OpenID

OpenID 2.0 Specification Approved

It looks like the OpenID Authentication 2.0 specification has finally been released, along with OpenID Attribute Exchange 1.0. While there are some questionable features in the new specification (namely XRIs), it seems like a worthwhile improvement over the previous specification. It will be interesting to see how quickly the new specification gains adoption.

While this is certainly an important milestone, there are still areas for improvement.

Best Practices For Managing Trust Relationships With OPs

The proposed Provider Authentication Policy Extension allows a Relying Party to specify what level of checking it wants the OpenID Provider to perform on the user (e.g. phishing resistant, multi factor, etc). The OP can then tell the RP what level of checking was actually performed.

What the specification doesn’t cover is why the RP should believe the OP. I can easily set up an OP that performs no checking on the user but claims that it performed “Physical Multi-Factor Authentication” in its responses. Any RP that acted on that assertion would be buggy.

This isn’t to say that the extension is useless. If the entity running the RP also runs the OP, then they might have good reason to believe the responses and act on them. Similarly, they might decide that JanRain are quite trustworthy so believe responses from myOpenID.

What is common in between these situations is that there is a trust relationship between the OP and RP that is outside of the protocol. As the specification gives no guidance on how to set up these relationships, they are likely to be ad-hoc and result in some OpenIDs being more useful than others.

At a minimum, it’d be good to see some best practices document on how to handle this.

Trusted Attribute Exchange

As mentioned in my previous article on OpenID Attribute Exchange, I mentioned that attribute values provided by the OP should be treated as being self asserted. So if the RP receives an email address or Jabber ID via attribute exchange, there is no guarantee that the user actually owns them. This is a problem if the RP wants to start emailing or instant messaging the user (e.g. OpenID enabled mailing list management software). Assuming the RP doesn’t want to get users to revalidate their email address, what can it do?

One of the simplest solutions is to use a trust relationship with the OP. If the RP knows that the OP will only transfer email addresses if the user has previously verified them, then they need not perform a second verification. This leaves us in the same situation as described in the previous situation.

Another solution that has been proposed by Sxip is to make the attribute values self-asserting. This entails making the attribute value contain both the desired information plus a digital signature. Using the email example, if the email address has a valid digital signature and the RP trusts the signer to perform email address verification, then it can accept the email address without further verification.

This means that the RP only needs to manage trust relationships with the attribute signers rather than every OP used by their user base. If there are fewer attribute signers than OPs then this is of obvious benefit to the RP. It also benefits the user since they no longer limited to one of the “approved” OPs.

Canonical IDs for URL Identifiers

I’ve stated previously that I think the support for identifier reuse with respect to URL identifiers is a bit lacking.  It’d be nice to see it expanded in a future specification revision.

OpenID Attribute Exchange

In my previous article on OpenID 2.0, I mentioned the new Attribute Exchange extension. To me this is one of the more interesting benefits of moving to OpenID 2.0, so it deserves a more in depth look.

As mentioned previously, the extension is a way of transferring information about the user between the OpenID provider and relying party.

Why use Attribute Exchange instead of FOAF or Microformats?

Before deciding to use OpenID for information exchange, it is worth looking at whether it is necessary at all.

There are existing solutions for transferring user data such as FOAF and the hCard microformat. As the relying party already has the user’s identity URL, it’d be trivial to discover a FOAF file or hCard content there. That said, there are some disadvantages to this method:

  1. Any information published in this way is available to everyone. This might be fine for some classes of information (your name, a picture, your favourite colour), but not for others (your email address, phone number or similar).
  2. The same information is provided to all parties. Perhaps you want to provide different email addresses to work related sites.
  3. The RP needs to make an additional request for the data. If we can provide the information as part of the OpenID authentication request, it will reduce the number of round trips that need to be made. In turn, this should reduce the amount of time it takes to log the user in.

Why use Attribute Exchange instead of the Simple Registration extension?

There already exists an OpenID extension for transferring user details to the RP, in the form of the Simple Registration extension. It has already been used in the field, and works with OpenID 1.1 too.

One big downside of SREG is that it only supports a limited number of attributes. If you need to transfer more attributes, you basically have two choices:

  1. use some other extension to transfer the remaining attributes
  2. make up some new attribute names to send with SREG and hope for the best.

The main problem with (2) is that there is no way to tell between your own extensions to SREG and someone else’s which will likely create interoperability problems if when an attribute name conflict occurs. So this solution is not a good idea outside of closed systems. This leaves (1), for which Attribute Exchange is a decent choice.

What can I do with Attribute Exchange?

There are two primary operations that can be performed with the extension:

  1. fetch some attribute values
  2. store some attribute values

Both operations are performed as part of an OpenID authentication request. Among other things, this allows:

  • The OP to ask the user which requested attributes to send
  • If the OP has not stored values for the requested attributes, it could get the user to enter them in and store them for next time.
  • The OP could use a predefined policy to decide what to send the RP. One possibility would be to generate one-time email addresses specific to a particular RP.
  • For store requests, the OP can ask the user to confirm that they want to store the attributes.

Fetching Attributes

An attribute fetch request is a normal authentication request with a few additional fields:

  • openid.ax.mode: this needs to be set to “fetch_request”
  • openid.ax.required: a comma separated list of attribute aliases that the RP needs (note that this does not guarantee that the OP will return those attributes).
  • openid.ax.if_available: a comma separated list of attribute aliases that the RP would like returned if available.
  • openid.ax.type.alias: for each requested attribute alias, the URI identifying the attribute type
  • openid.ax.count.alias: the number of values the RP would like for the attribute.
  • openid.ax.update_url: a URL to send updates to (will be discussed later).

The use of URIs to identify attributes makes it trivial to define new attributes without conflicting with other people (and as with XML namespaces, the attribute aliases are arbitrary). However, the extension is only useful if the OP and RP can agree on attribute types. To help with this, there is a collection of community defined attribute types at axschema.org.

As an example, imagine a web log that uses OpenID to authenticate comment posts. Rather than just printing the OpenID URL for the commenter, it could use attribute exchange to request their name, email, website and hackergotchi. The authentication request might contain the following additional fields:

openid.ns.ax=http://openid.net/srv/ax/1.0
openid.ax.mode=fetch_request
openid.ax.required=name,hackergotchi
openid.ax.if_available=email,web
openid.ax.type.name=http://axschema.org/namePerson
openid.ax.type.email=http://axschema.org/contact/email
openid.ax.type.hackergotchi=http://axschema.org/media/image/default
openid.ax.type.web=http://axschema.org/contact/web/default

In the successful authentication response, the following fields will be included (assuming the OP supports the extension):

  • openid.ax.mode: must be “fetch_response”
  • openid.ax.type.alias: specify the type URI for each attribute being returned.
  • openid.ax.count.alias: the number of values being returned for the given attribute alias (defaults to 1).
  • openid.ax.value.alias: the value for the given attribute alias, if no corresponding openid.ax.count.alias field was sent.
  • openid.ax.value.alias.n: the nth value for the given attribute alias, if a corresponding openid.ax.count.alias field was sent. The first attribute value is sent with n = 1.
  • openid.ax.update_url: to be discussed later.

For the web log example given above, the response might look like:

openid.ns.ax=http://openid.net/srv/ax/1.0
openid.ax.mode=fetch_response
openid.ax.type.name=http://axschema.org/namePerson
openid.ax.type.email=http://axschema.org/contact/email
openid.ax.type.hackergotchi=http://axschema.org/media/image/default
openid.ax.value.name=John Doe
openid.ax.value.email=john@example.com
openid.ax.count.hackergotchi=0

In this response, we can see the following:

  1. The user has provided their name and email
  2. They have not provided any information about their web site. Either the OP does not support the attribute or the user has declined to provide it.
  3. The use has explicitly stated that they have no hackergotchi (i.e. it is a zero-valued attribute).

Storing Attributes

Using the Attribute Exchange fetch request, it is possible to outsource management of pretty much all the user’s profile information to the OP. That said, the user will still need to update their profile data occasionally. Telling them to go to their OP to change things and then log in again is not particularly user friendly though.

Using the store request, the RP can let the user update their profile on site and then transfer the changes back to the OP. Like the fetch request, a store request is performed as part of an OpenID authentication request. The additional request fields are pretty much identical to a store response, except that openid.ax.mode is set to “store_request”.

In the positive authentication response, the RP can see whether the data was successfully stored by checking the openid.ax.mode response field. If the data was stored, then it will be set to “store_response_success”. If the data was not stored it will be set to “store_response_failure” and an error message may be found in openid.ax.error.

Asynchronous Attribute Updates

One downside of the Simple Registration extension is that it only transferred user details on login. This means that it is only possible to get updates to attribute values by asking the user to log in again. The Attribute Exchange extension provides a way to solve this problem in the form of the openid.ax.update_url request field.

When a “fetch_request” is issued with the openid.ax.update_url field set, a compliant OP will record the following:

  1. the claimed ID and local ID from the authentication request
  2. the list of requested attributes
  3. the update_url value (after verifying that it matches the openid.realm value of the authentication request).

The OP will then include openid.ax.update_url in the authentication response as an acknowledgement to the RP. When any of the given attributes are updated the OP will send an unsolicited positive authentication response to the given update URL. This will effectively be the same as the original authentication response (i.e. for the same claimed ID and local ID), but with new values for the changed attributes.

As there is no mention of unsolicited authentication responses in the main OpenID authentication specification, it is worth looking at what checking the RP should do. This includes:

  • Is this OP still authoritative for the claimed ID? This is checked by performing discovery on the claimed ID and verifying that it results in the same server URL and local ID as given in the response.
  • Did the message come from the OP? As with a standard response, there should be a signature for the fields. Since the OP does not know what association to use for the signature, a new private association will be used. By issuing a “check_authentication” request to the OP, the RP can verify that the message originated from the OP.

If these checks fail the RP should respond with a 404 HTTP error code, which tells the OP to stop sending updates. If the message is valid, the RP can update the user’s profile data.

Caveats

While the Attribute Exchange extension provides significant features above those provided by Simple Registration, but it still has its limitations:

  1. Any attribute values provided to the RP are self-asserted.
  2. Related to the above, there is no way for a third party to make assertions about attribute values.

For (1), the solution is to perform the same level of verification on the attribute value as if the user had entered it directly. So an OpenID enabled mailing list manager should verify the email address provided by attribute exchange before subscribing the user. In contrast, an OpenID enabled shop probably doesn’t need to do further verification of the user’s shipping address (since it is in the user’s best interest to provide correct information).

The exception to this rule is when there is some other trust relationship between the OP and RP. For instance, if the RP knows that the OP will only send an email address if it has first been validated, then it may decide to trust the email address without performing its own validation checks. This is most likely to be useful in closed systems that happen to be using OpenID for single sign-on.

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.

OpenID 2.0

Most people have probably seen or used OpenID. If you have used it, then it has most likely that it was with the 1.x protocol. Now that OpenID 2.0 is close to release (apparently they really mean it this time …), it is worth looking at the new features it enables. A few that have stood out to me include:

  • proper extension support
  • support for larger requests/responses
  • directed identity
  • attribute exchange extension
  • support for a new naming monopoly

I’ll now discuss each of these in a bit more detail

Extension Support

OpenID 1.1 had one well known extension: the Simple Registration Extension. An OpenID relying party (RP) would send a request with an openid.sreg.required field, and get back user information in openid.sreg.* fields from the OpenID Provider (OP). The RP and OP would just need to know that “openid.sreg” fields means that the simple registration extension is being used.

But what if I want to define my own extension? If my RP sends openid.foo.* fields, how does the OP know that it refers to my extension and not some other extension that happened to pick the same prefix?

OpenID 2.0 solves this problem by borrowing the idea of name space URIs from XML. If I am sending some openid.foo.* fields in an OpenID message, then I also need to send an openid.ns.foo field set to a URI that identifies the extension. This means that a message that sends the same data as openid.bar.* fields should be treated the same provided that openid.ns.bar is set to the extension’s name space URI.

As with XML name spaces, this allows us to piggy back on top of DNS as a way of avoiding conflicts.

Large Requests and Responses

OpenID 1.1 uses HTTP redirects as a way of transferring control between the RP and OP (and vice versa). This means that the upper limit on a message is effectively the same as the smallest upper limit on length of URLs in common web browsers and servers. Internet Explorer seems to have the lowest limit—2,083 characters—so it sets the effective limit on message size.

For simple authentication checks (what OpenID was originally designed for), this is not generally a problem. But once you start to introduce a few extensions, this limit can easily be reached.

OpenID 2.0 allows messages to be sent as an HTTP POST body which effectively removes the upper limit. The recommended way of achieving this is by sending a page to the user’s browser that contains a form that posts to the appropriate endpoint and contains the data as hidden form fields. The form would then get submitted by a JavaScript onload handler.

Directed Identity

For OpenID 1.1, the authentication process goes something like this:

  1. the user enters their identity URL into a form on the RP
  2. the RP performs discovery on that URL to find the user’s OP.
  3. the RP initiates an OpenID authentication request with that OP.

With OpenID 2.0, the discovery process may tell the RP that the URL identifies the OP rather than the user. If this happens, the RP proceeds with the authentication request using the special “http://specs.openid.net/auth/2.0/identifier_select” value as the identity URL. The OP will then fill in the user’s actual identity URL in the subsequent authentication response. As an additional step, the RP is then required to perform discovery on this URL to ensure that the OP is entitled to authenticate it.

There are a number of cases where this feature can be useful:

  1. An OpenID provider can give their users a single URL that will work for everyone. For instance, if AOL sets things up correctly, you’d be able to type “aol.com” into any OpenID 2.0 enabled site to log in with an AIM screen name.
  2. A privacy concious user could configure their own OpenID provider that will hand out different identity URLs to different RPs, similar to how some people use single-purpose email addresses today.
  3. If an RP requires that users use a particular OP, they could use directed identity to begin the authentication request without requiring the user to enter an identity URL.

Attribute Exchange Extension

The OpenID Attribute Exchange extension is like the simple registration extension on steroids. The major differences are:

  • Unlike the simple registration extension, the attribute exchange extension does not have a fixed set of attributes that can be transmitted. Instead it uses URIs to identify the attribute types, making it easy to define new attributes without causing conflicts. Of course an attribute is not particularly useful if no one else supports it, so there is a process set up to standardise common attribute types.
  • As well as receiving attribute values as part of an authentication response, an RP can request that an OP store certain attribute values. This is done as part of an authentication request, so the OP can verify that the user really wants to store the values.
  • The RP can request ongoing updates for the attributes it is interested in. As an example, if you stored your hackergotchi with your OP, changes to the image could be automatically pushed out to all sites you use that want to display that image.

Prop Up A New Naming Monopoly

With OpenID 2.0, a user is supposed to be able to enter an i-name in place of an identity URL in an RP, and be authenticated against the i-broker managing that name. So rather than entering an ugly URL, users can enter an ugly string starting with “=” or “@”.

All it costs to take advantage of this is US$12 per year (or US$55 for an organisation name). They claim that it will be possible to use an i-name in many contexts in the future, but for now it appears to be limited to (1) a subset of OpenID RPs, (2) a web form that people can use to send you emails and (3) an HTTP redirection to your website.

At this point, it seems that i-name support in OpenID is more important to the i-name crowd than the OpenID crowd. That said, the complexity is hidden by most of the existing OpenID libraries, so it’ll most likely get implemented by default on most RPs moving forward.

Conclusion

Overall OpenID 2.0 looks like a worthwhile upgrade, even if some parts like i-names are questionable.

Assuming the attribute exchange extension takes off, it should provide a much richer user experience. Imagine being able to update your shipping address in one place when you move house and having all the online retailers you use receive the updated address immediately. Or changing your email address and having all the bugzilla instances you use pick up the new address instantly (perhaps requiring you to verify the new address first, of course).

The improved extension support should also make it easier for people to experiment with new extensions without accidentally conflicting with each other, which should accelerate development of new features.

Canonical Shop Open

The new Canonical Shop was opened recently which allows you to buy anything from Ubuntu tshirts and DVDs up to a 24/7 support contract for your server.

One thing to note is that this is the first site using our new Launchpad single sign-on infrastructure. We will be rolling this out to other sites in time, which should give a better user experience to the existing shared authentication system currently in place for the wikis.