#protocols #ristretto #privacy #group #token #pass #challenge

no-std challenge-bypass-ristretto

A rust implemention of the privacy pass cryptographic protocol using the Ristretto group (WIP)

6 releases (3 stable)

1.0.2 Jan 18, 2024
1.0.1 Apr 18, 2022
1.0.0 Oct 28, 2021
1.0.0-pre.0 Feb 1, 2019
0.1.0-pre.2 Dec 14, 2018

#998 in Cryptography

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MPL-2.0 license

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challenge-bypass-ristretto Build Status

A rust implemention of the privacy pass cryptographic protocol using the Ristretto group.

This library utilizes the wonderful curve25519-dalek which is a pure-Rust implementation of group operations on Ristretto.

It is only an implementation of the cryptographic protocol, it does not provide a service or FFI for use by other languages.

This crate is still a work in progress and is not yet recommended for external use.


This library exposes some functions intended to assist FFI creation but does not implement a FFI itself.

For FFI see challenge-bypass-ristretto-ffi.

Blinded Tokens

As originally implemented in the challenge bypass server and extension repositories, blinded tokens enable internet users to anonymously bypass internet challenges (CAPTCHAs).

In this use case, upon completing a CAPTCHA a user is issued tokens which can be redeemed in place of completing further CAPTCHAs. The issuer can verify that the tokens are valid but cannot determine which user they were issued to.

This method of token creation is generally useful as it allows for authorization in a way that is unlinkable. This library is intended for use in applications where these combined properties may be useful.

A short description of the protocol follows, a more detailed writeup is also available.

The blinded token protocol has two parties and two stages. A client and issuer first perform the signing stage, after which the client is able to derive tokens which can later be used in the redemption phase.


The client prepares random tokens, blinds those tokens such that the issuer cannot determine the original token value, and sends them to the issuer. The issuer signs the tokens using a secret key and returns them to the client. The client then reverses the original blind to yield a signed token.


The client proves the validity of their signed token to the server. The server marks the token as spent so it cannot be used again.


WARNING this library has not been audited, use at your own risk!

Example Usage

See tests/e2e.rs.


Run cargo bench

Security Contract

This software attempts to ensure the following:

  1. The signing server / issuer cannot link the blinded token it sees during signing with the token preimage or other info that is used at the time of redemption.
  2. The client cannot create a valid signed token without performing the VOPRF protocol with the server. Each protocol run produces a single valid token which cannot be used to create additional valid tokens.

Given that:

  1. The client keeps the blind secret, at time of issuance only sends the blinded token and at time of redemption only sends the payload, verification signature and token preimage. The client verifies the DLEQ proof that tokens were signed by a public key which was committed to previously and not a key unique to the user. The client ensures that other out of band markers like IP addresses cannot be used to uniquely link issuance and verification.
  2. The server keeps the signing key secret. The server marks a token preimage as spent after the first successful redemption.


By default this crate uses std and the u64_backend of curve25519-dalek. However it is no-std compatible and the other curve25519-dalek backends can be selected.

The optional features include base64 and serde.

  • base64 exposes methods for base64 encoding / decoding of the various structures.
  • serde implements the serde Serialize / Deserialize traits.

merlin is an experimental feature that uses merlin to implement the DLEQ proofs. This diverges from the original protocol specified in the privacy pass paper. It is not yet stable / intended for use and is implemented in src/dleq_merlin.rs.


Install rust.


Run cargo build


Run cargo test


~54K SLoC