[Openid-specs-ab] Interaction between OIDC Registration and RFC7591

[Justin Richer]

We've come across an interesting interaction between related specs.

Our client software requests a "scope" value as part of its client 
metadata, as defined in RFC7591. The OIDC Registration spec does not 
define this metadata value, but of course allows it as an extension. Our 
server accepts this value as well.

We're testing against a server implementation that ignores the incoming 
"scope" metadata request entirely, since it's not explicitly listed in 
the OIDC Registration specification. Part of this ignoring process is 
that the server returns a registration object back to the client that 
omits the "scope" value. Our client, following the advice in RFC7591 and 
the OIDC Registration spec both, takes this response from the server to 
mean that it doesn't have a registered scope set with the server. This 
consequently causes our client to not send a "scope" value in the 
authorization request, which causes the server to fail because the 
"scope" is required.

I think the right solution to this is to revise the OIDC Registration 
specification to be a normative extension of RFC7591/RFC7592, as has 
been discussed previously on this list and, to my recollection, 
generally agreed on but not acted on yet. Software statements and other 
enhancements that are in RFC7591 would also be available as options 
without further effort.

In practice, most implementations that I've seen already mix the two 
specifications. This is the intended effect of having wire 
compatibility, of course. Changing the spec would align it better with 
reality, and help avoid cases like this one where strict interpretation 
leads to lack of interoperability.

  -- Justin