Exposing Protected Resource Metadata: Attributes & OpenId4VCI

Hey guys! Let's dive into an interesting discussion about exposing Protected Resource Metadata, especially in the context of the European Digital Identity Wallet (eudi-wallet) and the eudi-srv-pid-issuer. This is super relevant, and we need to make sure we're all on the same page.

Understanding Protected Resource Metadata

So, what's the big deal with Protected Resource Metadata? Well, it's all about providing a structured way to document the decisions and configurations of our implementation. Think of it as a detailed instruction manual for anyone (or anything) trying to interact with our protected resources. According to RFC9728, this metadata can be incredibly helpful, and while it's not explicitly required by OpenID for Verifiable Credential Issuance (OpenId4VCI), it can seriously streamline things and make life easier for everyone involved.

When we talk about exposing protected resource metadata, we're essentially talking about making certain attributes publicly available so that others can understand how our system works. This is particularly useful in complex systems where multiple components need to interact seamlessly. By exposing this metadata, we can reduce ambiguity, improve interoperability, and make debugging a whole lot less painful. Imagine trying to assemble a piece of furniture without the instructions – that's what it's like trying to integrate with a system that doesn't expose its metadata! Therefore, implementing this approach not only enhances transparency but also fosters a more collaborative and efficient ecosystem.

To make this even more concrete, let's consider a scenario where a relying party wants to request a verifiable credential from our issuer. Without metadata, the relying party would have to guess or manually configure various parameters, such as the supported scopes, authentication methods, and DPoP settings. This is not only inconvenient but also error-prone. By exposing the metadata, we provide a clear and machine-readable description of these parameters, allowing the relying party to automatically configure its request and ensure compatibility. This automated configuration significantly reduces the likelihood of integration issues and makes the overall process smoother and more reliable. It's like having a universal translator that ensures everyone understands each other, regardless of their native language.

Key Attributes for Consideration

Now, let’s get into the nitty-gritty of what attributes we should consider exposing. These attributes will provide a comprehensive overview of our protected resources and how they should be accessed.

1. resource

First up, we have the resource attribute. This should have the same value as the credential_issuer from the Credential Issuer metadata in OpenId4VCI. Essentially, it's a unique identifier for our protected resource. This consistency is crucial because it allows different components of the system to easily identify and reference the same resource. By aligning the resource attribute with the credential_issuer, we create a clear link between the resource being protected and the issuer responsible for it. This simplifies the overall architecture and makes it easier to reason about the system. It's like having a social security number for a resource – it provides a unique and unambiguous way to identify it.

2. authorization_servers

Next, we have authorization_servers. This should be identical to the authorization_servers from the Credential Issuer metadata in OpenId4VCI. This attribute lists the authorization servers that are responsible for issuing access tokens for our protected resource. In a typical OAuth 2.0 or OpenID Connect flow, an authorization server plays a critical role in verifying the identity of the client and granting access to protected resources. By exposing this attribute, we provide a clear indication of which authorization servers are trusted and authorized to issue tokens for our resource. This is important for security because it helps clients ensure that they are interacting with legitimate authorization servers and not falling victim to phishing or other attacks. It's like having a list of approved gatekeepers – you know that anyone not on the list shouldn't be trusted.

3. scopes_supported

Then comes scopes_supported. This is an array that lists the scopes defined by our Credential Issuer for accessing the Credential endpoint. Scopes are a fundamental concept in OAuth 2.0 and OpenID Connect. They define the level of access that a client is requesting and that the authorization server is willing to grant. By exposing the supported scopes, we provide clients with a clear understanding of what they can and cannot do with the access tokens they obtain. This is essential for ensuring that clients only request the minimum necessary permissions, which is a key principle of secure application design. It's like having a menu of permissions – you can only order what's on the menu.

4. bearer_methods_supported

We also have bearer_methods_supported. Initially, this would likely include header, assuming we continue to support bearer authentication. Bearer authentication is a common method for transmitting access tokens, where the token is included in the HTTP Authorization header. If we decide to remove support for bearer authentication in the future, we would need to update this attribute accordingly. This is important because it tells clients how to present their access tokens to our protected resource. It's like having instructions on how to use a key – you need to know where to insert it.

5. authorization_details_types_supported

For authorization_details_types_supported, we might initially set this to an empty array. This explicitly states that we don't support authorization details as defined in OpenId4VCI. Authorization details are a more advanced feature that allows clients to request access to specific resources or actions within a protected resource. By indicating that we don't support this feature, we simplify the implementation and reduce the complexity for clients. This is not to say that we will never support authorization details, but for the initial implementation, it may be a reasonable decision. It's like saying, "We don't offer this particular feature yet, but we might in the future."

6. dpop_signing_alg_values_supported

For dpop_signing_alg_values_supported, we'd have a non-empty array, likely including only what's described in HAIP (Hyperledger Aries Interop Profile). DPoP, or Demonstration of Proof-of-Possession, is a security mechanism that binds access tokens to a specific client, preventing them from being used by others. By specifying the supported signing algorithms, we ensure that clients use a compatible method for signing their DPoP proofs. This is a critical security measure that helps to prevent token theft and replay attacks. It's like having a specific lock and key – you need to use the right key to open the lock.

7. dpop_bound_access_tokens_required

Finally, there's dpop_bound_access_tokens_required. We should probably set this to false initially to indicate that we don't require DPoP-bound access tokens. However, if we decide to go all-in on the HAIP option and remove support for bearer access tokens, we should change this value to true. This is a key decision point that has significant implications for the security and complexity of our implementation. If we require DPoP-bound tokens, we can achieve a higher level of security, but it also adds complexity for clients. If we don't require them, we simplify the client integration process but potentially reduce security. It's a trade-off between security and usability that we need to carefully consider. It's like deciding whether to use a simple padlock or a high-security combination lock.

Impact of DPoP Requirement

The decision to require DPoP-bound access tokens is a significant one, guys. If we stick with false, we allow for simpler integration, but we might leave the door open for certain types of attacks. Setting it to true ramps up the security but adds complexity. We need to weigh these pros and cons carefully.

Think of it this way: requiring DPoP is like adding an extra layer of authentication. It ensures that the client possessing the access token is also the one authorized to use it. This prevents scenarios where an attacker might steal a token and use it to access protected resources. However, it also means that clients need to implement the DPoP protocol, which involves generating and signing cryptographic proofs. This can be a non-trivial task, especially for clients that are not familiar with these technologies.

On the other hand, not requiring DPoP simplifies the client integration process. Clients can simply include the access token in the Authorization header, as is common in many OAuth 2.0 implementations. This makes it easier for developers to get started and reduces the learning curve. However, it also means that we are relying on the security of the underlying transport layer (e.g., HTTPS) to protect the access token. If the token is intercepted, an attacker could potentially use it to access protected resources. Therefore, it's crucial to strike a balance between security and usability and choose the option that best fits our specific requirements and risk tolerance.

Conclusion

Exposing Protected Resource Metadata is a smart move for clarity and interoperability. By carefully considering these attributes, especially the DPoP requirement, we can create a more robust and user-friendly system. This will not only benefit our immediate implementation but also contribute to a more standardized and interoperable ecosystem for digital identity wallets. Remember, the goal is to make it as easy as possible for everyone to interact with our system while maintaining a high level of security.

By providing clear and comprehensive metadata, we empower developers to build secure and reliable applications that leverage the full potential of our protected resources. This, in turn, fosters innovation and drives the adoption of digital identity technologies. It's like providing a well-documented API – the easier it is to use, the more people will use it.

For further information on RFC9728, check out the RFC Editor website for detailed specifications.