Re: [Txauth] Revisiting the photo sharing example (a driving use case for the creation of OAuth)

Dick Hardt <> Tue, 04 August 2020 17:35 UTC

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From: Dick Hardt <>
Date: Tue, 4 Aug 2020 10:34:38 -0700
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To: Denis <>
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Subject: Re: [Txauth] Revisiting the photo sharing example (a driving use case for the creation of OAuth)
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I don't think it is productive to redefine terms that are well understood.

"The Client connects to the printing service (RS2) and the User
authenticates to the printing service (RS2)"

Does not answer the question of how the User authenticates to the RS. This
implies the user has an account at RS2. If that is the case, it is not just
acting in the role of a resource server.



On Tue, Aug 4, 2020 at 2:28 AM Denis <> wrote:

> Hi Dick,
> Hi Denis
> In your proposed flow, how does RS2 know who the user is that it is
> dealing with?
> The response is in the text: "The Client connects to the printing service
> (RS2) and the User authenticates to the printing service (RS2)".
> In OAuth, the AS authenticates the User for the RS. In the photo sharing
> example, the AS and RS are from the same organization, so the AS
> knowing what the RS was doing was not an issue.
> Nevertheless, a full description is still missing and might help to better
> understand the privacy issues.
> What you call a Client is not an OAuth Client, but is more of an agent for
> the User.
> RFC 6749 states:
> In OAuth, the client requests access to resources controlled by the
> resource owner and hosted by the resource server,
> and is issued a different set of credentials than those of the resource
> owner.
> This is how I use the term client which fits with the Client-Server model.
> However, RFC 6749 also states in section 1.1 (Roles) :
> client
>       An application making protected resource requests on behalf of the
> resource owner and with its authorization.
> This is clearly not how I use the term client.
> The term "User Agent" might grasp the concept, but I clearly like better
> the term "Client",
> as long as we define the use of this term in the context of GNAP.
> Denis
> Microsoft InfoCards comes to mind as a comparable architecture, and it
> seems aligned with what Tom is asking for.
> On Mon, Aug 3, 2020 at 12:35 AM Denis <> wrote:
>> Hello Dick,
>> This is a follow-up of the thread: "Reviewing
>> draft-hardt-xauth-protocol-11".
>> Hereafter are three exchanges between you and me which triggered this new
>> thread:
>> [Dick]    The photo sharing example was a driving use case for the
>> creation of OAuth.
>> [Denis]  We would need to revisit the original scenario and consider if
>> it can be addressed in a different way than the original way.
>> [Dick]   The use case is the same. Is there a different solution you are
>> proposing ?
>> My response is : Yes indeed, I have a different solution to address the
>> same use case.
>> RFC 6749 and draft-ietf-oauth-v2-1-00 both state:
>> For example, an end-user (resource owner) can grant a printing service
>> (client) access to her protected photos stored at
>> a photo-sharing service (resource server), without sharing her username
>> and password with the printing service. Instead,
>> she authenticates directly with a server trusted by the photo-sharing
>> service (authorization server), which issues the printing
>> service delegation-specific credentials (access token).
>> There are no further explanations.
>> Which information will be disclosed by the resource owner to the
>> authorization server to get "the printing service delegation-specific
>> credentials"
>> is not described. It is a private agreement between the AS and the RS. It
>> is more than likely that the authorization server will learn information
>> about which operation the resource owner is wishing to perform and where.
>> Since in OAuth, the access token is supposed to be opaque to the Client,
>> the user will be unable to make sure that her instructions have been
>> carefully followed.
>> RFC 6749 and draft-ietf-oauth-v2-1-00 both share a common point: they do
>> not contain a "Privacy Considerations" section.
>> Thus, the leakage of information to the AS is not discussed.
>> It is possible to revisit the original scenario by applying "Privacy by
>> design" principles.
>> The scenario can be solved by using an old data transfer method that has
>> been first described 32 years ago under the name
>> "Referenced Data Transfer (RDT)" in ECMA-131 (1988) and a few years later
>> in ISO/IEC 10740-2:1993.
>> RDT allows two servers to directly exchange large amounts of data under
>> the supervision of a client by communicating, through the client,
>> a reference generated by one server to the other server. RDT does not use
>> any Authorization Server (AS). This means that no AS is able
>> to act as Big Brother and this solves a major privacy concern.
>> * The three entities involved *
>> The Client supporting a User (was the Resource Owner).
>> The Photo-sharing service (was the Resource Server) : RS1.
>> The Printing service (was the client): RS2.
>> *Flow of operations with the Photo-sharing service (RS1) *The Client
>> first connects to the photo-sharing service (RS1) and the User
>> authenticates to the photo-sharing service (RS1).
>> RS1 to User: "Please select the operation to be performed".
>> Operation selected: "Print pictures using a third party printing service".
>> RS1 to User: "Please select a set of pictures to be printed".
>> The User selects the pictures.
>> RS 1 User consent: "Do you agree RS1 to communicate your selection of
>> pictures to a third party printing service" ?
>> If the User consents, RS1 to Client: "Here is the reference to be used by
>> your printing service to get the selected pictures".
>> * Flow of operations with the Printing service* (
>> *RS2) *
>> The Client connects to the printing service (RS2) and the User
>> authenticates to the printing service (RS2).
>>             *Note*: This allows to make sure that the user has an
>> account on RS2 so that further operations can be charged to this account
>>                       and that the prints can be sent to a known address.
>> RS2 to User: "Please select the operation to be performed".
>> Operation : "Print pictures to be received from a photo-sharing service ".
>> RS2 to User: "Please indicate your photo-sharing service".
>> The User responds: RS1.
>> "Please enter the reference to be used by RS2 to receive the set of
>> pictures from RS1".
>> The Client (or the User) enters the reference generated by RS1.
>> RS2 contacts RS1 using that reference in order to get the characteristics
>> and the thumbnails of the pictures to be printed (i.e. not yet the whole
>> pictures).
>> RS2 to client: What are your printing preferences for each picture (e.g.
>> number of prints, size, quality of the paper, resolution, margins, colours,
>> etc... ) ?
>> The User responds to all these questions.
>> RS 2 User consent: This operation will be charged XX € ? Do you agree ?
>> If the payment needs to be done on-line, then a payment phase is inserted.
>> * Continuation of the flow of operations *Final message from RS1 to the
>> Client: "Your selection of pictures will be soon transmitted to RS2".
>> Final message from RS2 to the Client: "Your prints will be soon on their
>> way".
>> RS2 to RS1 (asynchronous): transmit the set of pictures corresponding to
>> this reference.
>> * Some of the advantages of RDT*
>>    1. An end-user can grant a printing service (RS2) access to her
>>    protected photos stored at a photo-sharing service (RS1),
>>    without sharing her username and password with the printing service.
>>    2. Neither RS1, nor RS2 need to use or to trust any AS. This solves
>>    the Big Brother privacy issue.
>>    3. Any authentication method supported by RS1 or RS2 can be used by
>>    the User.
>>    4. The User can use any photo-sharing service (RS1) with any printing
>>    service (RS2), as long as they both support RDT.
>>    5. The User consent is first performed with the photo-sharing service
>>    (RS1) and then after with the printing service (RS2).
>>    6. The reference generated by RS1 will only be accepted by RS1 during
>>    a time period.
>>    7. The reference generated by RS1 allows RS2 to query first the
>>    thumbnails and then after the pictures selected by the User at RS1.
>>    8. The data transfer of the pictures selected at RS1 by the User is
>>    performed asynchronously from RS1 to RS2 as a back-office operation.
>> *Questions*: What would be the full scenario of this use case using
>> OAuth ? What about Privacy Considerations ?
>> Denis