Adding crit headers in JWTs makes the signature failed

[quentin castel]

Hello!

It's more a question in regards to your signature verifier:

I was troubleshooting why suddenly my JWTs are detected as invalid. I finally found the issue and this is due to line:

https://bitbucket.org/connect2id/nimbus-jose-jwt/src/409673c518b17f39d49a1ce17620b8cf1b58e005/src/main/java/com/nimbusds/jose/crypto/RSASSAVerifier.java#lines-155

if (! critPolicy.headerPasses(header)) {
            return false;
        }

I am not sure to understand why having 'crit' headers makes automatically the signature invalid?

Thanks!

[Vladimir Dzhuvinov]

JOSE has this notion of critical JWS / JWE headers, which if not understood by the recipient should cause sig validation / decryption to fail.

If you have critical headers that the application knows how to handle - list their names using the com.nimbusds.jose.crypto.RSASSAVerifier#RSASSAVerifier(java.security.interfaces.RSAPublicKey, java.util.Set<java.lang.String>) constructor.

If you have critical headers that act as inputs to the signature validation, override the RSASSAVerifier to take care of that.

https://tools.ietf.org/html/rfc7515#section-4.1.11

[quentin castel]

Perfect, thanks Vladimir. Really helpful.

I was thinking: Would it not be nice if the verify signature returns an exception like InvalidToken and returns as a message the root cause of the issue?

The error returns to the end user would be more explicit.

[Vladimir Dzhuvinov]

Packing extra info why the validation failed is a good idea.

https://bitbucket.org/connect2id/nimbus-jose-jwt/issues/306/

We could do that with exceptions. However, their performance to handle control flow like situations (invalid signature) is bad. Especially if the software needs to process a lot of tokens, and many of them can be invalid. This was addressed with caching them in the JWT processor, but that also has issues (we have a ticket for that).