The "kid" (key ID) Header Parameter is a hint indicating which key was used
to secure the [JWS][].  This parameter allows originators to explicitly signal a
change of key to recipients.  The structure of the "kid" value is
unspecified.  Its value MUST be a case-sensitive string.  Use of this Header
Parameter is OPTIONAL.

When used with a [JWK][], the "kid" value is used to match a [JWK][] "kid" parameter
value.

[JWK]: https://tools.ietf.org/html/rfc7517
[JWS]: https://datatracker.ietf.org/doc/html/rfc7515