The "alg" (algorithm) Header Parameter identifies the cryptographic
algorithm used to secure the JWS.  The JWS Signature value is not
valid if the "alg" value does not represent a supported algorithm or
if there is not a key for use with that algorithm associated with the
party that digitally signed or MACed the content.  "alg" values
should either be registered in the IANA "JSON Web Signature and
Encryption Algorithms" registry established by [JWA][] or be a value
that contains a Collision-Resistant Name.  The "alg" value is a case-
sensitive ASCII string containing a StringOrURI value.  This Header
Parameter MUST be present and MUST be understood and processed by
implementations.

A list of defined "alg" values for this use can be found in the IANA
"JSON Web Signature and Encryption Algorithms" registry established
by [JWA][]; the initial contents of this registry are the values
defined in Section 3.1 of [JWA][].

[JWA]: https://tools.ietf.org/html/rfc7518