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Subject: RE: [xacml] Updated policy template wiki


>>> 

A.2) The <AttributeDesignator> construct was, so far, meant to retrieve attributes values from (Subject, Resource, Environment) attributes. Are we introducing the possibility for the same construct to also retrieve attribute values from (policy-template) attributes?

<<< 

 

<AttributeDesignator> attribute references can be fulfilled by attributes carried in the auth request and by PIPs supporting the PDP. The list of values produced by evaluating an <AttributeDesignator> could contain some values from the auth request, some values from PIP1, some from PIP2, etc.

 

As Eric noted, static policy template reduction could still be done by having the policy template processor simply replace an <AttributeDesignator> referencing a particular attribute ID with a bag of static values to create a new policy-instance.  This will work fine for <Condition> expressions because any _expression_ currently taking an <AttributeDesignator> is already expecting a multivalue bag type argument. 

 

However, static substitution of bags for <AttributeDesignator> will not work for <Match> expressions, since <Match> accepts only <AttributeDesignator> and <AttributeSelector> elements in the 2nd argument, not in-place bags of values.

 

The equivalent of dynamic policy template reduction would require setting up a PIP with the PDP to provide the appropriate template-data values for a particular attribute ID when the PDP evaluates the <AttributeDesignator>.  The PIP would need to be able to distinguish between your two template cases below (Curtiss, Packard vs Curtiss, Spad). If the PIP knew the context of which policy ID an <AttributeDesignator> was being evaluated in, the PIP could make the decision of whether to return (Curtiss, Packard) or (Curtiss, Spad) to provide the semantics of the original policy template concept.  Knowledge of the PDP evaluation state by PIPs is not part of the XACML spec, but could be provided by an implementation.

 

I could imagine manipulating the policy ID to indicate what substitution group(s) the policy belongs to.  Policy template (ID = “ABC”) + parameter data set 1 => policy ID=”ABC/1”.  Policy template (ID = “ABC”) + parameter data set 2 => policy ID=”ABC/2”.  The policy bodies are identical, but their IDs differ.

 

A more generalized solution would be to tie the parameter substitution group selection to some artifact of the request (subject, resource) or the environment (PDP host organization).  PIP returns parameter data set 1 for the target attribute ID if the request context also contains a “green apple” attribute, else the PIP returns parameter data set 2.  PIP returns parameter data set 1 if the organization’s country is X, parameter data set 2 if the organization’s country is Y, etc.

 

But the PIP only needs to be that smart if the PDP is multi-tenant.  In the use case of a shared policy (template) needing to be customized to a particular organization’s specific details (parameter data), if the PDP is dedicated to that one organization, the PIP could simply be loading the specific details (parameter data) from a config file. No need for decision branches in the PIP at all.

 

-Danny

 

 

Danny Thorpe

Authorization Architect

Dell | Identity & Access Management, Quest Software

 

Quest Software is now part of Dell.

 

From: Jean-Paul Buu-Sao [mailto:jean-paul.buu-sao@tscp.org]
Sent: Tuesday, October 09, 2012 1:40 AM
To: Erik Rissanen; Steven Legg
Cc: Danny Thorpe; xacml@lists.oasis-open.org
Subject: RE: [xacml] Updated policy template wiki

 

For sake of clarity I would like to rephrase the thought process.

 

Here is the excerpt of a policy (TAA-1.1), expressing that the subject attribute "http://schemas.tscp.org/2012-03/claims/OrganizationID" must be in the bag {"Curtiss", "Packard"}

 

    <Condition FunctionId="urn:oasis:names:tc:xacml:1.0:function:and">

      <Apply xsi:type="AtLeastMemberOf" functionId="urn:oasis:names:tc:xacml:1.0:function:string-at-least-one-member-of">

        <Apply functionId="urn:oasis:names:tc:xacml:1.0:function:string-bag">

          <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">Curtiss</AttributeValue>

          <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">Packard</AttributeValue>

        </Apply>

        <AttributeDesignator AttributeId="http://schemas.tscp.org/2012-03/claims/OrganizationID" DataType="http://www.w3.org/2001/XMLSchema#string" />

      </Apply>

      (etc...)

    </Condition>

 

Here is the excerpt of a policy (TAA-1.2), expressing that the subject attribute "http://schemas.tscp.org/2012-03/claims/OrganizationID" must be in the bag {"Curtiss", "Spad"}

 

    <Condition FunctionId="urn:oasis:names:tc:xacml:1.0:function:and">

      <Apply xsi:type="AtLeastMemberOf" functionId="urn:oasis:names:tc:xacml:1.0:function:string-at-least-one-member-of">

        <Apply functionId="urn:oasis:names:tc:xacml:1.0:function:string-bag">

          <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">Curtiss</AttributeValue>

          <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">Spad</AttributeValue>

        </Apply>

        <AttributeDesignator AttributeId="http://schemas.tscp.org/2012-03/claims/OrganizationID" DataType="http://www.w3.org/2001/XMLSchema#string" />

      </Apply>

      (etc...)

    </Condition>

 

The two policies (TAA-1.1 and TAA-1.2) are similar, with the exception of the hard-coded values of the string bags used by the AtLeastMemberOf function. The idea is to have only one policy that does not contain these hard-coded values. Instead we have a construct that would substitute the appropriate values. In the example above: {“Curtiss”, “Packard”} for TAA-1.1, {“Curtiss”, “Spad”} for TAA-1.2.

 

My initial proposal was to using the element <AttributeValue> without qualifying its contents values, but by specifying a ParameterId attribute, that would allow to substitute the appropriate values:

 

    <Condition FunctionId="urn:oasis:names:tc:xacml:1.0:function:and">

      <Apply xsi:type="AtLeastMemberOf" functionId="urn:oasis:names:tc:xacml:1.0:function:string-at-least-one-member-of">

        <Apply functionId="urn:oasis:names:tc:xacml:1.0:function:string-bag">

          <AttributeValue ParameterId="organizations" DataType=http://www.w3.org/2001/XMLSchema#string/>

        </Apply>

        <AttributeDesignator AttributeId="http://schemas.tscp.org/2012-03/claims/OrganizationID" DataType="http://www.w3.org/2001/XMLSchema#string" />

      </Apply>

    (etc...)

    </Condition>

 

I understand that instead, and in order to achieving the same effect, Erik’s proposal is to replace the <AttributeValue ParameterId=…> construct with the more standard <AttributeDesignator> construct, given something like:

 

    <Condition FunctionId="urn:oasis:names:tc:xacml:1.0:function:and">

      <Apply xsi:type="AtLeastMemberOf" functionId="urn:oasis:names:tc:xacml:1.0:function:string-at-least-one-member-of">

        <Apply functionId="urn:oasis:names:tc:xacml:1.0:function:string-bag">

        <AttributeDesignator CategoryId="urn:oasis:names:tc:xacml:1.0:policy:" AttributeId="organizations" DataType="http://www.w3.org/2001/XMLSchema#string" />

        </Apply>

        <AttributeDesignator AttributeId="http://schemas.tscp.org/2012-03/claims/OrganizationID" DataType="http://www.w3.org/2001/XMLSchema#string" />

      </Apply>

    (etc...)

    </Condition>

 

If my interpretation is correct (at least on the intent, not on exact syntax; Erik, I apologize if I totally missed your point), then some questions arise:

 

A.1) How would the authorization engine know where to go and fetch the desired values (i.e. from the TAA-1.1 template-data or the TAA-1.2 template-data)?

A.2) The <AttributeDesignator> construct was, so far, meant to retrieve attributes values from (Subject, Resource, Environment) attributes. Are we introducing the possibility for the same construct to also retrieve attribute values from (policy-template) attributes? If so we need to make it explicit on the specification, I would think.

 

B) Can the two use-cases called out at the beginning of [https://wiki.oasis-open.org/xacml/Policy%20Template%20Profile] be supported by <AttributeDesignator>?

I assumed that use-case 1 requires a static reduction, that happens far before any actual authorization decision; hence <AttributeDesignator> cannot be used for the purpose of parameterization. If my assumption is correct, then we need a different mechanism to support use-case 1, which is damageable. My proposal was to find a single syntax that would equally support both use-cases.

 

 

Thanks,

Jean-Paul

 

 

-----Original Message-----
From: Erik Rissanen [mailto:erik@axiomatics.com]
Sent: Tuesday, October 09, 2012 08:32
To: Steven Legg
Cc: Jean-Paul Buu-Sao; Danny Thorpe; xacml@lists.oasis-open.org
Subject: Re: [xacml] Updated policy template wiki

 

Steven,

 

I am arguing against the whole concept of a template, thus there is no Policy Template Engine in my proposal.

 

What I am trying to say is that instead of adding a Policy Template Engine to the PDP before the policy is evaluated by the XACML engine, you can just use a regular XACML policy in the XACML engine.

 

You can solve the same use case this way. Say that you want a template which has a parameter called "foo" and then a Policy Template Engine which substitutes "foo" with values before the XACML engine. Instead you can have a regular XACML policy with attribute designator "foo" and a PIP which provides the value which would have been provided by the policy template engine.

 

Best regards,

Erik

 

 

On 2012-10-09 02:46, Steven Legg wrote:

> 

> Erik,

> 

> Sorry, I read too much into what you were saying. Please disregard my

> previous response.

> 

> On 8/10/2012 5:38 PM, Erik Rissanen wrote:

>> Steven,

>> 

>> Yes, just replace all parameters with attribute designators.

> 

> An issue with using attribute designators as parameters is that there

> is ambiguity for the Policy Template Engine to resolve. Some attribute

> designators in the policy template are "real" and are preserved as is,

> and the other attribute designators are actually parameters that need

> to be replaced by a bag of attribute values from the policy template

> data.

> 

> Your example only has one parameter as a designator and that

> designator uses the same URI for the Category and AttributeId. Were

> you intending that equivalence as a way of distinguishing an attribute

> designator that is actually a parameter, or is it just because you had

> to put something in the Category ? A Policy Template Engine could

> always look in the policy template data to work out which designators

> are really parameters, but I think it would be better if it were clear

> from the designator. I suggest using a special category URI just for

> parameters.

> 

> The attribute designators that are actually parameters should always

> use MustBePresent="true" so that an implementation that blindly

> evaluates a policy template as a normal policy will appropriately

> return an indeterminate decision.

> 

> It seems we have three proposals for representating parameters in

> policy

> templates:

> 

> (1) Just use attribute designators. This means that targets can't have

> parameters, but on-permit-apply-second can be used to get the same

> effect.

> Policy template data writers can determine whether a parameter is

> single-valued or multi-valued by seeing whether the corresponding

> attribute designator is wrapped in type-one-and-only.

> 

> (2) <ValueParameter> and <BagParameter> elements. Targets can have

> value parameters, but not bag parameters, though

> on-permit-apply-second can be used to get the same effect. It's

> obvious to policy template data writers what the parameters are and

> whether a parameter is single-valued or multi-valued.

> 

> (3) ParameterId XML attribute on <AttributeValue> elements. It's

> reasonably obvious to policy template data writers what the parameters

> are. An additional XML attribute to indicate whether the parameter is

> single-valued or multi-valued would make this solution cleaner.

> Targets can have value parameters. Targets could also have bag

> parameters provided the target is transformed by the Policy Template

> Engine. Alternatively, bag parameters in targets could be disallowed

> because on-permit-apply-second can be used to get the same effect.

> 

> I don't see a compelling need for transformations of conditions.

> Proposals

> (1) and (2) don't lend themselves to transformations, but it could be

> done.

> The difference in the proposals is mostly around the syntax for

> representing parameters.

> 

> Regards,

> Steven

> 

>> 

>> I don't understand why you need to evaluate the template twice. I

>> tried to understand it from the wiki, but to me it all looks like

>> simple value substitution, which can handled better with attribute

>> designators.

>> 

>> Best regards,

>> Erik

>> 

>> On 2012-10-08 04:32, Steven Legg wrote:

>>> 

>>> Erik,

>>> 

>>> On 5/10/2012 6:03 PM, Erik Rissanen wrote:

>>>> Jean-Paul,

>>>> 

>>>> I put in one such example on the wiki page. Search for

>>>> "on-permit-apply-second" and you will find it.

>>> 

>>> This strategy only works if there is one policy template data for

>>> the policy template. In general, there may be many policy template

>>> data per policy template. The example in the wiki has two: TAA-1.1 and TAA-1.2.

>>> 

>>> Although you didn't do it, I assume you meant to replace the

>>> parameters in the condition with attribute designators as well,

>>> i.e., designators for "organizations", "nationals" and "workEfforts".

>>> 

>>> The policy needs to be evaluated twice. The first time the attribute

>>> designator for PolicyIdOnResource needs to return

>>> "urn:curtiss:ba:taa:taa-1.1", the designator for "organizations"

>>> needs to return the bag of { "Curtiss", "Packard" }, the designator

>>> for "nationals" needs to return the bag of { "US", "GB" }, and

>>> "workEfforts" needs to return { "DetailedDesign", "Simulation" }.

>>> The second time, PolicyIdOnResource needs to return

>>> "urn:curtiss:ba:taa:taa-1.2", "organizations" needs to return {

>>> "Curtiss", "Spad" }, "nationals" needs to return { "US", "FR" } and

>>> "workEfforts"

>>> needs to return { "Integration", "Simulation" }.

>>> 

>>> Of course, we don't currently have a way to cause a policy to be

>>> evaluated multiple times with the attribute designators returning

>>> different bags each time. For one thing, the result of an attribute

>>> designator is fixed for the entire duration of the processing of an

>>> authorization request. The expansion of the policy template into a

>>> policy template instance for each policy template data achieves the

>>> same end.

>>> 

>>> The use of on-permit-apply-second is a good idea for removing the

>>> need for any special processing of parameters in targets.

>>> 

>>> Regards,

>>> Steven

>>> 

>>>> 

>>>> Best regards,

>>>> Erik

>>>> 

>>>> On 2012-10-04 23:01, Jean-Paul Buu-Sao wrote:

>>>>> 

>>>>> Greetings,

>>>>> 

>>>>> Apologies for missing the call today, as I was in a TSCP event,

>>>>> together with Gerry and David of Axiomatics.

>>>>> 

>>>>> I have been much interested in the last findings, and agree that

>>>>> if the “template” property that we are (all, I think) looking for

>>>>> could be achieved with standard the <AttributeDesignator>

>>>>> construct, rather than introducing new concepts, then this would

>>>>> be for the better.

>>>>> 

>>>>> May I suggest that, in order to verify this assertion (so to

>>>>> speak), some folks, such as Erik or Danny, would be kind enough to

>>>>> propose an alternate proposal to the sample found on our Wiki

>>>>> By the way, as a word of

>>>>> caution, please disregard the in-correctness of the XCAML 3.0 of

>>>>> section 1. of the example (yes the devil is in the details, and

>>>>> David shown me how this example could be made compliant).

>>>>> 

>>>>> Thanks in advance,

>>>>> 

>>>>> Jean-Paul

>>>>> 

>>>>> *From:*xacml@lists.oasis-open.org

>>>>> [mailto:xacml@lists.oasis-open.org] *On Behalf Of *Danny Thorpe

>>>>> *Sent:* Thursday, October 04, 2012 20:11

>>>>> *To:* Erik Rissanen; xacml@lists.oasis-open.org

>>>>> *Subject:* RE: [xacml] Updated policy template wiki

>>>>> 

>>>>> When the original proposal for policy templates was brought

>>>>> forward, I noted that simply replacing a single AttributeValue

>>>>> element with a list of values from the policy template <Parameter>

>>>>> would fail in <Match> expressions, since <Match> is very explicit

>>>>> about one value, one designator/selector.  I offered a transform

>>>>> to help produce valid match expressions.

>>>>> 

>>>>> AttributeDesignator does provide similar

>>>>> “insert-multiple-values-here” operations to the policy template

>>>>> substitution behavior, but I believe the suggestion of policy

>>>>> parameterization came up because of situations in which

>>>>> AttributeDesignator cannot be used. Comparing an  attribute

>>>>> against a static list of test values (specific to an organization

>>>>> or location and applied to a generic policy), for example, is a

>>>>> many-to-many comparison, but cannot be expressed in a <Match>

>>>>> element.

>>>>> 

>>>>> As we discussed on the TC call today, we’re finding more

>>>>> difficulties with parameter substitution the deeper we dig. Steven

>>>>> Legg noted in an earlier email that some Xacml functions that take

>>>>> single <AttributeValue> won’t work if multiple values are dropped

>>>>> in to replace the <AttributeValue>. This means some sort of

>>>>> _expression_ transform will be necessary in condition expressions as

>>>>> well to move policy templates forward.

>>>>> 

>>>>> In light of these increasing complexities and challenges, I’m

>>>>> beginning to agree with you that perhaps the policy template use

>>>>> case can better be addressed using the existing

>>>>> <AttributeDesignator>.

>>>>> 

>>>>> This would mean:

>>>>> 

>>>>> 1.Giving up parameterization behavior in <Match> expressions and

>>>>> moving that logic into conditions using <AttributeDesignator> to

>>>>> reference an attribute ID representing the parameterization data.

>>>>> 

>>>>> 2.Moving parameterization data from a static policy generator step

>>>>> to a PIP to fill <AttributeDesignator> references to a particular

>>>>> attribute ID with parameterization data in the PDP at auth request

>>>>> evaluation time.

>>>>> 

>>>>> Using <AttributeDesignator> instead of policy templates does

>>>>> impact the use case quite a bit because populating PIP data is not

>>>>> part of the Xacml spec. Policies could be shared between

>>>>> organizations per the use case, but how the parameterization data

>>>>> is applied to those policies would become a vendor-specific

>>>>> implementation detail.

>>>>> 

>>>>> I can see the attraction of parameterizing policies to allow

>>>>> up-front synthesis of specific policies, but as we say “the devil

>>>>> is in the details.” The details are winning. :/

>>>>> 

>>>>> -Danny

>>>>> 

>>>>> *Danny Thorpe *

>>>>> 

>>>>> Authorization Architect

>>>>> 

>>>>> Dell | Identity & Access Management, Quest Software

>>>>> 

>>>>> Quest Software is not part of Dell.

>>>>> 

>>>>> *From:*xacml@lists.oasis-open.org

>>>>> <mailto:[mailto:xacml@lists.oasis-open.org]> *On Behalf Of *Erik

>>>>> Rissanen

>>>>> *Sent:* Thursday, October 04, 2012 4:42 AM

>>>>> *Subject:* Re: [xacml] Updated policy template wiki

>>>>> 

>>>>> All,

>>>>> 

>>>>> I still fail to see why this is useful.

>>>>> 

>>>>> If you take a policy template, and replace each <Parameter> with

>>>>> an appropriate <AttributeDesignator>, then you get a regular XACML

>>>>> policy, and the PEP/PDP can "fill in"

>>>>> the "template" at runtime using normal XACML attributes.

>>>>> 

>>>>> Why do we need a new standard? In particular I would be opposed to

>>>>> "implementation option C", that is a PDP would construct the

>>>>> policy from the template at runtime.

>>>>> That's lots of heavy machinery for no gain.

>>>>> 

>>>>> Best regards,

>>>>> Erik

>>>>> 

>>>>> On 2012-09-20 20:25, Danny Thorpe wrote:

>>>>> 

>>>>>     I’ve updated the policy template wiki

>>>>>     with text about required Match _expression_ rewriting in

>>>>> parameter substitution and optional use of

>>>>>     AttributeDesignators and AttributeSelectors in Parameter data

>>>>> in dynamic policy template reduction

>>>>>     implementations.

>>>>> 

>>>>>     -Danny

>>>>> 

>>>>>     *Danny Thorpe *

>>>>> 

>>>>>     Product Architect | | *Quest Software*- /Now including the

>>>>> people and products of BiTKOO/ |

>>>>> 

>>>> 

>>> 

>> 

> 

 

 



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