work includes exploring blind signatures. Cloudflare's initial support for Privacy Pass, a privacy-preserving protocol leveraging zero-knowledge proofs, aimed to reduce the number of challenge pages presented to users accessing the web anonymously or through shared IPs. This helped improve the browsing experience for vulnerable users by allowing them to prove they are human across multiple sites on the Cloudflare network without revealing their identity or exposing customers to additional threats. The protocol is fully open source and available on GitHub.

All Cloudflare trails
  1. Reducing double spend latency from 40 ms to < 1 ms on privacy proxyThis post details the investigation and resolution of a 40ms latency issue in the double-spend check for Cloudflare's privacy proxy product. The root cause was identified as the interaction between Nagle's algorithm and delayed ACKs in TCP, which caused the `memcached get` command to be sent in multiple small packets, leading to a 40ms delay before the full command was processed. The fix involved using `BufWriter<TcpStream>` to buffer these small messages in user-space memory, allowing the entire command to be sent as a single packet, reducing latency to under 1ms.
  2. Privacy Pass: upgrading to the latest protocol versionThis post details the latest changes to the Privacy Pass protocol, including cryptographic updates and industry collaborations that shaped new standards like VOPRF and RSA Blind Signatures. It introduces public open-source implementations for Privacy Pass Origins, Issuers, and Attesters using Cloudflare Workers, and a new browser extension named 'Silk - Privacy Pass Client.' The post also discusses the protocol's evolution from its initial support in 2017 to its integration with Apple's Private Access Tokens and its expanded use cases for attestation beyond bot detection. It also announces the deprecation of the Privacy Pass API for configuration.
  3. Verify Apple devices with no installed softwareThis post announces the integration of Private Access Tokens (PATs) with Cloudflare Access. PATs leverage the Privacy Pass Protocol to allow device manufacturers (like Apple) to attest to a device's health and integrity without requiring client software installation. Cloudflare acts as both the Origin (requesting tokens) and the Issuer (generating tokens). This enables security teams to verify Apple devices (iOS initially) based on OS version, jailbreak status, and other factors before granting access to sensitive applications.
  4. Private Access Tokens: eliminating CAPTCHAs on iPhones and Macs with open standardsThis post introduces Private Access Tokens (PATs), an extension of the Privacy Pass protocol. PATs enable device vendors (like Apple) to attest to the authenticity of a device and user without sharing sensitive data with Cloudflare or the origin website. This allows Cloudflare's Managed Challenge platform to eliminate CAPTCHAs for supported devices, improving user experience and privacy. The post details the four parties involved (Origin, Client, Attester, Issuer) and the flow of token generation and validation.
  5. Privacy Pass v3: the new privacy bitsPrivacy Pass Extension v3.0 refactored to use TypeScript and a module bundler, improving maintainability and defining a clear API for cryptographic primitives. Provider-specific code is now treated as encapsulated software modules rather than configuration properties, allowing for greater flexibility and easier integration of new service providers. The core extension code is separated from provider-specific logic, and cryptographic primitives are factored out into a reusable module. The post also details the ongoing standardization efforts at the IETF for the Privacy Pass protocol, including work on public verifiability and the inclusion of metadata in tokens to mitigate hoarding attacks.
  7. Introducing Zero-Knowledge Proofs for Private Web Attestation with Cross/Multi-Vendor HardwareThis post introduces a new Zero-Knowledge Proof (ZKP) system for the browser to achieve private web attestation. Unlike existing WebAuthn attestation which reveals hardware make and model, this ZKP allows a website to verify the authenticity of a hardware security key without learning any identifying information about it. The system uses a proof that the attestation signature was generated by a key on a server-provided list, ensuring privacy by keeping identifying information on the client. The post details the challenges of existing attestation methods, the principles of ZKPs (using coin flipping and commitment schemes as examples), and the design considerations for an efficient browser-based ZKP system. It highlights the open-sourcing of the code and its potential applications beyond CAPTCHA alternatives.
  8. Helping build the next generation of privacy-preserving protocolsThis post introduces three new privacy-preserving protocols: Encrypted Client Hello (ECH) for fixing information leaks in HTTPS, Oblivious DNS-over-HTTPS (ODoH) for more private DNS, and OPAQUE for more secure password authentication. It highlights Cloudflare's ongoing tradition of supporting and developing technologies for a better Internet, citing past contributions to Universal SSL, TLS 1.3, QUIC, RPKI, NTS, Roughtime, and DNSSEC. The post emphasizes the importance of designing Internet protocols with user privacy in mind, distinguishing between data and metadata, and leveraging cryptography and secure proxies as tools for privacy.
  9. Supporting the latest version of the Privacy Pass ProtocolThis post details the release of Privacy Pass v2.0 browser extension and a new server backend using Cloudflare Workers. Key updates include: support for hCaptcha integration, improved cryptographic functionality with zero-knowledge proof validation and SSWU hashing to the P-256 curve, and enhanced key rotation mechanisms. The server backend utilizes the V8 engine for cryptographic operations and provides a public redemption API.
  10. Encrypt it or lose it: how encrypted SNI worksThis post introduces Encrypted SNI (ESNI) as an extension to TLS 1.3, building upon existing privacy features like Privacy Pass and encrypted DNS. It details how ESNI works by encrypting the SNI extension using a key derived from a public key published in DNS, preventing on-path observers from identifying visited websites. The post explains the cryptographic mechanisms involved (Diffie-Hellman) and the importance of TLS 1.3's encrypted certificate message. It also addresses the potential vulnerability of unencrypted DNS queries and the role of DNSSEC and DoT/DoH in mitigating this. Finally, it announces the free availability of ESNI for all Cloudflare zones and its upcoming support in Firefox Nightly.
  11. Encrypting SNI: Fixing One of the Core Internet BugsThis post announces the launch of Encrypted SNI (ESNI) across Cloudflare's network and its upcoming support in Firefox Nightly. ESNI aims to encrypt the Server Name Indication (SNI) extension, which previously leaked visited websites to ISPs and network observers. The protocol was developed in collaboration with industry partners and is being standardized as an IETF Draft RFC. A technical blog post on ESNI's implementation details and a testing tool are also mentioned.
  12. Verschlüsselung von SNI: Wie einer der großen Internet-Bugs behoben wurdeThis post announces the launch of Encrypted SNI (ESNI) across Cloudflare's network, marking a significant step in enhancing internet privacy. ESNI encrypts the Server Name Indication (SNI) field in TLS handshakes, preventing network observers (like ISPs) from seeing which website a user is visiting, even over HTTPS. This addresses a critical privacy leak in the existing encrypted web. Cloudflare collaborated with Apple, Fastly, and Mozilla on ESNI, and it is being standardized as an IETF draft RFC. Mozilla's Firefox is expected to be the first browser to support ESNI. The post also references the earlier launch of 1.1.1.1 as a privacy-enhancing DNS resolver and highlights the historical context of Cloudflare's commitment to making the web more private and secure, such as the introduction of Universal SSL.
  13. Introducing the Cloudflare Onion ServiceIntroduced Cloudflare Onion Services, which leverage HTTP Alternative Services to allow Tor Browser users to connect directly to Cloudflare's .onion addresses. This bypasses Tor exit nodes, improves security and performance, and enables more fine-grained rate-limiting by using circuit numbers to distinguish individual Tor connections. The implementation involves using a custom TLS 1.3 stack and relaying Client Hello packets to a local server that presents a certificate for the original hostname.
  14. Privacy Pass - “The Math”This post details the mathematical and cryptographic underpinnings of the Privacy Pass protocol, focusing on its evolution from RSA-based blind signing to an elliptic curve cryptography (ECC) based Oblivious Pseudorandom Function (OPRF) construction. It explains the blind signing phase and the redemption phase, highlighting the use of ECC for efficiency and the security considerations for unlinkability and preventing token inflation. The post introduces the core cryptographic primitives and protocol steps.
  15. Cloudflare supports Privacy PassThis post announces Cloudflare's support for the Privacy Pass protocol, a new privacy-preserving protocol developed in collaboration with researchers. It explains the problem of distinguishing legitimate users from bots, especially for those using anonymity services or shared IPs. The post details how Privacy Pass uses zero-knowledge proofs to allow users to prove their humanity across multiple sites anonymously, reducing the need to solve multiple challenges. It also highlights the open-source nature of the protocol and its availability as a browser extension and server implementation.