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