Signed JWT rejected: Another algorithm expected, or no matching key(s) found

Nimbus JOSE + JWT (Java) — BadJOSEException from DefaultJWTProcessor; also surfaced by Spring Security's NimbusJwtDecoder

What it means

Nimbus's key selector returned no candidate key for this token: either the token's `alg` is not one of the algorithms the selector accepts, or no key in the JWK set matches the token's `kid` and key type. Nimbus deliberately does not say which of the two it was. Behind Spring Security this is very often the RS256 default of `NimbusJwtDecoder` meeting an issuer that signs with ES256.

Why it happens

• The verifier expects RS256

  common

  Spring Security's `NimbusJwtDecoder` uses RS256 unless you configure algorithms (or it discovers them from the issuer's JWKS in newer versions). Keycloak, Auth0, Azure AD B2C and others can sign with ES256/PS256.

• The token's kid is not in the JWK set

  common

  A rotated key not yet published, a cached JWK set, or a JWK set URL belonging to another issuer or environment.

• Key metadata excludes the key

  occasional

  JWKs with `use: "enc"`, a different `alg`, or another key type (an RSA key set for an ES256 token) are filtered out by the matcher.

How to fix it

1. 1.

   Tell the decoder which algorithm the issuer uses

   `jwsAlgorithm(...)` (or `jwsAlgorithms(...)`) on the builder replaces the RS256 default. Keep the list to the issuer's real algorithms.

   @Bean
   JwtDecoder jwtDecoder() {
     return NimbusJwtDecoder.withIssuerLocation("https://issuer.example.com/realms/main")
         .jwsAlgorithm(SignatureAlgorithm.ES256) // the issuer's algorithm; RS256 is only the default
         .build();
   }

2. 2.

   Or configure it in application.yml

   Spring Boot exposes the same setting as `jws-algorithms`.

   spring:
     security:
       oauth2:
         resourceserver:
           jwt:
             issuer-uri: https://issuer.example.com/realms/main
             jws-algorithms: ES256

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