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Subject: Subject confirmation (was RE: Multiple subjects in SAML assertion s)

> From: Hallam-Baker, Phillip [mailto:pbaker@verisign.com]
> Subject: RE: Multiple subjects in SAML assertions
> Core 20 states:
> If a <Subject> element contains more than one subject 
> specification the
> issuer is asserting that the statement is true for all of the subjects
> specified. For example if both a <NameIdentifier> and a
> <SubjectConfirmation> element are present the issuer is 
> asserting that the
> statement is true of both parties.

This leaves us with no way in the standard to link a <SubjectConfirmation>
with a <NameIdentifier>, as Chris pointed out in his follow-up. It also
still leaves open the question I raised in my Multiple Subjects message
about what a Relying Party should do when faced with an
AuthenticationAssertion with more than one <NameIdentifier> in its

> Conditions is not the place to put subject confirmation.
> Phillip Hallam-Baker FBCS C.Eng.

A number of the suggested uses for the existing <SubjectConfirmation>
element feel like Conditions to me. For example, the SOAP profile that
Prateek presented at F2F4 put a ds:KeyInfo into <SubjectConfirmation> to
indicate that an assertion was only applicable in the context of a document
signed by the corresponding key. That, to me, certainly belongs in

Perhaps we need to clarify the distinction between:

(1) Authority asserts whatever about Subject, and only wants RelyingParty to
rely on this assertion if presented by holder-of-key (or similar mechanism)


(2) Authority asserts whatever about Subject, only wants RelyingParty to
rely on this assertion if presented by holder-of-key, *AND* asserts that
holder-of-key is Subject

and the slightly less strict

(3) Authority asserts whatever about Subject, and if RelyingParty cares, it
can verify that the presenter is the Subject by checking holder-of-key

I'm using the phrase "presented by holder-of-key" in a rather generic sense
to refer to any way that the RP can determine that a principal involved in a
process satisfies the <SubjectConfirmation> criteria - could be because that
principal signed a document containing the assertion, presented a SAML
Artifact (corresponding to the assertion) over an HTTP/SSL connection with
client certificate, etc.

My feeling is that (2) is what most people mean when they say
SubjectConfirmation, and I think that's as good a definition as any. I think
we still need to change the <Subject> schema to enforce the correspondence
between <SubjectConfirmation> and <NameIdentifier> elements (if there is

Bob Blakley's message
suggests that <SubjectConfirmation> is only meaningful in an assertion.
Unless I'm misreading his intent, it would not be possible to make a SAML
Protocol request, asking an attribute authority for attribute assertions
corresponding to a <SubjectConfirmation> subject. To me, this implies that
an assertion _must_ have a <NameIdentifier> in its subject if we want an RP
to be able to request other relevant assertions.

With that in mind, I'd be happy if we simplified the <Subject> schema to:

<element name="Subject" type="saml:SubjectType" />
<complexType name="SubjectType">
    <element ref="saml:NameIdentifier"/>
    <element ref="saml:SubjectConfirmation" minOccurs="0"/>

 - irving -

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