Identifier Reuse in OpenID 2.0

One of the issues that the OpenID 1.1 specification did not cover is the fact that an identity URL may not remain the property of a user over time. For large OpenID providers there are two cases they may run into:

  1. A user with a popular user name stops using the service, and they want to make that name available to new users.
  2. A user changes their user name. This may be followed by someone taking over the old name.

In both cases, RPs would like some way to tell the difference between two different users who present the same ID at different points in time.

The traditional method of solving this problem is to assign two identifiers to a user: a human friendly identifier and a persistent identifier (e.g. a UNIX user ID, a database row ID, etc). At any point in time, the human friendly identifier will point to a particular persistent identifier, but over time the relationship may not hold. Whenever a human-friendly identifier is presented, it is transformed to its persistent counterpart before storage.

With OpenID 1.1, Relying Parties are expected to use the canonicalised form of what the user enters to identify them. It is possible to redirect the human friendly identifier to a persistent one, but that is not particularly nice if you are trying to co-locate the user’s home page and OpenID.

OpenID 2.0: XRIs

The only solution to this problem in earlier drafts of OpenID 2.0 was to use XRIs. When resolving an XRI, the resulting XRDS document includes a persistent identifier in the element.

For example, resolving “=foo” gives us a canonical ID of “=!4EFC.841C.8012.E2F8″. If a user logs in to an RP with the former, the RP will record the latter. This means the following:

  1. If the user stops paying their $12/year and someone else registers “=foo”, that new user will have a different persistent ID so won’t be able to assume the identity.
  2. If the user registers another XRI pointing at the same persistent identifier, it will be considered equivalent.

OpenID 2.0: URL identifiers

But if you want to use URLs as identifiers, how do you solve the problem?

One solution that was shot down was to allow the <CanonicalID> element in the XRDS document for a URL OpenID. Apparently this was rejected because it would result in another round trip during the discovery process to find the endpoint for the persistent ID.

Instead, a feature was added to help detect the case where an identifier was recycled. As part of the positive authentication response, an OP is allowed to modify the claimed ID to include a fragment URI component. If the identifier gets reassigned, the OP is expected to return a different fragment.

This solves problem (1) but not problem (2). As it stands, the OpenID 2.0 specification doesn’t provide much guidance in letting a user change their human friendly URL identifier while maintaining the same identity.

A Solution

One solution to this problem is to make use of the directed identity feature of OpenID 2.0. Rather than making the user’s homepage their identifier, make it an OP identifier URL. This lets the OP decide on the final claimed identifier.

This allows the user to enter their home page (e.g. http://example.com/james), and have the RP record a persistent identifier (e.g. http://example.com/id/42). If the user changes their human friendly identifier, they’ll still be able to use existing services.

This solution does have a few downsides though:

  • Users can log in with any other user’s homepage URL since they all point at the same OP.
  • Supporting both OpenID 1.1 and 2.0 on the same URL will likely cause confusion, since 1.1 requests would record the human friendly identifier and 2.0 requests record the persistent identifier. If an RP upgraded to the 2.0 protocol, the user would appear to be a different person (which is one of the problems we are trying to avoid).

So it seems that there isn’t a good solution if you need to support OpenID 1.1. If anyone else has ideas, I’d be glad to hear them.

7 Comments

  1. Posted 12 November, 2007 at 1:55 pm | Permalink

    Wouldn’t your solution invalidate one of the major use-cases of OpenID, the ‘you need only one identifier’ one?

  2. Posted 13 November, 2007 at 6:15 am | Permalink

    Tassos: I don’t think anything I’ve suggested breaks the “single digital identity” idea of OpenID.

    As I said above, the main questions are (1) how to avoid someone else assuming an identity on OPs that allow identifier reuse and (2) on systems that allow a user to change their human-friendly identifier, can the user maintain their identity?

  3. Posted 13 November, 2007 at 8:17 am | Permalink

    James: By way of introduction I am the technical director for the PiP/SeatBelt products here at Verisign. I read with interest your article and I thought I would share with you what Verisign does. On the PiP we do not allow identifers to ever be re-used. Once an identifier has been claimed by a user we never allow for that to be re-issued in the system.

    Just thought you’d be interested.

  4. Posted 13 November, 2007 at 8:41 am | Permalink

    Identifier Recycling is an issue that all large identity providers face, and freeing up desirable usernames that aren’t being actively used is always a high priority.

    Section 11.5.1 of Draft 12 of the OpenID 2.0 spec recommends that OPs assign a unique url fragment to an OpenID url that changes when the OpenID changes ownership.

    I believe that if the OP of http://example.com/james responded with http://example.com/id/42, the RP would be required to preform discovery on http://example.com/id/42 to verify that the OP is authorized to make claims about http://example.com/id/42. This will add an extra round trip, adding more latency to the signin process. See section 11.2 for more info. I don’t believe that using url fragments as generation identifiers will require an extra round trip.

  5. Posted 13 November, 2007 at 9:14 pm | Permalink

    Garry: thanks for the info. That gets rid of the reuse problem, but does not address the renaming issue.

    Atom: I do realise that processing would require an additional round trip — I said as much in the article. Given that it wouldn’t affect things in the general case where is not used, I don’t really agree that this is reason enough to reject it.

    For instance, it requires no more round trips than the directed identity mode, and that is included in the specification.

  6. Roman Beslik
    Posted 15 November, 2007 at 9:45 am | Permalink

    >If the user registers another XRI pointing at the same persistent identifier, it will be considered equivalent.

    Does it mean that I can write under someone’s name? Assuming that I’ve registered an XRI pointing to someone’s persistent identifier.

  7. Posted 15 November, 2007 at 9:57 am | Permalink

    Roman: my understanding is that there are a few things preventing this:

    1. the XRI registry probably won’t let you do that.

    2. if you could do that, you probably wouldn’t be able to authenticate using the XRI. This is roughly equivalent to copying the openid.* meta tags from someone else’s web page — you would be creating a new OpenID for the other user, not creating a way to impersonate them.

    You’d be better off asking an XRI expert about this though. I have not read much about the implementation details.

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  1. By James Henstridge » OpenID 2.0 Specification Approved on 10 December, 2007 at 12:38 pm

    [...] stated previously that I think the support for identifier reuse with respect to URL identifiers is a bit lacking.  [...]