x5c is base64 only not b64-url-encoded

[shiup]

from the specification, https://www.rfc-editor.org/rfc/rfc7517#section-4.7

 The "x5c" (X.509 certificate chain) parameter contains a chain of one
   or more PKIX certificates [RFC5280].  The certificate chain is
   represented as a JSON array of certificate value strings.  Each
   string in the array is a base64-encoded (Section 4 of [RFC4648] --
   not base64url-encoded) DER [ITU.X690.1994] PKIX certificate value.

However the logic in openidc_pem_from_x5c(x5c), is performing a openidc_base64_url_decode on the payload, then base64 it again.

[bodewig]

many thanks!

[shiup]

@bodewig thank you for such a quick turnaround 🙇‍♀️