Re: [OAUTH-WG] [token-exchange] exchanging between issuers/domains

Phil Hunt <phil.hunt@oracle.com> Tue, 01 August 2017 17:59 UTC

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From: Phil Hunt <phil.hunt@oracle.com>
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Date: Tue, 1 Aug 2017 10:59:00 -0700
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To: Denis <denis.ietf@free.fr>
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Subject: Re: [OAUTH-WG] [token-exchange] exchanging between issuers/domains
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Denis,

Why is privacy a concern? OAuth is designed to have the Authorization Server be the issuer of tokens for a specific set of resource servers.  The AS represents users on the Resource server.  It does not represent users of the client - though they are often the same physical person, they are often different authenticated subjects.  

Of course, there are profiles of OAuth which change this relationship, but the foundational assumption in RFC6749 is the AS is usually associated with the RS.

Phil

Oracle Corporation, Identity Cloud Services Architect & Standards
@independentid
www.independentid.com <http://www.independentid.com/>phil.hunt@oracle.com <mailto:phil.hunt@oracle.com>
> On Aug 1, 2017, at 3:53 AM, Denis <denis.ietf@free.fr>; wrote:
> 
> Hello Brian,
> 
>> I don't think that's what I'm saying. Some of these concepts are difficult to reason about on a mailing list so I apologize for any miss or poor communication. 
>> 
>> When requesting a token, the resource or audience parameter can be used to indicate the target service where the client intends to use the token that it is requesting.  Audience is a logical name that says where the client wants to use the requested token. As a a logical name, the parties involved do need to know about the name. The resource parameter lets the client indicate to the AS/STS where it intends to use the issued token by providing the location, typically as an https URL, in the token exchange request in the same form that will be used to access that resource (again, an HTTPS URL). And the resource URL or audience can certinally indicate something that's external. Those parameters allow the AS/STS to determine where the token is going to be used (including externally) and produce the the appropriate token for that target based on configuration and policy.  The text in https://tools.ietf.org/html/draft-ietf-oauth-token-exchange-09#section-2.1 <https://tools.ietf.org/html/draft-ietf-oauth-token-exchange-09#section-2.1> about those parameters attempts to describe that in an intelligible way. Though there's likely always room for improvement.
> 
> Bear in mind, that they are cases where privacy is a concern, and for these cases the resource or audience parameter cannot be used to indicate the target service where the client intends to use the token that it is requesting.
> 
>> In general OAuth, the structure, content, style, etc. of access tokens is not defined. That stuff has to be agreed on between the AS and RS. 
> 
> RFC 7515 defines the major fields of a JWT. 
> 
>> Although Token Introspection (RFC 7662) <https://tools.ietf.org/html/rfc7662> has since been defined to give a more standardized option for the RS to query the AS for status and meta-information about an access token. Even with introspection, however, an RS effectively can only use access tokens from one AS because there's nothing standard provided by OAuth to indicate where the token is from when it's presented to the RS. 
> 
> RFC 7515 defines the "x5c" (X.509 Certificate Chain) Header Parameter in section 4.1.6: this parameter indicates where the token is from.
> 
>> For an AS/STS to validate an OAuth access token from a different AS, it is in a similar situation as an RS. 
> The key point is coming from the following proposed definition: "A Security Token Service (STS) is a service capable of validating and issuing security tokens".
> Up to now, the following definition applies: "A Security Token Service (STS) is a service capable of issuing security tokens". A given RS is free to trust (or not to trust) 
> any AS/STS.
> 
>> It would need to know the issuer of the access token - this is, I think, what you've pointed out with suggesting "subject_issuer" and "actor_issuer". 
> 
> I believe that I am now starting to understand why you made these suggestions. 
> 
>> There are maybe different ways that could be conveyed but some means at least would be needed to indicate the access token issuer. 
> 
> The "x5c" Header Parameter is such another way. When used, it should not conflict with any other parameter.
> 
>> Then the receiving AS/STS would have to call the issuing AS's token introspection endpoint (unless it somehow knew how to validate an access token from that issuer locally). The complexity of all that is one reason why token exchange scoped validation (and issuance) of access tokens to only access tokens from the AS/STS involved in the exchange (and not directly supporting OAuth access token to OAuth access token cross-domain exchanges). Also the assertion based authorization grants (RFC 7523 <https://tools.ietf.org/html/rfc7523> & 7522 <https://tools.ietf.org/html/rfc7522>) are largely intended to facilitate acquiring an access token from an external AS. The thinking (fro me anyway) was that token exchange would be used with a local STS to obtain an assertion suitable to be used at an external AS with an assertion grant to get an access token from that AS. That pattern is something that exists today. Cross domain could also be achieved with JWTs, for which a token type value of "urn:ietf:params:oauth:token-type:jwt" is defined.  
>> 
>> It's difficult to articulate but that's an attempt to explain how things are in the draft today and why.  
> 
> If we introduce relationships between AS/STSs, we are opening a pandora box where trust relationships is a concern and where privacy is also a concern.
> 
> Do we want a local AS/STS to be aware of all the RSs accessed by a client ? Do we want an external AS/STS to be aware of all the RSs accessed by a client ?
> What would mean a "local" AS/STS versus an "external" AS/STS ? It is from the point of view of the client or of the RS ?
> 
> It is normal that an AS/STS issuing access token knows some attributes related to its clients. Would it be appropriate if another AS/STS would know some attributes from "external" clients and, in addition, where the access tokens will be used ? We need to take care of not building a system where by construction "Big Brother would be watching you".
> 
> The core of problem is well beyond the simple addition of one or two parameters. 
> 
>> I guess I would have to defer to the larger working group here as to the question of if token exchange should support exchanges of an OAuth access token from a different AS for an OAuth access token issued by the AS/STS doing the exchange?
> 
> In order to progress on this topic, I believe that we first need an architecture paper with a clear description of the trust relationships and an identification of the privacy issues.
> 
> Denis
> 
>> On Sat, Jul 29, 2017 at 8:46 AM, Bill Burke <bburke@redhat.com <mailto:bburke@redhat.com>> wrote:
>> So, you're saying the STS has to define a subject_type for each external token the client wants to exchange from?  A type that is potentially proprietary and different between each and every STS?  On the opposite end, when you want to convert to an external token, the STS either has 3 options for the client to specify that it wants an external token.  1) a proprietary response type, 2) a proprietary resource scheme, 3) a proprietary audience scheme.
>> 
>> Don't you think at minimum, the token-exchange spec should define a standard way to do OAuth to OAuth cross-domain exchanges?  Right now cross-domain exchanges are proprietary and completely up to the target STS on how it wants the client to formulate a cross-domain exchange.  I still think a "subject_issuer" and "requested_issuer" are the clearest and simplest way to enable such an interaction.
>> 
>> 
>> On 7/28/17 6:28 PM, Brian Campbell wrote:
>>> The urn:ietf:params:oauth:token-type:access_token type is an "indicator that the token is a typical OAuth access token issued by the authorization server in question" (see near the end of section 3 <https://tools.ietf.org/html/draft-ietf-oauth-token-exchange-09#section-3>) so the issuer is the given STS in that case. Cross domain is possible by use of other token types that are not opaque to the STS where the issuer can be inferred from the token.
>>> 
>>> On Fri, Jul 28, 2017 at 3:27 PM, Bill Burke <bburke@redhat.com <mailto:bburke@redhat.com>> wrote:
>>> Thanks for replying,
>>> 
>>> The Introduction of the spec implies that inter-security-domain exchange is supported: "
>>> 
>>>  A Security Token Service (STS) is a service capable of validating and
>>>    issuing security tokens, which enables clients to obtain appropriate
>>>    access credentials for resources in heterogeneous environments or
>>>    across security domains.
>>> "
>>> 
>>> But with the current API if you want to exchange an external token to an internal one, there is no way for the STS to identify where the subject_token originated.  Are you saying that an STS cannot accept tokens from an external domain?
>>> i.e
>>> 
>>> subject_token: <opaque-string>
>>> 
>>> subject_token_type: urn:ietf:params:oauth:token-type:access-token
>>> 
>>> There's just no way for the STS to know where the subject_token came from because the subject_token can be completely opaque.  
>>> 
>>> Now, on the flip side, if you are converting from an internal token to an external one, the audience parameter is just too undefined.  For example, how could you specify that you want a token for an external client of an external issuer.  Client ids are opaque in OAuth, and issuer id isn't even something that is defined at all.  In OpenID connect, an issuer id can be any URL.
>>> 
>>> IMO, adding optional "subject_token_issuer" and "requested_issuer" parameters only clarifies and simplifies the cross-domain case.   If you don't like "issuer" maybe "domain" is a better word?
>>> 
>>> Thanks for replying,
>>> 
>>> Bill
>>> 
>>> 
>>> 
>>> 
>>> 
>>> 
>>> On 7/28/17 4:39 PM, Brian Campbell wrote:
>>>> In general, an instance of an AS/STS can only issue tokens from itself. The audience/resource parameters tell the AS/STS where the requested token will be used, which will influence the audience of the token (and maybe other aspects). But the issuer of the requested token will be the AS/STS that issued it. A cross domain exchange could happen by a client presenting a subject_token from a different domain/issuer (that the AS/STS trusts) and receiving a token issued by that AS/STS suitable for the target domain. 
>>>> 
>>>> 
>>>> 
>>>> On Fri, Jul 28, 2017 at 9:06 AM, Bill Burke <bburke@redhat.com <mailto:bburke@redhat.com>> wrote:
>>>> Should probably have a "subject_issuer" and "actor_issuer" as well as the "requested_issuer" too.
>>>> 
>>>> FYI, I'm actually applying this spec to write a token exchange service to connect various product stacks that have different and often proprietary token formats and architectures.
>>>> 
>>>> 
>>>> 
>>>> On 7/26/17 6:44 PM, Bill Burke wrote:
>>>> Hi all,
>>>> 
>>>> I'm looking at Draft 9 of the token-exchange spec.  How would one build a request to:
>>>> 
>>>> * exchange a token issued by a different domain to a client managed by the authorization server.
>>>> 
>>>> * exchange a token issued by the authorization server (the STS) for a token of a different issuer and different client.  In other words, for a token targeted to a specific client in a different authorization server or realm or domain or whatever you want to call it.
>>>> 
>>>> * exchange a token issued by a different issuer for a token of a different issuer and client.
>>>> 
>>>> Is the spec missing something like a "requested_issuer" identifier?  Seems that audience is too opaque of a parameter for the authz server to determine how to exchange the token.
>>>> 
>>>> Thanks,
>>>> 
>>>> Bill
>>>> 
>>>> 
>>>> 
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