This new converter takes a JSON Web Token, an algorithm (among the ones
specified for JWS tokens in RFC 7518) and a public key or a secret, and
it returns a verdict about the signature contained in the token. It does
not simply return a boolean because some specific error cases cas be
specified by returning an integer instead, such as unmanaged algorithms
or invalid tokens. This enables to distinguich malformed tokens from
tampered ones, that would be valid format-wise but would have a bad
signature.
This converter does not perform a full JWT validation as decribed in
section 7.2 of RFC 7519. For instance it does not ensure that the header
and payload parts of the token are completely valid JSON objects because
it would need a complete JSON parser. It only focuses on the signature
and checks that it matches the token's contents.
A JWT signed with the RSXXX or ESXXX algorithm (RSA or ECDSA) requires a
public certificate to be verified and to ensure it is valid. Those
certificates must not be read on disk at runtime so we need a caching
mechanism into which those certificates will be loaded during init.
This is done through a dedicated ebtree that is filled during
configuration parsing. The path to the public certificates will need to
be explicitely mentioned in the configuration so that certificates can
be loaded as early as possible.
This tree is different from the ckch one because ckch entries are much
bigger than the public certificates used in JWT validation process.
This helper function splits a JWT under Compact Serialization format
(dot-separated base64-url encoded strings) into its different sub
strings. Since we do not want to manage more than JWS for now, which can
only have at most three subparts, any JWT that has strictly more than
two dots is considered invalid.
The full list of possible algorithms used to create a JWS signature is
defined in section 3.1 of RFC7518. This patch adds a helper function
that converts the "alg" strings into an enum member.
