COSE Algorithm ID Update: -9 For OpenID4VP & HAIP?

This article delves into a proposal to update the OpenID4VP (OpenID for Verifiable Presentations) and HAIP (Health Assurance Interoperability Program) specifications regarding COSE (CBOR Object Signing and Encryption) algorithm identifiers. The current specifications primarily emphasize the use of the COSE algorithm identifier -7, which represents ECDSA (Elliptic Curve Digital Signature Algorithm) with SHA-256. The proposal suggests incorporating -9, an identifier that explicitly defines the elliptic curve used, namely P-256, alongside SHA-256. This addition aims to enhance interoperability and clarity within these frameworks. Let's examine the rationale behind this proposal, its potential benefits, and the implications it might have on the broader ecosystem.

Understanding the Current Landscape

Currently, OpenID4VP and HAIP specifications define specific cryptographic algorithms that issuers, verifiers, and wallets must support to ensure seamless and secure data exchange. These algorithms are crucial for validating digital signatures and ensuring the integrity and authenticity of verifiable presentations. Specifically, the specifications mandate support for ECDSA with SHA-256, identified by the JOSE (JSON Object Signing and Encryption) algorithm identifier ES256. In the COSE context, this corresponds to the algorithm identifier -7.

The OpenID4VP specification provides an example illustrating the relationship between JOSE and COSE algorithm identifiers. It explains that if the IssuerAuth structure contains an alg header with the value -7 (ECDSA with SHA-256) and is signed by a P-256 key, it aligns with an issuerauth_alg_values element of both -7 and -9. Here, -9 explicitly denotes ECDSA using the P-256 curve and SHA-256, as defined in the IETF JOSE Fully Specified Algorithms draft. This distinction is critical because while -7 implies the use of a P-256 key in this context, it doesn't explicitly state it, potentially leading to ambiguity or implementation inconsistencies.

HAIP further reinforces the requirement for ECDSA with P-256 and SHA-256, mandating that issuers, verifiers, and wallets must support this algorithm (ES256, COSE algorithm identifier -7) for validating various components within the HAIP ecosystem. This requirement underscores the importance of this specific cryptographic algorithm in ensuring secure and interoperable health information exchange.

The Case for Including COSE Algorithm Identifier -9

The core argument for including COSE algorithm identifier -9 alongside -7 is to promote clarity and avoid potential ambiguity in implementations. While -7, in the context of OpenID4VP and HAIP, typically implies the use of the P-256 curve, it does not explicitly state it. This implicit understanding can lead to inconsistencies in how different systems interpret and implement the specification. By explicitly allowing -9, which fully specifies the algorithm as ECDSA with the P-256 curve and SHA-256, the specification removes any room for misinterpretation.

Specifically, the inclusion of -9 caters to implementations that adhere to the principle of fully-defined algorithms. These implementations prefer explicit declarations of all algorithm parameters, including the specific elliptic curve used. This approach enhances security and reduces the risk of algorithm substitution attacks or other subtle vulnerabilities. By supporting both -7 and -9, the specification accommodates a wider range of implementation preferences and security considerations.

Furthermore, the change is relatively minor and backwards compatible. Systems that currently only support -7 would continue to function as expected. Systems that prefer or require fully-defined algorithms can now utilize -9 without violating the specification. This flexibility allows for a smoother transition and broader adoption of the OpenID4VP and HAIP standards.

Proposed Textual Change

To implement this change, the proposal suggests modifying the text in the HAIP specification to read:

(JOSE algorithm identifier ES256; COSE algorithm identifier -7 or -9, as applicable)

This simple modification explicitly allows for the use of either -7 or -9 as the COSE algorithm identifier for ECDSA with P-256 and SHA-256. The "as applicable" clause acknowledges that the choice between -7 and -9 may depend on the specific implementation and its requirements for algorithm explicitness.

Benefits of the Proposed Change

The inclusion of COSE algorithm identifier -9 offers several key benefits:

• Enhanced Interoperability: By explicitly allowing both -7 and -9, the specification reduces the potential for ambiguity and ensures that different implementations can seamlessly interoperate.
• Improved Security: Fully-defined algorithms, like the one identified by -9, enhance security by reducing the risk of algorithm substitution attacks and other vulnerabilities.
• Increased Flexibility: The change accommodates a wider range of implementation preferences, allowing developers to choose the algorithm identifier that best suits their needs.
• Backwards Compatibility: Existing systems that only support -7 will continue to function without modification.
• Clarity and Precision: Using -9 removes any ambiguity about the specific elliptic curve being used, promoting clearer and more precise implementations.

Potential Considerations

While the proposed change offers significant benefits, there are a few potential considerations to keep in mind:

• Specification Updates: The OpenID4VP and HAIP specifications would need to be updated to reflect the inclusion of -9. This update should clearly explain the rationale behind the change and provide guidance on when to use -7 versus -9.
• Implementation Guidance: Developers may benefit from additional guidance on how to choose between -7 and -9. This guidance could explain the trade-offs between conciseness and explicitness and help developers make informed decisions based on their specific requirements.
• Testing and Validation: New test cases may be needed to ensure that implementations correctly support both -7 and -9. These test cases should cover a range of scenarios to ensure interoperability and security.

Conclusion

The proposal to include COSE algorithm identifier -9 in the OpenID4VP and HAIP specifications is a valuable step towards enhancing interoperability, security, and clarity. By explicitly allowing both -7 and -9, the specification accommodates a wider range of implementation preferences and reduces the potential for ambiguity. This change is backwards compatible, relatively easy to implement, and offers significant benefits to the ecosystem. As OpenID4VP and HAIP continue to evolve, incorporating such improvements will be crucial for ensuring the widespread adoption and secure use of these important standards.

For further reading on COSE and its algorithm identifiers, you can refer to the IANA COSE registry.