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Subject: Re: Policy Labelling (was Re: Delegation and on-permit-apply-second)

Hi Steven,
Ok, so in your proposal there is no reduction graph. Early working drafts of the delegation profile were like this, so you are suggesting that we revert back to that old model.
The TC introduced the reduction graph because there are issues with the model you propose. The main reasons the TC moved on from the model you propose were (if I recall it correctly):
- We did not want to have the concept of a negative administrative authority, that is, someone can deny someone else to authorize an administrative request. I am not talking about combining policies and rules within an administrative policy(set) since this is just a matter of defining the scope of the administrative policy using XACML constructs. The problem is how to combine administrative policies from multiple authors, which might conflict. The way the reduction graph now works it simple and well defined.
- You have to deal with indeterminate values from both access policies and admin policies. This is not so simple if the policies themselves are not trusted with certainty. For instance, someone who puts an indeterminate policy at the top level could cause interference.
- The reduction graph search is also computationally efficient. Some forms of the early delegation model were NP-complete, but that was due to the ability to restrict more than one step in the delegation chain. I am not sure if there are any issues with your proposed model though. 

/Erik


On 2012-12-20 07:22, Steven Legg wrote:


Hi Erik,

On 20/12/2012 3:43 AM, Erik Rissanen wrote:

Hi Steven,

On 2012-12-19 06:29, Steven Legg wrote:


Hi Erik,

On 19/12/2012 2:08 AM, Erik Rissanen wrote:

Thanks Steven,
Ok, so the difference which I had not noticed in your proposal is that you advocate that any admin request 
should always start from the top,


You need to read more carefully. That was the fifth time I'd said it.
> while I say that the reduction graph is formed within the nested policy
set like before, except of course that we use labeling rather than categories to differentiate between admin 
and access policies.

By doing nested reduction graphs, there are two benefits as I see it:

- You don't need hybrids.
- Someone can neatly package a complete policy set with nested admin policies in full, without having to be concerned about how this might interfere with any other "top level" policies.
Could you refresh my memory again as to why you want to start reduction from the top? What are the benefits? 

I previously said this on the topic:

    ... "working out where
to put an administrative policy under the current scheme is a burden policy writers can do without. Take the simple case where User A delegates all rights to User D. This is a very simple administrative policy that basically says "if the delegate is User D, then Permit". The issue is where to put it. User A needs to place it (by copy or by reference) in every policy set where User D will potentially be creating access policies. Furthermore, as new policy sets are created that User D might put policies into, User A will have to put the administrative policy in there as well. Conversely, if User A isn't thorough or proactive in placing the administrative policy, then User D will be limited in the rights he or she can actually exercise. Compounding 
    the difficulties is the fact that in the general case, because of
multi-valued attributes and augmentation of the delegate category by a context handler, practially any untrusted policy can be authorized by any administrative policy. It all depends on the request context. It is only by making assumptions about certain attributes being single-valued or by knowing about correlations between the attributes of an access subject that we can begin to predict which untrusted policies will be authorized by which administrative policies in the absence of any specific request context. These are things that users shouldn't have to be concerned with when creating 
    administrative policies."
In the current scheme an administrative policy can only authorize an untrusted policy that is a sibling or a sibling of an ancestor. This is a severe limitation when the scope of an administrative policy is wider than any one of the untrusted 
policies that it authorizes.
It is correct that the admin policy has to be a sibling in the current scheme, and my proposed labeling scheme. I don't see this as a major issue, rather I find it nice, because it means that this allows 
recursive nesting of policies, which will not interfere with each other.
That is still possible with HybridPolicySet and evaluation from the top. The only difference between the original access request and any administrative requests subsequently spawned from it because of untrusted policies is that the administrative requests will have added categories for delegate and delegation-info. If the targets of the HybridPolicySet and its ancestor HybridPolicySets don't reference these categories (and there is no good reason for them to do so), then a PDP evaluating an administrative request will find its way back to the same HybridPolicySet where the untrusted policy was encountered. If the policy set hierarchy is constructed such that policy sets don't interfere with each other with respect to access requests, then they won't interfere with each other with respect to the corresponding 
administrative requests.
Mostly I find that I want to put the administrative policies higher up in the policy 
set hierarchy, because they have a wider scope.
I don't understand your second concern in the text you quoted above. It is true that XACML policies are written with a set of possible request contexts in mind. An admin policy can use any attributes it desires to constrain its applicability in any way, so it is quite clear which requests it will authorize for which delegates. It is true that the support relation between an access policy and an admin policy is resolved at runtime, so the links are not static. A policy may support another or not, depending on the situation/context. However, this is a feature, not a bug, since it allows an access policy author to "consolidate authority", that is he can write an access policy which is in itself greater in scope than any individual admin policy which authorizes him. Depending on the situation, the access policy would be 
supported by different admin policies.


That feature is not something I'm proposing changing. The limitation that
authorizing administrative policies have to be a sibling or descendant of
a sibling is just not in harmony with it. If a writer of an untrusted access policy doesn't know for certain which administrative policy or policies will 
authorize it, how can they be sure they have put it in a place that will
allow it to be authorized ? The writers of administrative policies have the similar problem of not being sure they have put the administrative policy in 
the places it needs to be to authorize the access policies yet to come.
Evaluating administrative requests from the top removes sibling adjacency as 
a concern and the feature operates unfettered.




I also said this:
[ In the current scheme ] "the effect of a nested AdminPolicy depends on where evaluation of the administrative request begins. Suppose that the nested AdminPolicy evaluates to Deny. If the evaluation of the administrative request started inside the AdminPolicySet, then the AdminPolicy has no effect because it doesn't add an arc to the reduction graph. If the evaluation of the administrative request started outside the AdminPolicySet, then the Deny result from the AdminPolicy is combined with the results from its siblings according to the combining algorithm of the AdminPolicySet. If that combining algorithm is deny-overrides, then the Deny result overrides any Permit results from the siblings. That sort of 
    variability will make delegation hard for users to understand."
Evaluating administrative requests from the top means that the result of an administrative policy is always combined with the results of its siblings according to the combining algorithm of the policy set in which it is found 
- with no exceptions.
Yes, that is true, but I would not expect anyone to use delegated admin policies inside an admin policy set. (In fact, we could forbid that explicitly.) Also, I am not sure it's a lot easier to grasp the big picture in case of nested untrusted admin policies even if the evaluation jumps up to the top level for each untrusted admin policy. It's going to melt most people's brain anyway. ;-)
In the current scheme, the reduction of an untrusted policy considers each of the sibling administrative policies by evaluating the administrative request against them. The effect of the reduction graph is to combine the results of 
those evaluations in a way that is similar to, though not exactly, the
permit-overrides combining algorithm. This is the only predictable way to
combine the results of administrative policies, whether I like it or not, and I don't. I particularly want to use deny-overrides. If I put the administrative policies inside an administrative policy set with the combining algorithm I want, then that's not reliable, for the reasons above. The practice might as well be forbidden if it can't be trusted, but either way, policy writers are condemned to only combine the results of administrative policies according to something like permit-overrides. Evaluating administrative policies from the top every time means that policy writers have the flexibility to use whatever combining algorithm they want, knowing that it will always work as specified. 




And I said this:
[ In the current scheme ] "evaluation of an administrative request always starts with the pair-wise consideration of siblings of the policy being reduced. It is a process that is different from the normal processing of a combining algorithm and, in fact, completely disregards the combining algorithm of the policy set that contains the access policy and its sibling administrative policies. Starting the evaluation of the administrative request at the top (just like an access request) would be simpler to manage and easier for users to get their heads 
    around."
When I read this, I am not sure, but do you imply that there is no reduction graph construction at the top 
level.
Correct. The administrative request is evaluated according to section 7.17 of the core, subject to the additional requirement that any access policy or access 
policy set is automatically NotApplicable.

> BTW, what does the reduction graph look like in your proposal?

There isn't one.

Regards,
Steven

> Which policies participate in it? How

is it built?

Best regards,
Erik


Regards,
Steven



Best regards,
Erik


On 2012-12-18 07:17, Steven Legg wrote:


Hi Erik,

On 18/12/2012 2:32 AM, Erik Rissanen wrote:

On 2012-12-14 04:28, Steven Legg wrote:

On 13/12/2012 11:36 PM, Erik Rissanen wrote:
> So why do you allow for the hybrid?
To overlay the administrative and access policy set hierarchies. Some 
folks have expressed a dislike for a complete separation.
I don't see that you need hybrids even if the admin hierarchy is overlaid. Can you give an example? 

Suppose I want to write a bunch of admin policies that all relate to
a resource with a resource-id of "printer", so I factor out the
resource-id test and wrap the admin policies in an admin policy set
like so:

AdminPolicySet {
    PolicySetId: PS1
    Target: resource-id="printer"
    AdminPolicy {
        PolicyId: P1
        ...
    }
    AdminPolicy {
        PolicyId: P2
        ...
    }
}
Along comes the delegate, who creates some access policies that relate to the resource with a resource-id of "printer". If the delegate factors 
out the resource-id test and wraps the access policies in an access
policy set it looks like this:

PolicySet {
    PolicySetId: PS2
    Target: resource-id="printer"
    Policy {
        PolicyId: P3
        ...
    }
    Policy {
        PolicyId: P4
        ...
    }
}
This represents a complete separation of access and administrative policy.
In my proposal I don't allow AdminPolicy in a PolicySet, or Policy in an AdminPolicySet, not because it is harmful, but because it is pointless. 
Recalling that I am proposing evaluating administrative requests from
the top, an AdminPolicy in a PolicySet will never get evaluated because the PolicySet is not applicable to an administrative request. Likewise, 
a Policy in an AdminPolicySet will never get evaluated because the
AdminPolicySet is not applicable to an access request. In order to take advantage of the common targets for the two policy sets above and merge 
them into one I need a policy set that is applicable for both access
requests and administrative requests. That thing is the HybridPolicySet: 

HybridPolicySet {
    PolicySetId: PS3
    Target: resource-id="printer"
    AdminPolicy {
        PolicyId: P1
        ...
    }
    AdminPolicy {
        PolicyId: P2
        ...
    }
    Policy {
        PolicyId: P3
        ...
    }
    Policy {
        PolicyId: P4
        ...
    }
}

Regards,
Steven

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