Re: [OAUTH-WG] Issuers, Discovery Docs & Brands

Joseph Heenan <joseph.heenan@fintechlabs.io> Wed, 20 May 2020 16:16 UTC

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From: Joseph Heenan <joseph.heenan@fintechlabs.io>
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Date: Wed, 20 May 2020 17:16:44 +0100
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Cc: Dave Tonge <dave.tonge@momentumft.co.uk>, oauth <oauth@ietf.org>, Openid-specs-fapi <openid-specs-fapi@lists.openid.net>
To: Vladimir Dzhuvinov <vladimir@connect2id.com>
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Subject: Re: [OAUTH-WG] Issuers, Discovery Docs & Brands
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Thanks for your thoughts Vladimir!

The client_id based solution I wasn’t previously aware of - unfortunately it doesn’t solve the problem for app2app, as the mobile OS selects the app to use based purely on the URL (and in at least the iOS case will not offer the user a choice if multiple apps claim to handle the same url).

I think some kind of mapping like you suggest will work and fallback, I wonder about a structure in the authorization server metadata something like this:

{
  ...
  "alternative_authorization_endpoints": [
    {
      "authorization_endpoint": "https://loadsamoney/business/auth",
      "description": "loadsmoney business banking customers",
      "logo_url": "https://loadsamoney/business/logo.png"
    },
    {
      "authorization_endpoint": "https://loadsamoney/consumer/auth",
      "description": "loadsmoney personal customers",
      "logo_url": "https://loadsamoney/consumer/logo.png"
    }
  ]
}

And as you say, the existing authorization_endpoint can be a fallback for clients that are unaware of the new spec or prefer the simpler option of just using a single authorization endpoint. Supporting the new spec would allow a better UX though so there’s advantages to client to do so.
> Speaking of mTLS, I'm not sure how the "mtls_endpoint_aliases" can be sensibly combined with the proposed multi-brand spec.
> 

I think that particular part is not really an issue as mtls isn’t used at the authorization endpoint.

Thanks

Joseph


> On 20 May 2020, at 16:07, Vladimir Dzhuvinov <vladimir@connect2id.com> wrote:
> 
> Hi Dave,
> 
> In the absence of such a "multi-brand" spec we have tackled this issue in the past by letting the "brand" be encoded in the client_id. An alternative scenario is to do a "brand" lookup by client_id. Then let the AS render the "branded" authZ endpoint.
> 
> You're probably aware the mTLS spec is allowing for endpoint aliases, so this is not the first time such as need has occurred:
> 
> https://tools.ietf.org/html/rfc8705#section-5 <https://tools.ietf.org/html/rfc8705#section-5>
> One could devise a similar JSON object with mappings "label" - "authorization_endpoint".
> 
> Clients that are aware of the new spec will look it up, those that are not will fall back to the std "authorization_endpoint".
> 
> Speaking of mTLS, I'm not sure how the "mtls_endpoint_aliases" can be sensibly combined with the proposed multi-brand spec.
> 
> 
> 
> Vladimir
> 
> 
> 
> On 20/05/2020 15:07, Dave Tonge wrote:
>> Dear OAuth WG
>> 
>> We have an issue <https://bitbucket.org/openid/fapi/issues/255/certification-clarification-request> in the OpenID FAPI Working Group that we believe affects the wider OAuth community.
>> 
>> In summary: what is the recommended approach to discovery (RFC8414) for Authorization Servers who support multiple "brands" .
>> 
>> If brands are completely separate, then it seems sensible that each brand must have its own `issuer` and therefore its own discovery document at the correct location (i.e. brand 1 would have an issuer of "https://as/brand1 <https://as/brand1>" and a discovery document available at  https://as/.well-known/ <https://as/.well-known/>oauth-authorization-server/brand1).
>> 
>> However in the real world it is not always so simple. We have many existing implementations in UK open banking that support multiple authorization endpoints. Here is an example (thanks to @Joseph Heenan <mailto:joseph.heenan@fintechlabs.io> )
>> 
>> > Bank “loadsamoney” has one idp and, for internet banking, one “login page” for both business and personal customers.
>> 
>> > They have separate mobile apps for business/personal, and are required to support app2app. This means they will definitely be exposing multiple authorization endpoints (as there’s a 1:1 mapping of authorization endpoints to mobile apps) - the choice is how they do this.
>> 
>> > Their choices are:
>> 
>> > 1. Multiple discovery endpoints (one for business, one for personal), each with a different authorization endpoint, multiple issuers (if their vendor allows this)
>> > 2. Single discovery endpoint, single issuer, multiple authorization endpoints listed in one discovery doc (one for business, one for personal) some of which are hardcoded by the 3rd party
>> > 3. Multiple discovery endpoints each with a different authorization endpoint, same issuer in all cases (breaks RFC8414 and OIDC Discovery)
>> 
>> Option 3 is invalid and that leaves us with options 1 and 2. 
>> Option 1 can be problematic as often it is in reality the same `issuer` behind the scenes.
>> 
>> We would like to get feedback on this issue and potentially an extension to RFC8414 to allow the definition of multiple authorization endpoints.
>> 
>> Thanks in advance
>> 
>> Dave Tonge
>> Co-Chair FAPI WG
>> Open ID Foundation
>> 
>> 
>