Friday Mar 28, 2008

RDFAuth: sketch of a buzzword compliant authentication protocol

Here is a proposal for an authentication scheme that is even simpler than OpenId ( see sequence diagram ), more secure, more RESTful, with fewer points of failure and fewer points of control, that is needed in order to make Open Distributed Social Networks with privacy controls possible.

Update

The following sketch led to the even simpler protocol described in Foaf and SSL creating a global decentralized authentication protocol. It is very close to what is proposed here but builds very closely on SSL, so as to reduce what is new down to nearly nothing.

Background

Ok, so now I have your attention, I would like to first mention that I am a great fan of OpenId. I have blogged about it numerous times and enthusiastically in this space. I came across the idea I will develop below, not because I thought OpenId needed improving, but because I have chosen to follow some very strict architectural guidelines: it had to satisfy RESTful, Resource oriented hyperdata constraints. With the Beatnik Address Book I have proven - to myself at least - that the creation of an Open Distributed Social Network (a hot topic at the moment, see the Economist's recent article on Online social network) is feasible and easy to do. What was missing is a way for people to keep some privacy, clearly a big selling point for the large Social Network Providers such as Facebook. So I went on the search of a solution to create a Open Distributed Social Network with privacy controls. And initially I had thought of using OpenId.

OpenId Limitations

But OpenId has a few problems:

  • First it is really designed to work with the limitations of current web browsers. It is partly because of this that there is a lot of hopping around from the service to the Identity Provider with HTTP redirects. As the Tabulator, Knowee or Beatnik.
  • Parts of OpenId 2, and especially the Attribute Exchange spec really don't feel very RESTful. There is a method for PUTing new property values in a database and a way to remove them that does not use either the HTTP PUT method or the DELETE method.
  • The OpenId Attribute Exchange is nice but not very flexible. It can keep some basic information about a person, but it does not make use of hyperdata. And the way it is set up, it would only be able to do so with great difficulty. A RESTfully published foaf file can give the same information, is a lot more flexible and extensible, whilst also making use of Linked Data, and as it happens also solves the Social Network Data Silo problems. Just that!
  • OpenId requires an Identity Server. There are a couple of problems with this:
    • This server provides a Dynamic service but not a RESTful one. Ie. the representations sent back and forth to it, cannot be cached.
    • The service is a control point. Anyone owning such a service will know which sites you authenticate onto. True, you can set up your own service, but that is clearly not what is happening. The big players are offering their customers OpenIds tied to particular authentication servers, and that is what most people will accept.
As I found out by developing what I am here calling RDFAuth, for want of a better name, none of these restrictions are necessary.

RDFAuth, a sketch

So following my strict architectural guidelines, I came across what I am just calling RDFAuth, but like everything else here this is a sketch and open to change. I am not a security specialist nor an HTTP specialist. I am like someone who comes to an architect in order to build a house on some land he has, with some sketch of what he would like the house to look like, some ideas of what functionality he needs and what the price he is willing to pay is. What I want here is something very simple, that can be made to work with a few perl scripts.

Let me first present the actors and the resources they wish to act upon.

  • Romeo has a Semantic Web Address Book, his User Agent (UA). He is looking for the whereabouts of Juliette.
  • Juliette has a URL identifier ( as I do ) which returns a public foaf representation and links to a protected resource.
  • The protected resource contains information she only wants some people to know, in this instance Romeo. It contains information as to her current whereabouts.
  • Romeo also has a public foaf file. He may have a protected one too, but it does not make an entrance in this scene of the play. His public foaf file links to a public PGP key. I described how that is done in Cryptographic Web of Trust.
  • Romeo's Public key is RESTfully stored on a server somewhere, accessible by URL.

So Romeo wants to find out where Juliette is, but Juliette only wants to reveal this to Romeo. Juliette has told her server to only allow Romeo, identified by his URL, to view the site. She could have also have had a more open policy, allowing any of her or Romeo's friends to have access to this site, as specified by their foaf file. The server could then crawl their respective foaf files at regular intervals to see if it needed to add anyone to the list of people having access to the site. This is what the DIG group did in conjunction with OpenId. Juliette could also have a policy that decides Just In Time, as the person presents herself, whether or not to grant them access. She could use the information in that person's foaf file and relating it to some trust metric to make her decision. How Juliette specifies who gets access to the protected resource here is not part of this protocol. This is completely up to Juliette and the policies she chooses her agent to follow.

So here is the sketch of the sequence of requests and responses.

  1. First Romeo's user Agent knows that Juliette's foaf name is http://juliette.org/#juliette so it sends an HTTP GET request to Juliette's foaf file located of course at http://juliette.org/
    The server responds with a public foaf file containing a link to the protected resource perhaps with the N3
      <> rdfs:seeAlso <protected/juliette> .
    
    Perhaps this could also contain some relations describing that resource as protected, which groups may access it, etc... but that is not necessary.
  2. Romeo's User Agent then decides it wants to check out protected/juliette. It sends a GET request to that resource but this time receives a variation of the Basic Authentication Scheme, perhaps something like:
    HTTP/1.0 401 UNAUTHORIZED
    Server: Knowee/0.4
    Date: Sat, 1 Apr 2008 10:18:15 GMT
    WWW-Authenticate: RdfAuth realm="http://juliette.org/protected/\*" nonce="ILoveYouToo"
    
    The idea is that Juliette's server returns a nonce (in order to avoid replay attacks), and a realm over which this protection will be valid. But I am really making this up here. Better ideas are welcome.
  3. Romeo's web agent then encrypts some string (the realm?) and the nonce with Romeo's private key. Only an agent trusted by Romeo can do this.
  4. The User Agent then sends a new GET request with the encrypted string, and his identifier, perhaps something like this
    GET /protected/juliette HTTP/1.0
    Host: juliette.org
    Authorization: RdfAuth id="http://romeo.name/#romeo" key="THE_REALM_AND_NONCE_ENCRYPTED"
    Content-Type: application/rdf+xml, text/rdf+n3
    
    Since we need an identifier, why not just use Romeos' foaf name? It happens to also point to his foaf file. All the better.
  5. Because Juliette's web server can then use Romeo's foaf name to GET his public foaf file, which contains a link to his public key, as explained in "Cryptographic Web of Trust".
  6. Juliette's web server can then query the returned representation, perhaps meshed with some other information in its database, with something equivalent to the following SPARQL query
    PREFIX wot: <http://xmlns.com/wot/0.1/>
    SELECT ?pgp
    WHERE {
         [] wot:identity <http://romeo.name/#romeo>;
            wot:pubkeyAddress ?pgp .
    } 
    
    The nice thing about working at the semantic layer, is that it decouples the spec a lot from the representation returned. Of course as usage grows those representations that are understood by the most servers will create a de facto convention. Intially I suggest using RDF/XML of course. But it could just as well be N3, RDFa, perhaps even some microformat dialect, or even some GRDDLable XML, as the POWDER working group is proposing to do.
  7. Having found the URL of the PGP key, Juliette's server, can GET it - and as with much else in this protocol cache it for future use.
  8. Having the PGP key, Juliette's server can now decrypt the encrypted string sent to her by Romeo's User Agent. If the decrypted string matches the expected string, Juliette will know that the User Agent has access to Romeo's private key. So she decides this is enough to trust it.
  9. As a result Juliette's server returns the protected representation.
Now Romeo's User Agent knows where Juliette is, displays it, and Romeo rushes off to see her.

Advantages

It should be clear from the sketch what the numerous advantages of this system are over OpenId. (I can't speak of other authentication services as I am not a security expert).

  • The User Agent has no redirects to follow. In the above example it needs to request one resource http://juliette.org/ twice (2 and 4) but that may only be necessary the first time it accesses this resource. The second time the UA can immediately jump to step 3. [but see problem with replay attacks raised in the comments by Ed Davies, and my reply] Furthermore it may be possible - this is a question to HTTP specialists - to merge step 1 and 2. Would it be possible for a request 1. to return a 20x code with the public representation, plus a WWWAuthenticate header, suggesting that the UA can get a more detailed representation of the same resource if authenticated? In any case the redirect rigmarole of OpenId, which is really there to overcome the limitations of current web browsers, in not needed.
  • There is no need for an Attribute Exchange type service. Foaf deals with that in a clear and extensible RESTful manner. This simplifies the spec dramatically.
  • There is no need for an identity server, so one less point of failure, and one less point of control in the system. The public key plays that role in a clean and simple manner
  • The whole protocol is RESTful. This means that all representations can be cached, meaning that steps 5 and 7 need only occur once per individual.
  • As RDF is built for extensibility, and we are being architecturally very clean, the system should be able to grow cleanly.

Contributions

I have been quietly exploring these ideas on the foaf and semantic web mailing lists, where I received a lot of excellent suggestions and feedback.

Finally

So I suppose I am now looking for feedback from a wider community. PGP experts, security experts, REST and HTTP experts, semantic web and linked data experts, only you can help this get somewhere. I will never have the time to learn these fields in enough detail by myself. In any case all this is absolutely obviously simple, and so completely unpatentable :-)

Thanks for taking the time to read this

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