Re: [Txauth] A model with a User Consent Element (with a clean figure)

[Denis <[e-mail]>]

Hi Fabien,

It would have been appreciated that you kept the original message in 
your response. I have copied it again at the end of this email.

Comments are between the lines.

> Hi Denis,
>
> I think it's interesting, but also very different to XYZ/XAuth so it 
> raises many questions ;-)
> The figure is impossible to read.

Use a PC. Copy and paste and then use the Courier font. On my PC (with 
the clear figure) it was perfect.

> So let me try to summarize the suggested approach, with a concrete 
> example, to make sure we understand well:
>
> *0. The client authN to the AS (in whatever way is supported)*
> Ex : client is a corporate financing called "finapp". finapp contacts 
> AS0 for authentication (say an openbanking service).
> User is John Doe, CFO at NeedMoney Inc. (+ other identity claims if 
> needed, maybe some verified credential from NeedMoney Holding that 
> John is indeed CFO).
>
> /Dear John, /
> /to access to your finapp, please identify yourself through your 
> prefered openbanking account./
> /Thanks/

If I understand you correctly,  finapp is a local application e.g. on 
your smartphone.

> *1. The client contacts a RS in a discovery phase, which includes the 
> selection of (at least) an operation, for which the RS returns the 
> required authZ attributes *
> Ex : finapp needs to use NeedMoney's data to evaluate how much credit 
> it can offer.
>
> Op1 : compute the credit rating, from RS1 (this is outsourced to an 
> external credit analyst), through the external service's own AS1.
> But to do that, RS needs your historic bank statements.
> Op2 : get your list of banks, RS2 (as registered within finapp), 
> through openbanking AS0 and retrieve the bank statements :
> Op3a : get historic data from his main bank, RS2a (say an 
> international bank), through openbanking AS0
> Op3b : same from a second bank account, RS2b (say a local bank), 
> through openbanking AS0

Why don't you make your very first example a little bit more complicated 
? with RS1, RS2a, RS2b, ... AS0, AS1, ...

:-)

The intent of the /first /email was to discuss a /basic /model and to 
place the highlights on the way to capture the *user's consent*
in an interoperable manner without letting know to any RS or AS the 
choices of the user. This is a fundamental feature of the model.
In XAuth, the user's consent is not formalized in the protocol : "User 
consent is /often /required at the GS".

> *2. User consent *
> RS1 aggregates the list of attributes required (from all RS) and sends 
> it to finapp.
> /Dear John, /
> /To evaluate your credit request, we need the following information: /
> /- your list of bank accounts (retrieved from your finapp account)/
> /- the associated banking statements over the past 12 months (from 
> each bank)/
> /- we'll pass that data to the credit agency, which will return your 
> credit score /
> /Do you agree ?/
>
> John approves (or not..., maybe he'll agree only for one specific 
> bank), via finapp directly
> (I like that, albeit in a more traditional flow, I'm also separating 
> the UI from the rest of the protocol of XYZ, and it works too).

As described, the user could simply push to the RS the banking 
statements over the past 12 months (from each bank).
The user consent is not about : "/Do you agree that/ /we pass the data 
to the credit agency, which will return your credit score"
/since no attributes nor ASs are involved in the question./
/

I guess you want the user to get access tokens targeted for RS2x so that 
each bank will accept to disclose his banking statements over the past 
12 months.

The consent is whether the user accepts to get access tokens from some 
of his banks targeted for RS2x for the following operation:
"Retrieval of the past 12 months banking statements" which corresponds 
to an API for each bank and then to send these access tokens to RS1.

In practice, the client (e.g. using FIDO) will connect transparently to 
each of the appropriate AS from the banks and will get the requested 
access tokens
with a requested validity period of about 5 minutes.

> *3. Requests to the protected resources *
> The client gets the access tokens and uses the services for which 
> access was granted.
>
> *Analysis: (maybe I didn't get everything right, if so let me know) *
> The trust model is focused around the relationship between the enduser 
> (John) and his application (finapp), which seems fine.

No. The trust model is not making a focus on that specific relationship. 
BTW, no access token is necessarily needed by the user to be able to use 
finapp.

> => I see some potential issues :
>
> a. it will be really difficult for an end user to understand what AS0 
> and AS1 are, why they're different, and why he needs to authenticate 
> to each of them.
> How do you enable a federated experience? (especially as there could 
> be many)

I fear that you have not fully captured what the user consent is about. 
See the above explanations. In addition, there is no concept of federation.

> b. deciding what is the main RS (here RS1) to be called by the client 
> seems very critical, as it is the one that needs to orchestrate 
> everything.
> This seems a very hierarchical and imperative model which seems 
> somewhat counter intuitive in terms of developer experience (as least
> as it is made today, we clearly don't go into so much details). The 
> call hierarchy may quickly become very complex, which may also become
> a problem when separate services evolve.

The client calls the main RS (here RS1). What may happen next is fully 
dependant upon the operation that the user is willing to perform and
this is unpredictable (since the back end service may change at any 
point of time).

> c. RS1 gets all the information required to access all sub-resources, 
> and therefore gets also a lot of responsibility (and power). But from 
> finapp's
> point of view, it is the one that has the relationship with the user 
> and is providing the core value proposition, while RS1 is just an 
> external service.

  So is it really a problem ?

> d. multi-user (common B2B scenario): John wants to authorize a read 
> access to his finapp account to his external auditor, Ann (who is not 
> a direct user
> of finapp herself, but might already be registered by openbanking 
> AS0). How do you do that? Does it require the access token itself to 
> be able to delegate rights?

The intent of the short description I sent was to describe two simple 
scenarios, so that we could start discussing about them.
At this point, the intent is not to cover all the scenarios you may 
dream of.

> e. more generally, a threat model would be required, as there are many 
> more interactions now.

There are less interactions than in XAuth: there is no protocol between 
ASs and RSs, nor between ROs and ASs.

Before a threat model, a trust model is needed. Do we have a trust model 
for XAuth ?
Unfortunately not, since the word "trust" is absent in the main body of 
draft-hardt-xauth-protocol-12.

In this model, the trust relationships are as follows:

  * The user trusts its client.
  * If a user has an account opened with an AS, then he trusts that AS
    to deliver the requested and genuine attributes into an access token.
  * A RS may trust one or more ASs for one or more types of attributes
    _and_ for performing a given operation.
  * A RS may be administered remotely by one or more RO.

_Note_: for authentication, a RS may accept either FIDO or one or more 
types of attributes from one or more ASs.

Denis

> Cheers,
> Fabien
>

This is a new thread.


Preamble: This model is quite different from the XAuth model.
In particular, a RO has no relationship with any AS and a Client does 
not need to be associated with any AS prior to any access to a RS.

A key point of this model is that the user's consent is handled locally 
by the Client and hence no AS nor RS is handling a man machine interface
for the user consent. This allows to support locally the user consent 
for multiple ASs while keeping all ASs ignorant about the choices of the 
user
made for accessing a particular RS.
*

**+--------++------------+
|User||Resource|
||| Owner (RO) |
+--------++------------+
|\|
|\|
|\|
|\|
+-----------++---------------++------------+
||---->| Authorization |||
|| (2) |Server (AS)|||
||<----||||
|Client|+---------------+||
||-------------------------->|Resource|
|User|(1)|Server|
|Consent|<--------------------------|(RS)|
|element|||
||-------------------------->||------>
||(3)||(4)
||<--------------------------||<------
+-----------++------------+
*
The flow of operations is as follows:

The Client (which may have been previously authenticated using FIDO) 
contacts the RS and after some dialogue with the RS selects an operation
that it wants to perform on the RS (1a). Note that it may also indicate 
directly the operation that it wants to perform on the RS without any 
prior dialogue.
In return (1b), the RS informs the Client about which attributes are 
needed by the RS for performing the requested operation and from which 
Attributes Servers
they may be obtained.

This information is specifically marked to indicate that it shall be 
handled by the "User Consent element" from the Client.
The presentation of that information is up to the man machine interface 
supported by the "User Consent element" from the Client.

The user can see which attributes are requested by the RS for performing 
the requested operation and, if it consents, the Client contacts one or 
more
appropriate Authorization Servers (2a). The user consent is hence 
captured locally by the Client (i.e. there is no dialogue with any AS 
nor any RS).

When the Client got the access tokens from these authorization servers 
(2b), it sends all of them in a single request to the RS (3a).

End of the story for a simple access


Start of a subsequent story for a delegation case

Let us now suppose that the RS is unable to fulfil the request by its 
own and that it needs to contact another RS. RS1 contacts RS2 (4a) and 
indicates the operation
that it wants to perform on RS2 (that operation may not be the same as 
the original operation). In return (4b), RS2 informs RS1 about which 
attributes are needed
by RS2 for performing the requested operation and from which Attributes 
Servers they may be obtained. RS1 forwards that information to the Client.

This information is marked to indicate that it shall be handled by the 
"User Consent element" from the Client. The presentation of that 
information is up to the man machine
interface from the Client. The user can see which attributes are 
requested by RS2 for performing the new requested operation and, if it 
consents, the Client contacts one or more
appropriate Authorization Servers. The user consent is hence captured 
locally by the "User Consent element" from the Client. (i.e. there is no 
dialogue with any AS, nor RS1, nor RS2).

When the Client got the access token(s) from the authorization 
server(s), it sends all of them in a single request to RS1. RS1 then 
forwards the additional access token(s) to RS2.


Some observations:

The user nor the Client are linked with any particular AS. A user may 
use today an AS of the Bank of America and may change tomorrow to the 
Bank of Missouri.
As soon as he will be registered with the Bank of Missouri, he will be 
able to get access tokens from the AS of the Bank of Missouri. The AS of 
Bank of America
has not been able to know where its access tokens have been used. This 
will be the same for AS of the Bank of Missouri. There is no need for 
any direct dialogue
between any AS and any RS at the time a client is making an access. 
There is no need for any RO to contact any AS.

This model has been constructed following a "Privacy by Design" approach.

Denis