Fixing SD-JWT VC Signature Verification With X5c

by Alex Johnson 49 views

Introduction

This article delves into a critical issue identified in the verification of SD-JWT VC (Selective Disclosure JSON Web Token Verifiable Credentials) signatures, specifically when using the x5c (X.509 Certificate Chain) header parameter. The problem arises from an incorrect implementation in the library that attempts to validate the issuer (iss) value against the Subject Alternative Name (SAN) entries of the leaf certificate. This behavior, while aligned with an older draft specification, deviates from the updated requirements in draft 10 of the SD-JWT VC standard.

In this comprehensive guide, we will explore the background of this issue, the differences between draft 9 and draft 10 specifications, and the necessary steps to rectify the implementation. We aim to provide a clear understanding of the problem and its resolution, ensuring that developers and users can accurately verify SD-JWT VC signatures.

Background of the Issue

The core of the issue lies in how the library handles the verification of the issuer's public key when the x5c header is present in the SD-JWT. In earlier drafts of the SD-JWT VC specification (specifically, draft 9), a stringent requirement was imposed on the validation process. According to this draft, if the iss value was an HTTPS URI, the recipient had to:

  1. Obtain the public key from the end-entity certificate in the x5c header.
  2. Validate the X.509 certificate chain.
  3. Ensure that the iss value matched either a uniformResourceIdentifier SAN entry or the domain name in the iss value matched a dNSName SAN entry of the end-entity certificate.

This approach, while providing a robust validation mechanism, introduced complexities and potential bottlenecks in the verification process. It necessitated additional steps to extract and compare the iss value against SAN entries, which could be resource-intensive and time-consuming.

The Shift from Draft 9 to Draft 10

The SD-JWT VC specification underwent a significant revision with the transition from draft 9 to draft 10. One of the key changes was the simplification of the issuer verification process when using x5c. The updated specification, as outlined in draft 10, states:

X.509 Certificates: A mechanism to retrieve the Issuer's public key using the X.509 certificate chain in the SD-JWT header. When the header of the Issuer-signed JWT contains the x5c header parameter, the recipient uses the public key from the end-entity certificate of the certificates from the x5c header parameter and validates the X.509 certificate chain accordingly. In this case, the Issuer of the Verifiable Credential is the subject of the end-entity certificate.

This revision removed the requirement to match the iss value against SAN entries in the certificate. Instead, it mandates that the recipient should use the public key from the end-entity certificate to validate the SD-JWT's signature and ensure the X.509 certificate chain's validity. The issuer of the Verifiable Credential is now simply considered the subject of the end-entity certificate.

This change streamlined the verification process, making it more efficient and less prone to errors. It also aligned the specification with common practices in X.509 certificate usage, where the subject of the certificate is the primary identity associated with the certificate.

Impact of the Incorrect Implementation

The existing library implementation, which adheres to the draft 9 requirements, introduces several issues:

  • Verification Failures: SD-JWT VCs issued according to draft 10 may fail verification because the library incorrectly attempts to match the iss value against SAN entries.
  • Performance Overhead: The unnecessary check for SAN entries adds computational overhead to the verification process, slowing it down.
  • Compliance Issues: The implementation is not compliant with the current SD-JWT VC specification (draft 10), potentially leading to interoperability problems.

These issues underscore the importance of updating the library to align with the latest specification. Failure to do so can result in incorrect verification outcomes, performance degradation, and compliance violations.

Identifying the Problematic Code

The specific code segment responsible for the incorrect behavior is within the NimbusSdJwtVcVerifierFactory#keySource(NimbusSignedJWT): SdJwtVcIssuerPublicKeySource method. This method is responsible for determining the source of the issuer's public key for signature verification. In the current implementation, when an x5c header is present, the method incorrectly attempts to validate the iss value against SAN entries in the end-entity certificate.

To rectify this, we need to modify this method to align with the draft 10 specification. The updated implementation should:

  1. Extract the public key from the end-entity certificate in the x5c header.
  2. Validate the X.509 certificate chain.
  3. Use the extracted public key for signature verification without attempting to match the iss value against SAN entries.

Steps to Correct the Implementation

To address this issue, the following steps are necessary:

  1. Locate the NimbusSdJwtVcVerifierFactory#keySource Method: Identify the method within the library's codebase.
  2. Modify the Method: Update the method to remove the logic that checks the iss value against SAN entries. Instead, focus on extracting the public key and validating the certificate chain.
  3. Implement Certificate Chain Validation: Ensure that the certificate chain validation is performed correctly. This typically involves verifying the chain of trust from the end-entity certificate to a trusted root certificate.
  4. Use the Public Key for Verification: Use the extracted public key to verify the signature of the SD-JWT VC.
  5. Test the Implementation: Thoroughly test the updated implementation with various SD-JWT VCs, including those issued according to draft 10, to ensure correct verification.

Detailed Code Modification

To provide a clearer understanding, let's outline the code modifications required (Note: The exact code will depend on the specific library implementation):

Original Code (Illustrative)

public SdJwtVcIssuerPublicKeySource keySource(NimbusSignedJWT jwt) throws  {
    if (jwt.getHeader().getX509CertChain() != null) {
        // ... existing code ...
        // Incorrectly checks iss against SAN entries
        validateIssAgainstSan(jwt.getJWTClaimsSet().getIssuer(), endEntityCert);
        // ...
    }
    // ...
}

Updated Code (Illustrative)

public SdJwtVcIssuerPublicKeySource keySource(NimbusSignedJWT jwt) throws  {
    if (jwt.getHeader().getX509CertChain() != null) {
        // ... existing code ...
        // Remove the incorrect check
        // validateIssAgainstSan(jwt.getJWTClaimsSet().getIssuer(), endEntityCert);
        // ...
    }
    // ...
}

The key change here is the removal of the validateIssAgainstSan call, which is responsible for the incorrect behavior. The updated code will now proceed with signature verification using the public key from the end-entity certificate without attempting to match the iss value against SAN entries.

Testing the Solution

After implementing the changes, it is crucial to thoroughly test the solution. The testing process should include:

  • Unit Tests: Write unit tests to verify the behavior of the keySource method and the signature verification process.
  • Integration Tests: Conduct integration tests to ensure that the library works correctly with other components of the system.
  • SD-JWT VC Test Suite: Utilize a comprehensive SD-JWT VC test suite to cover various scenarios and edge cases.

Specifically, the tests should cover:

  • SD-JWT VCs with x5c headers issued according to draft 10.
  • SD-JWT VCs without x5c headers.
  • SD-JWT VCs with invalid signatures or certificate chains.

Conclusion

The issue of incorrect behavior in SD-JWT VC signature verification when using x5c highlights the importance of staying current with specification updates and ensuring correct implementation. By understanding the differences between draft 9 and draft 10 of the SD-JWT VC standard and implementing the necessary code modifications, developers can ensure accurate and efficient verification of SD-JWT VCs.

The steps outlined in this article provide a comprehensive guide to identifying and resolving this issue. By following these recommendations, you can enhance the security and reliability of your SD-JWT VC implementation.

For further information on SD-JWT and related standards, please refer to the official documentation and specifications. You can find more details on draft-ietf-oauth-sd-jwt-vc.