Here's the use case I have in mind. Suppose I have an account with a Shibboleth Identity Provider and suppose I want to self-query the IdP for attributes. Consider the following assumptions: - I possess a username/password (it is very unlikely that I possess an X.509 credential trusted by the IdP). - Fact: There is no attribute query handler that ships with the Shib2 IdP that can be used in this scenario. - To my knowledge, there is no existing attribute query client that can self-query a Shib IdP for attributes (e.g., clients that implement the current OGF profile can not be used because few, if any Shib IdPs today support an X.509 PKI). - It is easier to implement/deploy an attribute query client and corresponding attribute query handler based on my existing campus credential (username/password) than it is to implement/deploy a system based on an X.509 PKI. - The attribute assertion retrieved from the IdP MUST be a holder-of-key assertion, in any case. If you agree with these assumptions, you are led to the same solution I outlined in my comments, that is, a client that authenticates via HTTP basic auth or WS-Security Username Token Profile. The SSL/TLS client certificate is indeed a self-signed certificate (or more generally, an untrusted X.509 end-entity certificate) used solely to provide a key that the IdP can bind to the assertion (holder-of-key). Internet2 has submitted a profile to the OASIS SSTC that addresses the above problem for Web SSO. NCSA will submit a profile to the OASIS SSTC that solves the corresponding non-browser use case. This opens the door for the implementation/deployment described above and in the comments. I'm sorry if all of this is untimely, but that's the way it goes, I'm afraid. It didn't occur to me that this might be a possible path forward until Internet2 submitted their profile to OASIS. Comments inline below... Tom Scavo NCSA On Mon, Sep 15, 2008 at 10:50 AM, David Chadwick <d.w.chadwick@kent.ac.uk> wrote:
concerning your comment 1b. X.509 authentication is assumed, I am slightly confused by this one.
The whole purpose of the OASIS SAML V2.0 Deployment Profiles for X.509 Subjects is that quote "it specifies how a principal who has been issued an X.509 identity certificate is represented as a SAML Subject, how an assertion regarding such a principal is produced and consumed,..."
That addresses the VOMS use case but not the Shibboleth use case, so I would argue that is only half the story.
It would therefore be perverse, would it not, to assume that a principal with an X.059 certificate should use any other method to authenticate to the IDP/AA.
Well, that's the whole point. If you assume the user possesses a trusted X.509 certificate (as in VOMS), then that's one thing, but if you relax that assumption and consider other possible use cases (such as Shibboleth), other profiles present themselves.
Your proposed solution does use the X.509 certificate to authenticate, since you need to be sure that the caller possesses the private key that matches the public key in the certificate. Therefore the AA/IDP does know the DN of the user (providing it trusts the CA that issued the cert). If the AA/IDP does not trust the CA, then the user might as well issue self signed certificates.
That's exactly right. I'm asking you to imagine a client configured for SSL/TLS client authn with a self-signed certificate. In fact, we have implemented just such a client.
But the OASIS spec says "has been issued an X.509 certificate" so we can assume that the CA is known and trusted.
Well, I wrote those words so it's okay for me to say they are just plain wrong :-)
But what you appear to be concerned about is that the public key and DN in the certificate are unknown to the IDP/AA, therefore the latter is unable to authenticate the caller *as being one of its existing users*, so does not know which attributes to release to him/her.
All I'm saying is that the SSL/TLS client certificate does NOT authenticate the user. Instead the user authenticates with his/her existing Shibboleth credentials (username/password).
But the IDP/AA can still authenticate the user. It is just that the user is unknown to it. Therefore one solution would be for the CA that issued the presumably short lived certificate with a random DN (if it was a long lived certificate then the AA/IDP could use the DN as its user identifier) to also insert into the certificate the username/identifier of the user that is known to the AA/IDP. In this way the AA/IDP can know that the caller holds the private key, and is known by the particular username in the certificate. Would this solve your problem?
No, not at all. The goal is to convert my username/password into a signed, holder-of-key SAML assertion. That can be done without introducing an X.509 PKI at the Shibboleth IdP.
Tom Scavo wrote:
Please find attached some comments regarding the "Use of SAML to Retrieve Authorization Credentials." I haven't fully reviewed this document, but these are the comments I can offer at this time.
Tom Scavo NCSA
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***************************************************************** David W. Chadwick, BSc PhD Professor of Information Systems Security The Computing Laboratory, University of Kent, Canterbury, CT2 7NF Skype Name: davidwchadwick Tel: +44 1227 82 3221 Fax +44 1227 762 811 Mobile: +44 77 96 44 7184 Email: D.W.Chadwick@kent.ac.uk Home Page: http://www.cs.kent.ac.uk/people/staff/dwc8/index.html Research Web site: http://www.cs.kent.ac.uk/research/groups/iss/index.html Entrust key validation string: MLJ9-DU5T-HV8J PGP Key ID is 0xBC238DE5
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