libpabc

Privacy-preserving Attribute-based Credentials

C implementation of BBS+ blind signatures using BLS12-381.

A problem with the concept of aggregated claims in combination with JWTs is excessive disclosure of information. While anonymous credentials such as Camenisch-Lysyanskaya [1] using blind signature schemes such as BBS+ can be used to enable selective disclosure of attributes, such signature schemes are not explicitly defined for the OIDC Aggregated Claims standard. For the DISSENS project, we have implemented this library that supports non-interactive zero knowledge proofs for the use case of self-sovereign identitiy. In particular, we have implemented PABC credentials in re:claimID.

With PABC credentials, SSI systems can support users to effectively selectively disclose attributes from a credential without invalidating the issuer’s signature.

How To Build

Dependencies:

• doxygen
• libb64
• gmp
• jansson
• relic (pulled by cmake)

Build library

$ mkdir build && cd build && cmake ../
$ make install (may need sudo)

How to use

Step 0: Prepare setup

Create an issuer

pabc-issuer --create-issuer TESTissuer

Create a set of parameters

pabc-issuer --issuer TESTissuer \
            --create-parameter TESTparams \
            --attributes Akey,Bkey,Ckey

Export / Import of issuer. Issuer needs to be public. Users and verifiers need to import it.

JSON_PP=$(pabc-issuer --params TESTparams --export)

Step 1: A user can now create credential requests

pabc-user --import-params "${JSON_PP}" --params TESTparams

Create a user

pabc-user --params TESTparams --create-user TESTuser

Set attributes

pabc-user --params TESTparams --user TESTuser --set-attr "Akey=Aval"
pabc-user --params TESTparams --user TESTuser --set-attr "Bkey=Bval"

Get a nonce/challenge to use for the CR.

JSON_NONCE=$(pabc-issuer --get-nonce)
echo "JSON_NONCE: ${JSON_NONCE}"

Generate a credential request

JSON_CR=$(pabc-user --params TESTparams --user TESTuser --create-cr "${JSON_NONCE}")
echo "JSON_CR: ${JSON_CR}"

Step 2:

Issuer now checks CR. If issuer is ok with CR -> generate credential

JSON_CERT=$(pabc-issuer --issuer TESTissuer \
                        --params TESTparams \
                        --expected-nonce "${JSON_NONCE}"\
                        --sign "${JSON_CR}")
echo "JSON_CERT: ${JSON_CERT}"

Step 3:

User can now create blinded proofs.

JSON_PROOF=$(pabc-user --params TESTparams \
                       --user TESTuser     \
                       --signed-cred "${JSON_CERT}" \
                       --reveal-attrs "Akey,Ckey")
echo "JSON_PROOF: ${JSON_PROOF}"

Step 4:

pabc-verifier --import-params "${JSON_PP}" --params TESTparams

Verify proof

pabc-verifier --params TESTparams --check "${JSON_PROOF}" && echo "SUCCESS :)"

Structure

include/pabc/*.h

This folder contains public API headers.

pabc.h

Include this in your project to make use of libpabc.

pabc_json_creds.h

This header provides a wrapper for raw pabc credentials that adds additional meta information. You probably want to make use of these functions in your project. All functions here are prefixed with pabc_cred_.

pabc_json_constants.h

This header defines JSON key names used throughout libpabc.

src/*

This folder contains the actual implementation. The files prefixed with pabc- implement a proof-of-concept CLI.

tests/* contains several test implementations

Setup

See setup_test for general system setup and creation of issuer key pair.

Credential Request

See cred_request_test for user key pair creation and credential request.

Issue Credential

See cred_issue_test for issuing a credential.

Proof / Presentation

See proof_test for creating a (blinded) proof/presentation.

Verification

See verify_test for verification of a proof/presentation.

cli_example.sh

This bash script demonstrates how to use the CLI. Run with ../tests/cli_example.sh from your build directory.

Disclaimer

libpabc is meant to be a research sandbox in which we can (re)implement protocols and potentially extend and modify functionality under the hood to support research projects. It is NOT a production grade solution and should not be used as such.

Implementations may not be correct or secure. Use at your own risk. This project makes use of the RELIC toolkit for cryptography which considers itself "at most alpha-quality software".

Coding Style

Please use the provided uncrustify.cfg.

References

1. J. Camenisch, M. Drijvers, and A. Lehmann. “Anonymous attestation using the strong diffie hellman assumption revisited”. In: International Conference on Trust and Trustworthy Computing. Springer. 2016, pp. 1–20.
2. Au, Man Ho, et al. "Constant-size dynamic k-times anonymous authentication." IEEE Systems Journal 7.2 (2012): 249-261.