6. Key Identification

6. Key Identification

It is necessary for the recipient of a JWS to be able to determine the key that was employed for the digital signature or MAC operation. The key employed can be identified using the Header Parameter methods described in Section 4.1 or can be identified using methods that are outside the scope of this specification. Specifically, the Header Parameters "jku", "jwk", "kid", "x5u", "x5c", "x5t", and "x5t#S256" can be used to identify the key used. These Header Parameters MUST be integrity protected if the information that they convey is to be utilized in a trust decision; however, if the only information used in the trust decision is a key, these parameters need not be integrity protected, since changing them in a way that causes a different key to be used will cause the validation to fail.

The producer SHOULD include sufficient information in the Header Parameters to identify the key used, unless the application uses another means or convention to determine the key used. Validation of the signature or MAC fails when the algorithm used requires a key (which is true of all algorithms except for "none") and the key used cannot be determined.

The means of exchanging any shared symmetric keys used is outside the scope of this specification.

Also, see Appendix D for notes on possible key selection algorithms.