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Subject: RE: Representing anonymous Subjects
Bob - Well! Let's have a go.
Bob wrote ...
"I think a bearer assertion is an assertion which refers indexically to an
unnamed subject and does not require an authenticator."
Tim replies ...
By my understanding, SAML separates this thought into two unrelated topics: anonymity and bearer authentication. When it comes to anonymity, look at this fragment from "core 14" ...
<xsd:element name="Subject" type="saml:SubjectType"/>
<xsd:complexType name="SubjectType">
<xsd:choice maxOccurs="unbounded">
<xsd:element ref="saml:NameIdentifier" minOccurs="0" maxOccurs="unbounded"/>
<xsd:element ref="saml:Authenticator" minOccurs="0" maxOccurs="unbounded"/>
<xsd:element ref="saml:AssertionSpecifier" minOccurs="0" maxOccurs="unbounded"/>
</xsd:choice>
This allows for the name to be absent, both because the structure is a "choice" and because the NameIdentifier has a minOccurs of zero. I realize, of course, that strict anonymity is more complicated than just leaving the name blank. But, SAML doesn't aspire to solve such tricky issues.
Now let's look at bearer authentication. In my view, the Authenticator element MUST be present in an authentication assertion. But, it may have no associated authentication data. Consider this fragment ...
<xsd:element name="Authenticator" type="saml:AuthenticatorType"/>
<xsd:complexType name="AuthenticatorType">
<xsd:sequence>
<xsd:element name="Protocol" type="uriReference" maxOccurs="unbounded"/>
<xsd:element name="Authorizationdata" type="string" minOccurs="0"/>
<xsd:element ref="ds:KeyInfo" minOccurs="0"/>
</xsd:sequence>
</xsd:complexType>
The "protocol" element would identify the "Assertion bearer" authentication scheme, and the "AuthorizationData" (which should say "AuthenticationData") would be absent - allowed by the minOccurs = zero.
Bob wrote ...
"I think the mechanism by which an artifact refers to an assertion should not vary depending on which subject reference type is used within the assertion."
Tim replies ...
The structure of the artifact is not defined in our schema. It appears in the Bindings document. It is a 64 bit assertion identifier. The corresponding assertion carries the same value in its AssertionID element ...
<xsd:attribute name="AssertionID" type="saml:IDType" use="required"/>
So, the artifact references an assertion the same way regardless of how the subject is referenced in the assertion.
Bob wrote ...
"But surely an artifact should be able to refer to an assertion with ANY subject reference type"
Tim replies ...
Absolutely. But, as far as I know, the only use case, and the only "profile" defined in the Bindings document, that uses an artifact is the Browser profile, and it happens also to use the "Assertion bearer" Authentication protocol.
Best regards. Tim.
-----Original Message-----
From: George_Robert_Blakley_III@tivoli.com
[mailto:George_Robert_Blakley_III@tivoli.com]
Sent: Monday, August 20, 2001 4:17 PM
To: tim.moses@entrust.com
Cc: security-services@lists.oasis-open.org
Subject: RE: Representing anonymous Subjects
Maybe. Can you give me an example of the specific schema which
accomplishes this? I have a hard time translating English
into XML (or the other direction, for that matter...) Thanks,
--bob
Bob Blakley (email: blakley@us.tivoli.com phone: +1 512 436 1564)
Chief Scientist, Security and Privacy, Tivoli Systems, Inc.
Tim Moses <tim.moses@entrust.com> on 08/20/2001 02:47:24 PM
To: security-services@lists.oasis-open.org
cc:
Subject: RE: Representing anonymous Subjects
Bob - I agree with you on all fronts, and the schema accommodates these
ideas. There is one exception, though. Reference your second paragraph
... if the "bearer assertion" is an "authentication assertion", then it
MUST contain an authenticator. Now, the "authenticator" may have no value;
i.e. its authentication protocol identifier is "assertion bearer", but it
is an authenticator nonetheless, and it says: "if a subject presents an
artifact referencing this assertion, then he/she is the owner of the
attributes in (or linked to) this assertion".
There is a browser profile that has no artifact, then the bearer
authentication assertion must still have an authenticator (as above). In
this case, the authenticator says: "if a subject presents this assertion,
then he/she is the owner of the attributes in (or linked to) this
assertion".
No?
Best regards. Tim.
-----Original Message-----
From: George Robert Blakley III [mailto:blakley@us.tivoli.com]
Sent: Monday, August 20, 2001 3:04 PM
To: Tim Moses
Cc: security-services@lists.oasis-open.org
Subject: RE: Representing anonymous Subjects
I think we're having two different discussions at the same time.
I think a bearer assertion is an assertion which refers indexically to an
unnamed subject and does not
require an authenticator.
It should be possible to generate such an assertion without having to
generate an artifact AT ALL.
I think the mechanism by which an artifact refers to an assertion should
not vary depending on which
subject reference type is used within the assertion.
I DO NOT think that the assertion should refer to the artifact; the
reference should be strictly one-way
(from the artifact to the assertion). But surely an artifact should be
able to refer to an assertion with
ANY subject reference type... right?
--bob
Bob Blakley (email: blakley@us.tivoli.com phone: +1 512 436 1564)
Chief Scientist, Security and Privacy, Tivoli Systems, Inc.
Tim Moses <tim.moses@entrust.com> on 08/20/2001 01:38:58 PM
Please respond to Tim Moses <tim.moses@entrust.com>
To: security-services@lists.oasis-open.org
cc:
Subject: RE: Representing anonymous Subjects
Bob - Are you just arguing that the assertion should reference the artifact
(in its "Assertion bearer" element), as well as the artifact referencing
the assertion? All the best. Tim.
-----Original Message-----
From: George Robert Blakley III [mailto:blakley@us.tivoli.com]
Sent: Monday, August 20, 2001 2:32 PM
To: Tim Moses
Cc: security-services@lists.oasis-open.org
Subject: RE: Representing anonymous Subjects
Tim,
Right... regardless of this though it seems to me that an implementation
which would support this could easily be
achieved and I don't see any downside to doing it in a way which would
preserve our flexibility in this regard.
I could make a more abstract argument here. If "bearer" is a way of
describing a subject, in the same way
that "name" or "authenticator" is a way of describing a subject, we should
use the same mechanism to
describe "bearer" subjects which we use to describe other types of
subjects, so that the processing of
subject descriptors can be localized to a single piece of logic.
:-)
--bob
Bob Blakley (email: blakley@us.tivoli.com phone: +1 512 436 1564)
Chief Scientist, Security and Privacy, Tivoli Systems, Inc.
Tim Moses <tim.moses@entrust.com> on 08/20/2001 01:22:58 PM
Please respond to Tim Moses <tim.moses@entrust.com>
To: security-services@lists.oasis-open.org
cc:
Subject: RE: Representing anonymous Subjects
Bob - I don't believe the bindings group currently considers your scenario
"in scope". That would change if we had a use case, of course. Best
regards. Tim.
-----Original Message-----
From: George Robert Blakley III [mailto:blakley@us.tivoli.com]
Sent: Monday, August 20, 2001 2:19 PM
To: Hallam-Baker, Phillip
Cc: Marlena Erdos; security-services@lists.oasis-open.org
Subject: RE: Representing anonymous Subjects
Phill,
I "sort of agree" -- I think that whether "bearer" is the default or not
should be up to the specifier of the binding,
which is to say that I think "bearer" ought to be a first-class subject
option like the other three, and that it ought
not be possible to specify any of these options by simply omitting the
element -- you ought to have to explicity
choose a subject representation and put the element in in order to get
anything.
As far as implementation of the "bearer" element is concerned, I guess what
I'm worried about is that if we
want to be able to pass assertions along a chain of calls and achieve the
same effect as delegation but by
the mechanism of passing bearer assertions, we can't do that unless the
"bearer" nature of the assertion appears
inside the assertion structure itself.
Or am I missing something?
PS I realize this is really dangerous, but there are trust structures where
it makes sense INSIDE a distributed
application with several components.
--bob
Bob Blakley (email: blakley@us.tivoli.com phone: +1 512 436 1564)
Chief Scientist, Security and Privacy, Tivoli Systems, Inc.
"Hallam-Baker, Phillip" <pbaker@verisign.com> on 08/20/2001 12:48:45 PM
Please respond to "Hallam-Baker, Phillip" <pbaker@verisign.com>
To: George Robert Blakley III/Austin/IBM@IBMUS, Marlena
Erdos/Austin/Contr/IBM@IBMUS
cc: security-services@lists.oasis-open.org
Subject: RE: Representing anonymous Subjects
Actually 'Bearer' does appear in section 4 as one of the Authenticator
protocols.
My recollection of the F2F was that bearer was mentioned but not
specifically brought up as a topic. Tim wanted bearer to be possible but
was
not that bothered how it was implemented. I did not want bearer to be the
default - for much the same reason that Dave O. is proposing to squelch
defaulting to wildcard.
Phill
Phillip Hallam-Baker FBCS C.Eng.
Principal Scientist
VeriSign Inc.
pbaker@verisign.com
781 245 6996 x227
> -----Original Message-----
> From: George Robert Blakley III [mailto:blakley@us.tivoli.com]
> Sent: Monday, August 20, 2001 1:06 PM
> To: Marlena Erdos
> Cc: security-services@lists.oasis-open.org
> Subject: Re: Representing anonymous Subjects
>
>
> Hmmmm.....
>
> At FTF3 we took it as implied that we were going to have a
> "bearer" option
> in the subject specifier.
>
> However we were pretty sure that our thinking on the topic
> was confused; to
> quote from FTF3 minutes:
>
> >ISSUE:[F2F#3-35]: BobB said in an aside that there is something wrong
> >with at least some of the occurrences of our use of the "bearer" in
> >Subject specifiers in all of the above constructs. Need to get him to
> >elaborate in detail on what his observations are.
>
> I didn't do this, but it also appears that "bearer" has
> disappeared from
> the schema , probably because I couldn't
> find our whiteboard mentions of it at all in the minutes (!)
>
> We need to add it back and discuss how to get it right.
>
> I agree with Marlena that we don't currently capture the concept:
>
> >Summary: I don't think we currently have a good way of representing
> >either "bearer" assertions or anonymous assertions gotten
> via an artifact.
> > A possible solution is the extension of the Subject field to
> >include an "anonymous" type. But I'm no
> >XML expert. I'm hoping that if others agree that there is a need,
> >we can work out the XML specifics.
>
> I see three possibilities:
>
> (1) A distinguished name, to be used within the "name" field
> of a subject
> definition.
>
> (2) A distinguished public "key" field to be used with the
> "holder of key"
> (is it called "authenticator" now?) field
>
> (3) A type-distinct fourth subject option, called "bearer".
>
> I don't like either (1) or (2) -- I'd rather have (3). I
> don't necessarily
> think that bearer = anonymous, however, since in either an
> authentication
> assertion or an attribute assertion, other asserted data may in fact
> identify
> the subject. However you could USE a bearer token to
> implement anonymity.
>
> Now I need to think about what the issues are and how to get
> it right....
>
>
> --bob
>
> Bob Blakley (email: blakley@us.tivoli.com phone: +1 512 436 1564)
> Chief Scientist, Security, Tivoli Systems, Inc.
>
>
> Marlena Erdos <marlena@us.ibm.com> on 08/20/2001 12:59:25 AM
>
> To: security-services@lists.oasis-open.org
> cc:
> Subject: Representing anonymous Subjects
>
>
>
> Dear SAML'ers,
>
> Recent discussions of the http binding have brought up
> two flows that result in assertions about (potentially) anonymous
> subjects. These flows either involve an artifact (or "handle" in
> Shibboleth) that is used to retrieve an assertion about the
> browser user, or involve the assertion being "redirected" (so
> to speak)
> through the user's browser (via interesting uses of javascript
> and "post").
> Various discussions have flowed about impersonation
> countermeasures and other aspects of these http-based flows.
> I won't repeat them here. Instead, I'd like to raise up the
> question of the "Subject" element in the assertion.
>
> In both flows (artifact or "bearer" assertion), it is possible for
> the assertion to have an anonymous (or pseudonymous) subject. (Note
> that the assertion could be either an authN or attribute assertion.)
> Currently, the Subject element in the SAML schema (version 14)
> doesn't seem to have a graceful way of dealing with the sorts of
> anonymous subjects that arise from the http binding.
> (I'll continue to say "anonymous subjects" to refer to both
> anonymity and
> pseudonymity, and to mean the http binding case.) Here's
> what it does
> allow for (to the extent that I understand the schema correctly).
> I'll follow with a brief discussion on why none of the options seem
> to have a good fit for the anonymous subject case.
>
> The Subject element allows for 3 different types of identifiers. It
> allows for
> -- a string "name" qualified by a security domain
> -- an "authenticator" field (mostly related to a subject
> authenticating with a key)
> -- a reference to another assertion or inclusion
> ofthat assertion
>
> Why these don't really fit:
> - The string name could be used to convey the contents of
> the handle
> or the "assertion id" part of the artifact, but neither of these is in
> any way a subject 'name'. I suppose that the RP could use the fake
> name as a real name in its continued processing, but that is
> conceptually incorrect, and it feels like looking for trouble to me.
> (One concrete difficulty is how an RP could meaningfully have
> the notion
> of "anonymous" in an authZ policy if it can't tell the difference
> between a fake name referring to an anonymous user and a real
> user name.)
> - The authenticator field strikes me as being mostly related to
> subjects with keys, but Phill (in a phone call with me) suggested
> that it could be used for the anonymous browser user case. I feel
> reluctant about this because the association between an anonymous
> browser user and an assertion is just that -- an association.
> To refer
> to it as an authentication just seems like a distortion of
> both the term authentication and what is really going on from a
> processing standpoint. (That is, the RP validates the association of
> user and assertion rather than validating the identity of the user. )
> - The reference to an assertion or inclusion of one doesn't seem
> to apply, because anonymous assertions in the http binding are (often)
> the initial ones. There is not necessarily any other
> assertion to refer
> to (and there would still be the question of representing anonymity
> in that assertion!).
>
>
> Summary: I don't think we currently have a good way of representing
> either "bearer" assertions or anonymous assertions gotten via
> an artifact.
> A possible solution is the extension of the Subject field to
> include an "anonymous" type. But I'm no
> XML expert. I'm hoping that if others agree that there is a need,
> we can work out the XML specifics.
>
>
> Regards,
> Marlena
> IBM/Tivoli
>
> PS My apologies if I've misunderstood the meaning of the fields in
> the current schema. I welcome enlightenment.
>
>
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