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

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.
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. 


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. ;-) 


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