[anakinj]

Description

JWT::EncodedToken called key_finder before looking at the allowed algorithms, so alg: none or HS256 tokens that could never verify against an RS256-only check still triggered a key lookup, which is often a remote JWKS request. JWT.decode already rejects these first.

#valid_signature? now returns false without calling key_finder when the token's alg is not allowed, so #verify_signature! raises JWT::VerificationError. That is the error these tokens already raised, so it differs from the JWT::IncorrectAlgorithm suggested in the issue, to keep existing rescues working and the predicate from raising.

An empty algorithm list skips the check, so a key finder can still supply a JWK that determines the algorithm. A key_finder token whose alg header does not match the allowed algorithm used to verify and is now rejected. The key: path is unchanged, because an existing spec expects it to tolerate that mismatch.

valid_signature? also clears any earlier verification state up front, so a rejected recheck can no longer leave payload readable. Custom algorithms must match the token header through valid_alg?, as they already must in JWT.decode, so one with a symbol or nil alg no longer verifies through a key_finder. An unknown algorithm name still raises JWT::VerificationKeyError, so a configuration typo is not reported as a bad signature.

Fixes #781

Checklist

Before the PR can be merged be sure the following are checked:

• There are tests for the fix or feature added/changed
• A description of the changes and a reference to the PR has been added to CHANGELOG.md. More details in the CONTRIBUTING.md

Posted by Claude on behalf of @anakinj.

[Copilot]

🟢 Approval recommended

No active review issues remain for the change to reject disallowed token algorithms before key lookup.

No open issues 🎉

What changed in this PR

This PR updates JWT::EncodedToken to reject disallowed token algorithms before invoking a key finder, avoiding unnecessary key lookups and remote JWKS requests.

Changes:

• Checks allowed algorithms before key lookup while preserving JWK-derived algorithm selection and direct-key behavior.
• Clears previous signature-verification state before rechecking and adds regression coverage.
• Documents the behavior change and sets RuboCop’s class-length limit to 120.

File	Description
spec/​jwt/​encoded_token_spec.rb	Tests early rejection, algorithm compatibility, verification-state reset, and JWK fallback.
lib/​jwt/​encoded_token.rb	Adds the algorithm precheck and resets signature-verification state.
CHANGELOG.md	Documents the key-finder behavior change.
.rubocop.yml	Sets the maximum class length to 120.

_{Review effort: Balanced}