RE: [xacml] Re: Policy Labelling (was Re: Delegation and on-permit-apply-second)

[Hal Lockhart <hal.lockhart@oracle.com>]

I am sure I don't understand all the subtleties of this debate, but it seems to me that for the organization maintaining a large policy hierarchy it will be easier to have a single tree with both admin and access policies than two trees (or the equivalent). 

The reason is that the policy set hierarchy is used to partition the various scopes of concern within the organizations. It seems to me that the constructs used to partition the space for the purposes of applicable policy will usually be the same as those used to partition the space for scope of administrative responsibility. With two trees, its seems to me that often it will be necessary to make synchronized parallel changes to both trees, which is a good thing to avoid if possible.

For example, suppose parts of the policy tree pertain to different subsidiaries within a corporation. It is likely that administrative responsibilities will also be divided by subsidiary. If we need to change a Target which selects server from some subsidiary, it is likely the same change will be required in the admin tree as well.

Hal

> -----Original Message-----
> From: Erik Rissanen [mailto:erik@axiomatics.com]
> Sent: Thursday, November 29, 2012 7:33 AM
> To: Steven Legg
> Cc: xacml@lists.oasis-open.org
> Subject: [xacml] Re: Policy Labelling (was Re: Delegation and on-
> permit-apply-second)
> 
> Hi Steven,
> 
> On 2012-11-28 06:28, Steven Legg wrote:
> >
> > Hi Erik,
> >
> > On 27/11/2012 8:43 PM, Erik Rissanen wrote:
> >> Hi Steven,
> >>
> >> See inline.
> >>
> >> On 2012-11-05 05:42, Steven Legg wrote:
> >>>
> >>> Hi Erik,
> >>>
> >>> On 1/11/2012 6:40 PM, Erik Rissanen wrote:
> >>>> Hi Steven,
> >>>>
> >>>> I do think there is some practical value in being able to collect
> >>>> any types of policies into a single policy set, but I agree that
> >>>> the value probably is not that big. In come cases where someone
> >>>> would hand over a policy set "here are all my access and admin
> >>>> policies", they would have to hand over two separate sets of
> >>>> policies instead.
> >>>>
> >>>> And, yes, when I think about it more, the evaluation logic change
> >>>> is small. And there is actually one additional benefit. The
> >>>> explicit marking of a policy as an admin policy is probably
> simpler
> >>>> conceptually to users than to use weird attribute category names.
> >>>
> >>> I agree completely.
> >>>
> >>>>
> >>>> So, I would agree with you to these changes.
> >>>
> >>> Reflecting on what you say here and what you were saying during the
> >>> TC meeting makes me think you are assuming more than the minimal
> >>> change required to dispense with category prefixing, in which case
> >>> there are some technical choices to be discussed. I'll start by
> >>> describing the minimal change.
> >>>
> >>> The reduction procedure, as currently described by the
> >>> administration profile, begins when the PDP encounters an
> >>> applicable, untrusted access policy. Let's call it 'A'. A reduction
> >>> graph is formed by considering each ordered pair of policies in the
> policy set that contains 'A'.
> >>> Let's
> >>> call that policy set 'B'. To even get to the point of evaluating
> >>> access policy 'A', policy set 'B' must be an access policy set
> >>> (which is to say, it has a target that can only be satisfied by an
> access request).
> >>> For policy 'A' to be authorized at all there must be at least one
> >>> sibling policy of 'A' that is an administrative policy. Let's call
> >>> it policy 'C'.
> >>> So an access policy set like 'B' must be allowed to have children
> >>> that are either access policies (like 'A') or administrative
> >>> policies (like 'C').
> >>> In the current scheme we distinguish between access policies and
> >>> administrative policies by whether or not they have attribute
> >>> designators referring to prefixed categories. In the new scheme we
> >>> would tell the difference by the element name of the policy, e.g.,
> >>> either <Policy> or <AdministrativePolicy>, and in the process
> >>> exclude the unhelpful possibility of hybrid policies.
> >>>
> >>> It is also possible for a sibling of policy 'B' to be an
> >>> administrative policy set, which is to say, a policy set with a
> >>> target that can only be satisfied by an administrative request. We
> >>> can likewise distinguish between access policy sets and
> >>> administrative policy sets by the element name, e.g., either
> >>> <PolicySet> or <AdministrativePolicySet>. Let's say we have an
> >>> administrative policy set 'D' that is a sibling of policy 'A'. Both
> >>> policy 'C' and policy set 'D' are considered when reducing policy
> >>> 'A'. In which case they are evaluated with respect to an
> >>> administrative request.
> >>> Policy
> >>> set 'D' is only ever considered when processing an administrative
> >>> request, so any access policy or access policy set children of
> >>> policy set 'D'
> >>> will
> >>> never be evaluated. We might as well disallow such children.
> >>>
> >>> In summary, what we have for the minimal change is <PolicySet>
> >>> elements that may have <Policy>, <AdministrativePolicy>,
> <PolicySet>
> >>> and <AdministrativePolicySet> children, and
> >>> <AdministrativePolicySet> elements that may have only
> >>> <AdministrativePolicy> and <AdministrativePolicySet> children.
> >>>
> >>> The fact that an access policy set can have administrative child
> >>> policies and policy sets is a non-issue for most policy set
> >>> combining algorithms.
> >>> In the current scheme the administrative children would always
> >>> evaluate to NotApplicable when evaluating an access request. In the
> >>> new scheme the administrative children would be skipped (or
> >>> automatically
> >>> NotApplicable)
> >>> when evaluating an access request. Either way, the result of the
> >>> combining algorithm is unaffected. The one exception is the
> >>> on-permit-apply-second combining algorithm because it limits a
> >>> policy set to exactly two children.
> >>> If, for an access policy set, those children are both access
> >>> policies, then in practice they have to be trusted policies because
> >>> there is no room for any administrative policies to authorize
> >>> untrusted policies (note this is also the case with the current
> >>> administration profile). If either child of an access policy is an
> >>> administrative policy, then the policy set will never be
> applicable.
> >>> To make the on-permit-apply-second combining rule more useful in an
> >>> administrative setting, the restriction to two children should be
> >>> relaxed to two access children and any number of administrative
> >>> children when the combining rule is used in an access policy set.
> >>> The "first" and "second" children should be taken to mean the first
> >>> and second access children, or we can insist that any
> administrative
> >>> children must come after the two access children. What to do in the
> >>> case of an administrative policy set with the on-permit-apply-
> second
> >>> combining rule is not so obvious. If we limit it to two
> >>> administrative children, then those children would have to be
> >>> trusted in practice. We could say that the first two children are
> >>> considered by the combining algorithm and any subsequent children
> >>> are only used if reduction of untrusted children is required.
> >>>
> >>
> >> I think the above "minimal change" seems good. I don't think it is a
> >> big problem that the on-permit-apply-second is meaningful only in
> >> context of trusted children, since it is intended to solve the issue
> >> of a condition in policies, and for that purpose everything work
> >> fine.
> >>
> >>> That covers the minimal change. The more substantial change is a
> >>> complete separation of access and administrative policies and
> policy
> >>> sets.
> >>> That is,
> >>> a <PolicySet> can only have <Policy> and <PolicySet> children, and
> >>> an <AdministrativePolicySet> can only have <AdministrativePolicy>
> >>> and <AdministrativePolicySet> children. An access request would
> >>> initially be assessed against the collection of access policies and
> >>> access policy sets.
> >>> The administrative policies and policy sets would become involved
> if
> >>> there are any untrusted access policies or policy sets to be
> >>> reduced. A revision of the reduction procedure would be necessary
> >>> and I see two main possibilities.
> >>>
> >>
> >> I don't like the above since this introduces a top level construct
> in
> >> the PDP which is different from any of the regular XACML policy
> >> constructs.
> >
> > As opposed to the current scheme that introduces a mid-level
> construct
> > (the reduction graph built from pair-wise assessment) which is
> > different from the regular XACML policy set evaluation ?
> >
> > > It is a fact that the initial request to the PDP is an access
> > > request,
> >> so the access request must match the "top level policy construct" in
> >> the PDP.
> >
> > It still would.
> >
> > > Also, if the administrative
> >> profile is going to be of any use, this top level policy construct
> >> also has to also be able to contain administrative policies.
> >
> > Only if you assume that everything has to be wrapped up in a single,
> > physical policy set.
> >
> 
> It would, in a broad sense. It does not have to be a policy set
> strictly. Naturally, in your proposal it would be something different.
> It would be nice if we would not need this "different thing".
> 
> > An important aspect of the policy labelling solution is that
> > administrative policies and policy sets are automatically
> > NotApplicable when evaluating access requests, and access policies
> and
> > policy sets are automatically NotApplicable when evaluating
> > administrative requests. Section 7.17 of the core specification has
> > this to say about request evaluation:
> >
> >     "the PDP is defined by a policy-combining algorithm and a set of
> > policies
> >      and/or policy sets. The PDP SHALL return a response context as
> if
> > it had
> >      evaluated a single policy set consisting of this policy-
> combining
> > algorithm
> >      and the set of policies and/or policy sets".
> >
> > We could assume that this virtual, single policy set is exempt from
> > the separation of access policy and administrative policy. The
> > children could be access policies and/or access policy sets and/or
> > administrative policies and/or administrative policy sets. Some of
> > these children will be automatically NotApplicable depending on
> > whether it is an access request or an administrative request that is
> > being evaluated.
> >
> > Alternatively, we could (with respect to option 2) assume that 7.17
> > applies to access request evaluation and write similar text to cover
> > administrative request evaluation. That is, the PDP is defined by a
> > policy-combining algorithm and a set of access policies and/or access
> > policy sets, and a policy-combining algorithm (maybe the same one)
> and
> > a set of administrative policies and/or administrative policy sets.
> >
> > > Thus the top level policy construct is something different from the
> > above policy
> >> constructs which you listed.
> >>
> >> That adds more specification and implementation cost, compared with
> a
> >> PDP which simply starts with a selected top level policy set, which
> >> is semantically no different than any other policy set in XACML.
> >
> > In the context of option (2), and given that an implementation
> chooses
> > to restrict the "set of policies and/or policy sets" in 7.17 to
> > exactly one policy set, is it really that difficult to have a
> selected
> > top level access policy set as the place for the PDP to start for an
> > access request and also a selected top level administrative policy
> set
> > as the place for the PDP to start for an administrative request ?
> 
> No, it's not difficult, but it means that delegation will only work at
> the top level. A sub policyset cannot use delegation within itself in
> that case, which is a restriction for a collaborative policy authoring.
> 
> >
> >>
> >>> The first possibility is that we keep the idea of a reduction
> graph,
> >>> only
> >>> instead of the nodes in the graph being the siblings of the
> >>> untrusted access
> >>> policy being reduced, the nodes are the standalone administrative
> >>> policies
> >>> and policy sets. In effect, every access policy has every
> standalone
> >>> administrative policy or policy set as an implied sibling when
> >>> forming the
> >>> reduction graph. We have the choice when reducing an untrusted
> >>> administrative
> >>> policy whether to consider the actual siblings of that
> >>> administrative policy
> >>> when forming the reduction graph (call this (1.a)), or to form the
> >>> graph
> >>> from the standalone administrative policies and policy sets (call
> >>> this (1.b)),
> >>> just as we would for untrusted access policies.
> >>>
> >>> The second possibility (call this (2)) is to do away with the
> reduction
> >>> graph and simply assess an administrative request against the
> >>> collection
> >>> of administrative policies and policy sets just as we assess an
> access
> >>> request against the collection of access policies and policy sets.
> >>> If we
> >>> find an untrusted policy (in either collection), then we form the
> >>> necessary administrative request and assess it against the
> >>> collection of
> >>> administrative policies and policy sets.
> >>>
> >>> An effect of options (1.b) and (2) is that in the process of
> >>> reducing an
> >>> administrative policy we may encounter that very same
> administrative
> >>> policy.
> >>> If we do, then we treat it as NotApplicable rather than recursively
> >>> reducing
> >>> it. To do otherwise would open the possibility of self-authorizing,
> >>> untrusted
> >>> policies. Note that this effect doesn't arise with the delegation
> graph
> >>> because policies aren't paired with themselves.
> >>>
> >>> Also with options (1.b) and (2) we don't need to do anything about
> the
> >>> on-permit-apply-second combining rule. Administrative policies no
> >>> longer
> >>> need to be siblings of the untrusted policies they authorize, so
> the
> >>> on-permit-apply-second combining rule can still insist on having
> >>> exactly
> >>> two children.
> >>>
> >>> I would very much like us to choose option (2) because 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. Options (1.b) and (2) mean that they don't
> >>> have to
> >>> be. Option (2) is the conceptually simpler of the two, and can
> >>> simulate (1.b)
> >>> if desired.
> >>>
> >>
> >> I don't agree that option 2 is simpler conceptually. I would say
> it's
> >> the opposite since it introduces
> >> another type of policy combining at the top level.
> >
> > Option (1.b) calls for forming a reduction graph from the top level
> > administrative
> > policies and/or policy sets, which is quite different from what 7.17
> > requires
> > for access requests. Option (2) is just like 7.17, except the
> policies or
> > policy sets will be different ones.
> >
> >>
> >> Also, being able to mix in admin policies in nested access policies
> >> is useful for creating hierarchical
> >> structures, where a delegated policy contains nested delegation
> >> itself. This sort of recursion is useful for
> >> being able to import a full policy set of a sub-organization which
> >> internally might want to use delegation.
> >
> > I'm sceptical about the utility of that. Importing policy from a
> > sub-organization
> > isn't necessarily going to be easy. For example, a user of the
> > sub-organization
> > may have "read" access to everything, which is alright in the scope
> of
> > the
> > sub-organization, but may not be appropriate in the scope of the
> parent
> > organization. Access rights will generally need to be reconsidered,
> which
> > will involve rewriting and reorganizing policy for both the parent
> > organization
> > and the sub-organization, and the administrative policies won't be
> > immune to that.
> > Even if restricting the scope of the sub-organization's
> administrative
> > policies
> > to the sub-organization's access policies is useful, importing the
> > policy of
> > a sub-organization is a relatively infrequent event. Option (2) is
> > about making
> > delegation easier to use all the time. Keeping the reduction graph so
> > that
> > importing a policy set from a sub-organization is (maybe) easier is
> > making the
> > wrong trade off.
> >
> 
> I think collaborative policy authoring, whereby anything can be nested,
> is valuable.
> 
> > Also, with option (2) the parent organization would be importing an
> > access policy
> > set and a separate administrative policy set. If there is a simple
> way
> > to distinguish
> > the issuers in the sub-organization from issuers in the parent
> > organization (e.g.,
> > by the domain name in their email addresses), then this could be
> added
> > to the
> > target of the administrative policy set, effectively restricting the
> > scope of the
> > sub-organization's administrative policies to the sub-organization's
> > access
> > policies, even though they are separated.
> >
> 
> But it would put everything at the top level. I would think that
> recursion and nesting would be useful to handle very large policy sets.
> 
> > Regards,
> > Steven
> >
> >>
> >> Best regards,
> >> Erik
> >>
> >>> Regards,
> >>> Steven
> >>>
> >>>>
> >>>> Best regards,
> >>>> Erik
> >>>>
> >>>>
> >>>> On 2012-11-01 06:14, Steven Legg wrote:
> >>>>>
> >>>>> Hi Erik,
> >>>>>
> >>>>> On 31/10/2012 11:35 PM, Erik Rissanen wrote:
> >>>>>> Hi Steven,
> >>>>>>
> >>>>>> Sorry about the slow response to this.
> >>>>>>
> >>>>>> As I see it, there are two benefits to the category prefixing:
> >>>>>>
> >>>>>> 1. A policy can be mixed admin/access.
> >>>>>
> >>>>> How is that a particular benefit when any such mixed policy (or
> >>>>> policy set) can be
> >>>>> rewritten as a pair of a pure access policy and a pure
> >>>>> administrative policy, together
> >>>>> having exactly the same effect ? A policy that authorizes itself
> >>>>> is pointless, and
> >>>>> merging an access policy with an administrative policy that
> >>>>> authorizes unrelated
> >>>>> access policies is weird and confusing, so why do it ?
> >>>>>
> >>>>>>
> >>>>>> 2. We don't need to change the XACML evaluation logic as much.
> To
> >>>>>> implement the delegation model, the PDP
> >>>>>> can use the normal XACML evaluation on admin requests, while
> >>>>>> building the delegation graph.
> >>>>>>
> >>>>>> I think the second is a significant benefit. It would
> >>>>>> significantly complicate both the administrative
> >>>>>> spec
> >>>>>> and the implementation if we need to define additional modes of
> >>>>>> XACML policy evaluation.
> >>>>>
> >>>>> The delegation model is already a significant departure from
> >>>>> normal XACML evaluation,
> >>>>> what with building the delegation graph and forming
> administrative
> >>>>> requests. Those
> >>>>> things would still be there. The complexity trade off is between
> >>>>> category prefixing
> >>>>> (which is not yet complete because it has unresolved issues) and
> a
> >>>>> simple boolean
> >>>>> test during request processing to see if the kind of the policy
> >>>>> matches the kind
> >>>>> of the request (i.e., access versus administrative). That test is
> >>>>> simple and efficient
> >>>>> compared to category prefixing.
> >>>>>
> >>>>>>
> >>>>>> What about changing the prefix so it works with all cases?
> >>>>>
> >>>>> The only thing that comes to mind that would be syntactically
> >>>>> valid is a
> >>>>> prefix to the scheme name. For example, http://example.com/foobar
> >>>>> becomes
> >>>>> delegated-http://example.com/foobar.
> >>>>>
> >>>>> > Or changing the admin category to something else
> >>>>>> than it being based on a prefix?
> >>>>>
> >>>>> That's difficult without making changes to the core syntax.
> >>>>>
> >>>>> > By means of some other kind of algorithm, or maybe an explicit
> >>>>> table?
> >>>>>
> >>>>> Labelling the policies would be much easier than a managing a
> >>>>> mapping table.
> >>>>>
> >>>>> Regards,
> >>>>> Steven
> >>>>>
> >>>>>>
> >>>>>> Best regards,
> >>>>>> Erik
> >>>>>>
> >>>>>>
> >>>>>> On 2012-09-03 07:03, Steven Legg wrote:
> >>>>>>>
> >>>>>>> Hi Erik,
> >>>>>>>
> >>>>>>> On 1/09/2012 1:23 AM, Erik Rissanen wrote:
> >>>>>>>> I don't think we change the delegation model in this way since
> >>>>>>>> it would mean changing the core schema,
> >>>>>>>> which
> >>>>>>>> we really don't want to do at this stage.
> >>>>>>>
> >>>>>>> I originally suggested an XML attribute for the label, which
> >>>>>>> would need
> >>>>>>> a change to the core schema, but there is another way to label
> the
> >>>>>>> policies without changing the core schema and that is
> substitution
> >>>>>>> groups for Policy and PolicySet. For example:
> >>>>>>>
> >>>>>>>     <xs:element name="AdministrativePolicy"
> type="xacml:PolicyType"
> >>>>>>>                 substitutionGroup="xacml:Policy"/>
> >>>>>>>
> >>>>>>>     <xs:element name="AdministrativePolicySet"
> >>>>>>> type="xacml:PolicySetType"
> >>>>>>> substitutionGroup="xacml:PolicySet"/>
> >>>>>>>
> >>>>>>> In a way this is better than a simple XML attribute because it
> >>>>>>> is more
> >>>>>>> visually distinctive. It would be desirable for
> >>>>>>> AdministrativePolicy
> >>>>>>> and AdministrativePolicySet to be in the same namespace as
> Policy
> >>>>>>> and PolicySet, but if they have to be in their own namespace so
> >>>>>>> be it.
> >>>>>>>
> >>>>>>> > Also, I think we did consider labeling policies like this in
> >>>>>>> early
> >>>>>>>> stages of the delegation work. I don't remember right now why
> >>>>>>>> we did not go that way, but I suspect
> >>>>>>>> there
> >>>>>>>> was a reason.
> >>>>>>>
> >>>>>>> I've found and reported on a bunch of problems with category
> >>>>>>> prefixing,
> >>>>>>> so there are reasons to reconsider using labelling. If the
> >>>>>>> purpose of
> >>>>>>> the prefixing is just to ensure that administrative policies
> aren't
> >>>>>>> applicable when evaluating access requests, and vice versa,
> then
> >>>>>>> labelling is a simpler and cleaner way to achieve the same end.
> >>>>>>>
> >>>>>>> > Probably that the current mechanism is more flexible.
> >>>>>>>
> >>>>>>> It's more flexible in that it allows hybrid policies that are a
> bit
> >>>>>>> access policy and a bit administrative policy. I can't think of
> a
> >>>>>>> good reason to want to do that, and in any case, it is always
> >>>>>>> possible
> >>>>>>> to split a hybrid policy into a pure access policy and a pure
> >>>>>>> administrative policy. I would happily forego this dubious
> >>>>>>> flexibility
> >>>>>>> to get rid of category prefixing and its problems.
> >>>>>>>
> >>>>>>> Regards,
> >>>>>>> Steven
> >>>>>>>
> >>>>>>>
> >>>>>>
> >>>>>
> >>>>
> >>>
> >>
> >
> 
> 
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