Crescent

version 0.4

Crescent is a library to generate proofs of possession of JSON Web Tokens (JWT) and mobile Driver's Licenses (mDL) credentials. By creating a proof for a JWT or mDL, rather than sending it directly, the credential holder may choose to keep some of the claims in the credential private, while still providing the verifier with assurance that the revealed claims are correct and that the underlying credential is still valid.

This repository contains the Crescent library and a sample application consisting of a JWT issuer, a setup service, a browser extension client and client helper service, and a web server verifier. some external dependencies have been forked into this project; see the NOTICE file for details

Disclaimer: This code has not been carefully audited for security and should not be used in a production environment.

Documentation

A report describing Crescent is available on the IACR ePrint Archive.

Crescent: Stronger Privacy for Existing Credentials (DRAFT)
Christian Paquin, Guru-Vamsi Policharla and Greg Zaverucha
December 2024

Setting up

To setup the library, see the instructions in /circuit_setup/README.md; to setup the sample application, see sample/README.md.

To check that the library has been setup correctly, run

cd creds
cargo test --release

Enabling symlinks with git on Windows

This project uses symlinks to share directories within the project. On Windows, symlinks require administrator privileges. Git can be configured to create project symlinks when cloning the repository. To enable symlinks with git, run the following command:

git config --global core.symlinks true

If you have already cloned the repository, you can delete and re-clone the repository for the symlinks to be created or manually create the link by running the following CMD command in the project root directory:

mklink /J circuit_setup\circuits-mdl\circomlib circuit_setup\circuits\circomlib

Verify circuit_setup\circuits-mdl\circomlib is now a directory.

Running the demo steps from the command line

There is a command line tool that can be used to run the individual parts of the demo separately. This clearly separates the roles of prover and verifier, and shows what parameters are required by each. The filesystem is used to store data between steps, and also to "communicate" show proofs from prover to verifier.

The circuit setup must be completed first, by running

cd circuit_setup/scripts
./run_setup.sh rs256
./run_setup.sh mdl1
cd ../../creds

Circuit setup will copy data (parameters etc.) into creds/test-vectors/.

The individual steps are

• zksetup Generates the (circuit-specific) system parameters
• prove Generates the Groth16 proof for a credential. Stored for future presentation proofs in the "client state"
• show Creates a fresh and unlinkable presentation proof to be sent to the verifier
• verify Checks that the show proof is valid

and we can run each step as follows

cargo run --bin crescent --release --features print-trace zksetup --name rs256
cargo run --bin crescent --release --features print-trace prove --name rs256
cargo run --bin crescent --release --features print-trace show --name rs256
cargo run --bin crescent --release --features print-trace verify --name rs256

The --name parameter, used in circuit setup and with the command-line tool, specifies which credential type is used, the two examples are rs256, a JWT signed with RSA256, and mdl1 a sample mobile driver's license.

Note that the steps have to be run in order, but once the client state is created by prove, the show and verify steps can be run repeatedly.

Contributing

This project welcomes contributions and suggestions. Most contributions require you to agree to a Contributor License Agreement (CLA) declaring that you have the right to, and actually do, grant us the rights to use your contribution. For details, visit https://cla.opensource.microsoft.com.

When you submit a pull request, a CLA bot will automatically determine whether you need to provide a CLA and decorate the PR appropriately (e.g., status check, comment). Simply follow the instructions provided by the bot. You will only need to do this once across all repos using our CLA.

This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.

Trademarks

This project may contain trademarks or logos for projects, products, or services. Authorized use of Microsoft trademarks or logos is subject to and must follow Microsoft's Trademark & Brand Guidelines. Use of Microsoft trademarks or logos in modified versions of this project must not cause confusion or imply Microsoft sponsorship. Any use of third-party trademarks or logos are subject to those third-party's policies.