[OGSA-AUTHZ] Comments: Use of SAML to Retrieve Authorization Credentials

[Tom Scavo]

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 at 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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>>  ogsa-authz-wg at ogf.org
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>
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>
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