[OGSA-AUTHZ] some thoughts on attribute mapping of an XACML request context
David Chadwick
d.w.chadwick at kent.ac.uk
Wed Mar 7 08:57:43 CST 2007
Hi Takuya
Here is my input for todays telecon
Takuya Mori wrote:
> Hi,
>
> The following is some thought on attribute mapping of an XACML request
> context. I hope it will be a good input for today's discussion.
>
> Thanks,
> Takuya
>
> ----
> Feb. 13, 2007
> Takuya Mori
>
> a note on attribute mapping of an XACML request context
>
> 1. Subject Attributes
> Any attributes asserted in credentials of a subject attached to a
> request message should be taken into the consideration of the authz
> decision. Therefore, such attributes should be embedded into an XACML
> request context as its subject attributes.
>
> The followings are an example of such credentials:
> - X.509 Certificates
> -- public key certificates
> -- attribute certificates
> -- proxy certificates
> - VOMS Attribute Certificates (actually, it's a kind of X509 attr
> certs)
> - privilege credeitnals such as PERMIS, CAS, ... credentials
> - others? Username/Password Tokens?, Kerberos Tickets?
I generally agree with you, but note that there are no PERMIS specific
credentials, they are all pure X.509 ACs.
> - ...
>
> [AuthzFunc] document defines four types of interactions between
> authorization functional components. In case of two PEP Context
> Handler Models, because the validity of the credentials are validated
> on the PDP side, the whole credentials of the subject should be passed
> to the PDP in an XACML request context. Therefore, it is needed to
> define a subject attribute format of the XACLM request context which
> can hold a binary credential within it.
>
> On the other hand, in case of two PEP Context Handler Models, the
> validity of the credentials are validated on the PEP side and the
> validated attributes are passed to the PDP in an XACML request
> context. Therefore, there is a need to define a rule to map such
> validated attributes into XACML subject attributes.
>
> As a conclusion, regarding with our [XACMLProfile] document, what we
> need for our subject attribute profile is:
> 1) a way to embed a binary credential into a subject attribute of
> an XACML request context.
Agreed. I suggest that we can give a rule to say
i) base64 encode the credential, then use the <SubjectAttributeDesignator>
> 2) a way to map an attribute of a generic credential such as X.509
> public key certificate into a subject attribute of an XACML
> request context.#
We can use the XACML spec for this. We should encourage people to use
standard LDAP attribute types wherever possible, since this is supported
directly by XACML. The "AttributeId" field is set to the RFC number and
attribute name e.g.for the user password LDAP attribute defined in
RFC2256 it would be
http://www.ietf.org/rfc/rfc2256.txt#userPassword
For X.509 attributes that are not already defined in LDAP, we can use
the OID urn format, which I think is
urn:oid:<dotted number notation for OID>
> 3) a way to map an attribute of popular credential formats such
> as VOMS attribute certificate, Shibboleth attribute assertions,
> etc... The scope of credential types to support by
> the [XACMLProfile] document should be defined based on the
> requirements by the community.
We can use the OID format for these where OIDs exist.
>
> In case of NAREGI, X.509 proxy certificates as well as VOMS attribute
> certificates are used for authentication and authorization. And
> NAREGI's AuthZ service is based on "PEP Context Handler + Push" model
> defined in the [AuthzFunc] document. The subject attributes used in
> NAREGI for AuthZ are shown below.
>
> - UserDN:
> ID: "urn:oasis:names:tc:xacml:1.0:subject:subject-id"
> Value: User's "Globus ID" (in String format)
> - VOMS VO Name Attribute:
> ID: “naregi:vo:voName”
> Value: VO Name (in URI format)
> - VOMS Group Attribute:
> ID: “naregi:vo:group”
> Value: Group Name (in String format)
> - VOMS Role Attribute:
> ID: “naregi:vo:role”
> Value: Role Name (in String format)
> - VOMS Capability Attribute:
> ID: “naregi:vo:capability”
> Value: Capability Name (in String format)
>
> The IDs described above are tentative and it is preferable to have
> standardized IDs for them.
I agree we need to have standard names for these. This is something the
group will need to discuss and agree upon.
>
> 2. Resource Attributes
> [XACMLProfile] document specifies that the resource attribute of an
> XACML request context should be the value of wsa:To header of an
> incoming request message, which is an endpoint reference of a service
> interface to which the request is sent. [OGSA-WSRFBasicProfile]
> docuemnt defines a set of profiles to embed reference properties into
> an endpoint reference which enables to specify a WS-Resource, a
> stateful resource behind a service interface of a Web Service.
> Therefore, the profile specified in the current [XACMLProfile]
> document is sufficient for specifying a WS-Resource in an XACML
> request context.
>
> There may be a case where some service providers need to make an
> authorization decision on information appeared in a message body. For
> example, a user who is submitting a job to an job execution service
> can request some amount of computing resources in a JSDL document and
> a service owner of the job execution service would like to make an
> authorization decision based on the amount of comuting resources
> requested in the JSDL document. Because information which appeared in
> a message body is consumed by a service, the authorization decision
> discussed here can be said as application level authorization. In my
> opinion, a support for such authorization decision on middleware level
> is helpful for application developers because not all such the
> developers can design authorization mechanisms for their application.
Agreed. This would be up to the application PEP to pick up the correct
information for the PDP
>
> In case of NAREGI's AuthZ service, in order to address the requirement
> for application level authorization, Resource Attribute Filtering
> Mechanism, a flexible and configurable method to specify a mapping
> between resource descriptions in a request message and resource
> attributes of an XACML request context is introduced. RAFM is
> configured by a Resource Mapping Description which defines such
> mapping rule and only a owner of a service can configure it.
>
> The following is an example of a Resource Mapping Description.
>
> List.1: An example of a Resource Mapping Description
> ----
> 01: <resource_maps>
> 02: <namespaces>
> 03: <namespace prefix="ns1"
> value="http://schemas.ggf.org/jsdl/2005/06/jsdl"/>
> 04: </namespaces>
> 05: <map>
> 06: <input_element>//ns1:Resources/ns1:TotalResourceCount/ns1:Exact
I think you mean TotalCPUCount
> </input_element>
> 07: <xacml_resource_attribute_id>naregi:TotalResourceCount
> </xacml_resource_attribute_id>
> 08: </map>
> 09: </resource_maps>
> ----
> 02..04: a <namespaces> element defines namespace declarations that
> are used in an XPath expression in a <input_element> element.
> 06: a value of an <input_element> is an XPath expression for
> specifying a resource description in a message body of a request
> message.
> 07: a value of an <xacml_resource_attribute_id> element is an resource
> attribute id that will be used in an XACML request context
> to specify the resource specified by the XPath expression in the
> line 06.
>
> The description above will make the RAFM to convert the fragment of
> an JSDL document shown in List.2 to the XACML resource attribute
> described in List.3.
>
> List.2: a fragment of an JSDL document
> ----
> 01: <Resources>
> 02: <TotalCPUCount>
> 03: <Exact>1</Exact>
> 04: </TotalCPUCount>
> 05: </Resources>
> ----
> List.3: an example of a resource attribute of an XACML request context
> ----
> 01: <Resource>
> 02: <Attribute AttributeId="naregi:TotalResourceCount">
> 03: <AttributeValue>1</AttributeValue>
> 04: </Attribute>
> 05: </Resource>
> ----
>
> There may be an issue that a malcious user may specify a spoofed value
> in a message body which can cause the authorization component to make
> a false decision on the service access. In my opinion, in order to
> provide some service to a user, a service owner need to trust a
> content of the reqest message in some extent, because the content will
> be used for some computation behind the service. Therefore, the
> information in the request message can also be used for authorization
> decision with the same level of the trust that the computation relys
> on at the service owner's choice.
I agree
David
>
>
> A. References
> [XACMLProfile] Use of XACML Request Context to access a PDP,
> David W. Chadwick et al.
> [AuthzFunc] Functional Components of Grid Service Provider
> Authorisation Service Middleware, David W. Chadwick et al.
>
>
>
> ------------------------------------------------------------------------
>
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--
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David W. Chadwick, BSc PhD
Professor of Information Systems Security
The Computing Laboratory, University of Kent, Canterbury, CT2 7NF
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Email: D.W.Chadwick at kent.ac.uk
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