add PKCE verifier for OIDC

[Rorical]

https://www.rfc-editor.org/rfc/rfc7636.html
To fix the error "Could not exchange code for the token" when using the PKCE method, a verifier should be generated and used during the authentication process.

• read the CONTRIBUTING guidelines
• raised a GitHub issue or discussed it on the projects chat beforehand
• added unit tests
• added integration tests
• updated documentation if needed
• updated CHANGELOG.md

Summary by CodeRabbit

• New Features

  • Enhanced OpenID Connect (OIDC) registration and callback process with Proof Key for Code Exchange (PKCE) for improved security.
  • Introduced a new data structure, RegistrationInfo, to manage OIDC-related information effectively.

• Bug Fixes

  • Improved validation processes to ensure secure handling of authorization code exchanges.

[coderabbitai]

Walkthrough

The changes enhance the OpenID Connect (OIDC) registration and callback process by implementing Proof Key for Code Exchange (PKCE). A new RegistrationInfo structure is introduced to store both the machine key and the PKCE verifier. The RegisterOIDC and OIDCCallback functions are modified to handle this new structure, improving the security of the authorization code exchange process.

Changes

File	Change Summary
hscontrol/oidc.go	Introduced PKCE support by adding a verifier to the registration cache and modifying related functions.
hscontrol/types/node.go	Added a new RegistrationInfo struct to encapsulate MachineKey and Verifier for OIDC validation.

Sequence Diagram(s)

sequenceDiagram
    participant User
    participant Client
    participant OIDCProvider
    participant Headscale

    User->>Client: Initiate OIDC registration
    Client->>Headscale: RegisterOIDC(machKey, verifier)
    Headscale-->>Client: Store RegistrationInfo

    User->>OIDCProvider: Authorise
    OIDCProvider-->>User: Redirect with auth code
    User->>Client: Redirect back with auth code
    Client->>Headscale: OIDCCallback(authCode, regInfo)
    Headscale->>Headscale: Validate RegistrationInfo
    Headscale->>OIDCProvider: Exchange auth code for token
    OIDCProvider-->>Headscale: Return access token
    Headscale-->>Client: Return access token

Loading

🐰 In the meadow, a change so bright,
With PKCE, we leap to new heights.
RegistrationInfo, a clever new friend,
Ensures our security will never bend.
Hopping along, we celebrate this feat,
In the world of OIDC, we can't be beat! 🌼

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Recent review details

Configuration used: .coderabbit.yaml
Review profile: CHILL

Commits

Files that changed from the base of the PR and between fe68f50 and 64fd5f4.

Files selected for processing (2)

• hscontrol/oidc.go (6 hunks)
• hscontrol/types/node.go (1 hunks)

Additional comments not posted (6)

hscontrol/types/node.go (1)

562-566: LGTM!

The new RegistrationInfo struct looks good. It groups the MachineKey and Verifier fields, which seem to be related to OIDC registration. The field names are clear and the types appear appropriate based on the context.

Grouping this information in a struct can help in organizing the code and passing the registration data around more easily.

hscontrol/oidc.go (5)

136-144: LGTM!

Generating a PKCE code verifier and storing it alongside the node key in the state cache is a good security practice. It enhances the security of the authorization code exchange process by preventing certain attacks.

---

188-212: Proper state validation!

The code segment correctly retrieves the state from the registration cache and validates it. Appropriate error responses are generated for invalid or expired states, which helps maintain security.

---

313-315: Secure token exchange!

Using the stored verifier when exchanging the authorization code for an access token is a good security practice. It ensures that the authorization code was obtained from a valid authorization request, preventing certain attacks.

---

477-487: Simplified node validation!

Using the RegistrationInfo directly instead of retrieving the machine key from the state cache simplifies the code and reduces the chances of errors. This is a good refactoring.

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549-549: Consistent machine key retrieval!

Returning the machine key from the RegistrationInfo is consistent with the changes made to the validateNodeForOIDCCallback function. This maintains the integrity of the refactoring.

---

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[kradalby]

I imagine this can go in and be rebased on top of #2020, whenever that is ready.

[kradalby]

@Rorical if you have time to rebase this in the next few days, then it might be able to make it into the next release.

[Rorical]

This could be resolved soon. I'm away from my headscale instance for months and haven't realized current status of this project, so I need some time to check what's going on.

Based on the purpose of pkce, I recommend to add a oidc config parameter EnablePKCE, since pkce will only provide a extra layer of security and totally compitable with currect setup. However, user should have the option to enable or disable this feature.

[Rorical]

Testing and documentation will be added soon