Hi Steve,
This is first chance I've had to look at this. I basically agree with
the way you have
described the problem. It sounds like you are saying:
- since both "/a/b/c/d" and "/b/a/c/d" produce the same set of
attributes
in the request context, that these two cases are indistinguishable
I think that is true that the requests are indistinguishable, but maybe
not the
policies.
First, this is no longer a single hierarchy problem: "/a/b/c/d" is one
hierarchy,
where "a" is the root node, and "/b/a/c/d" is another hierarchy, where
"b" is
the root node. I don't think this would be allowed in the DAG, because
/a/b
and /b/a would create a loop which is forbidden, so it would have to be
a forest. However, let's leave that aspect aside for now.
If we look at the policies, btw, Anne did a preliminary document on some
use cases, which may help a bit. See attachment to this email:
http://lists.oasis-open.org/archives/xacml/200406/msg00033.html
Doc is here:
http://lists.oasis-open.org/archives/xacml/200406/pdf00003.pdf
In that doc, check section 4.2 which shows an example policy using
ancestors.
In any event, let's consider how we'd write a Policy that specified
"descendants of 'a'". What we could do is collect the attributes for
"a",
namely, in the case "/a/b/c/d", the attributes are:
- self: "a"
- parent: null
- ancestors: null
- ancestors-or-self: "a"
Similarly for "b"
- self: "b"
- parent: "a"
- ancestors: "a"
- ancestors-or-self" "a","b"
For the node in the request, we have:
- self: "d"
- parent: "c"
- ancestors: "a","b","c"
- ancestors-or-self: "a","b","c","d"
In XACML 2.0 there is a set function in section A.3.11 called:
urn:oasis:names:tc:xacml:1.0:function:type-intersection
where "type" can be any XACML datatype
this function returns a bag that contains the set of nodes common to
both bags
that are input.
So, for Policy descendants-of-a we could compare the ancestors-or-self
bags
of the request node and the policy node, which would produce a bag:
{"a"}.
For Policy descendants-of-b the intersection would produce a bag:
{"a","b"}.
Now there is also a function:
urn:oasis:names:tc:xacml:1.0:function:type-bag-size
this function returns an integer.
Possibly, one could then write a policy-combining algorithm that would
include
this integer in the combining, where the greater integer implies the
"lower" policy,
which would then take precedence?
For the "2nd hierarchy" you'd have 2 similar policies. However, I am
not sure what
kind of precedence could be meaningfully created by whether the node
was higher
in hierarchy 1 than it was in hierarchy 2. So, let's leave that
discussion aside as well
for now until we reach agreement on the single hierarchy.
Thanks,
Rich
Steve Bayliss wrote:
003401cbc8fa$4d5ef8c0$0301010a@asusp4t533"
type="cite">
Message
Hi Rich
Thanks, I think my understanding of how
this can work is nearly there.
There's a point I'd like to clarify to make
sure I haven't misunderstood something.
For clarification, this is using the
ancestor/parent scheme but not using the full path identifiers for the
ancestors/parents (I can see if the full path identifiers are used then
there is no ambiguity):
If my assumption is correct then the given set
of parent/ancestor attributes could also result if the hierarchy was
/b/a/c/d - so if policies were present that specied "all descendents of
b" and "all descendents of a" then it would not be possible to
determine which was the "lower" policy and therefore which takes
precedence?
I'm not sure I follow the logic: in the case
given
descendants of "a" are "c" and "d",
and descendants of "b" are "a", "c", "d".
The scenario
here is as follows:
i) The request
from the PEP is for resource "d"
ii) The context
handler builds the set of parent/ancestor attributes
iii) There are
policies specifying 1) descendents of "a", 2) descendents of "b", 3)
descendents of "c)
iv) The policy
combining algorithm is "lowest policy wins"
v) the policy
engine needs to determine the order of precedence of these policies in
order to combine the policies according to the combining algorithm
So, if the
hierarchy was /a/b/c/d, then in (ii) this would result in the context
handler building the following attributes for the resource "d"
ancestors:
bag{a,b,c}
parent: c
ancestors or
self: bag{a,b,c,d}
If the
hierarchy was in fact /b/a/c/d, then the context handler would build:
ancestors:
bag{a,b,c}
parent: c
ancestors or
self: bag{a,b,c,d}
So, the same
attributes in both cases, so I believe the engine won't be able to
determine the hierarchy of "a" and "b" and therefore the precedence of
policies 1 and 2.
This comes back
to my assumption that the policy engine is working on the set of
resource attributes provided by the context handler (or can request
these resource attributes via an attribute query).
Does this make
sense?
Steve
Hi Steve,
I looked over your comments earlier and think we are converging,
but now I will try to respond to your points inline.
Thanks,
Rich
Steve Bayliss wrote:
006401cbc83a$d3a25280$0301010a@asusp4t533"
type="cite">
Hi
Rich
Many thanks for this. I can see that I maybe need to add
some clarity in terms of identifiers, nodes and hierarchies to avoid us
discussing at cross purposes. So I'll
try and do that first, then further comment inline to your responses.
And for further
clarification I'll leave aside any more comments on multiple
hierarchies and assume a single rooted hierarchy for this example.
The nodes in the example
have identifiers a, b, c, d - these identifiers are unique within the
scope of the system.
The expression
/a/b/c/d
is meant to state that these
resources a, b, c, d are organised in a hierarchy, so that b is the
child of a, c the child of b etc - this "path" is not a node id per se;
but a URI containing this path could be generated as part of the
hierarchical "path" URI scheme, as a "resource-id".
So a resource-id (using this
scheme) of eg file:///a/b/c/d
refers to the node d (and also gives information about its ancestors
and their relationships within the hierarchy).
I think this is the
assumption you've made below.
I agree. So, both "d",
and "file:///a/b/c/d" identify the same node,
since "d", by itself is unique, and the other also
shows where "d" is in the "hiearchy" in which these nodes have been
assigned. For concreteness, if a,b,c,d were
some sort of physical device, then a,b,c,d might be their unique serial
numbers, which identifies each device,
but the devices themselves have no inherent relation to each other. The
hierarchical relation is a property that
is added later. This has been generally covered in earlier emails, so
nothing really new, just indicating how
the identity of a node "may" also include its position in the
hierarchy, but this is not required, as there are other
"identities" that may represent the node that are independent of any
hierarchy.
006401cbc83a$d3a25280$0301010a@asusp4t533"
type="cite">
Further comments inline.
Steve
Hi Steve,
You raise some interesting questions. I am finding that it is necessary
to qualify
my answers significantly, because the problem statements appear, at
least to me,
somewhat ambiguous, and rather than try to answer all possible
interpretations
of the problem, I opt to select one definitive interpretation and maybe
only mention
the others for reference. I think this particular problem, the
hierarchical profile, is
particularly prone to this difficulty, because there is so much
inherent flexibility in
hierarchical interpretation and it can be applied in many different
ways. So, bear
with me as I try to provide useful answers inline.
Thanks,
Rich
Steve Bayliss wrote:
Hi
Rich
This
is most useful, so thank you for your insight. It's prompted me to
read through both the 2.0 and 3.0 HRP specs again in some detail.
In
terms of being more specific, a use-case I am thinking of is a
hierarchy where policies are applied at different levels in the
hierarchy, with the lower-level policies overriding higher-level ones.
That's pretty
clear, sounds like a reasonable problem to be trying to solve: define
policies
on hierarchical members (nodes) that override policies of ancestor
nodes that apply to all
descendant nodes.
So a
made-up example would be a single hierarchy:
/a/b/c/d
Ok, this can
be considered to be the URI " '/' + pathname" part of a specific
node-id.
Or possibly what you are saying is that this is the node-id of a
hierarchy with a single
node, namely the one specified.
I will assume we are talking about a single-root, single-parent
hierarchy, where the
root node is "a" and the "made-up example" refers to a specific node
within this
hierarchy. (Maybe this seems like hair-splitting, but I have found
unless all the
details are explicitly nailed down that the discussion quickly diverges
with each
of the people in the discussion working from a different set of
assumptions, which,
imo, ends up being a waste of everyone's time because effectively no
one really
understands what the others are saying. Don't mean to soapbox, but as
this email
progresses I think you will see a substantial number of potential
divergence points.)
I think we're in agreement here, and do appreciate the point you're
making about the potential distinction.
ok
006401cbc83a$d3a25280$0301010a@asusp4t533"
type="cite">
Policy
1 states "allow read access to Subjects with an attribute value of
'public' to all descendents of 'a'" (so a policy resource-id regex of
/a/.*)
Agreed.
Policy
2 states "deny read access to Subjects with an attribute value of
'public' and an IP address of 'x.x.x.x' to all descendents of 'b' (ie
regex .*/b/.*)
Agreed with
following qualification: I assume that Policy 2 could also be stated
as "/a/b/*" instead of ".*/b/.*". Not sure why you chose the latter.
The reason for
this distinction is that the policy is to apply to descendents of b,
rather than descendents of b when b is a child of a. A use case for
this may be reorganisation of the hierarchy so that b is no longer a
child of a.
I think I get your
point. Starting with the collection a,b,c,d, if we are interested in
the descendants
of "b", then that is not determined until someone puts together the
hierarchy. However, once the
hierarchy has been assigned, i.e. at any given point in time, then the
list of descendants can be
determined. Also, at some later discrete time, the hierarchy could have
been rearranged and the
list of descendants will be different, in general, from the list at the
previous time.
006401cbc83a$d3a25280$0301010a@asusp4t533"
type="cite">
A
request for "d" from a Subject 'public' with IP address "x.x.x.x"
should result in a "deny" based on Policy 2 overriding Policy 1.
I think all that
can be said here is a request for "/a/b/c/d" should result in
- a Permit from Policy 1 and
- a Deny from Policy 2.
The ambiguity
of saying "d" as opposed to "/a/b/c/d" comes from the initial problem
statement,
where I assumed there could be multiple nodes in the hierarchy. If you
also had a node named
"/a/b/d" and then stated a request for "d", I am not sure if want all
nodes with node-name "d"
or if you really want the particular "/a/b/c/d".
Hopefully
the clarification with respect to nodes and identifiers resolves this
ambiguity. The user is requesting resource d, that results in a
resource-id of file://a/b/c/d
using the hierarchical resource profile path URI scheme.
Agree. The fact that
you said "these
identifiers are unique within the scope of the system"
means
there will only be one "d", and, as above, at any point in time it may
show up at different points
in the hierarchy, depending on whether the hierarchy has been
re-arranged.
006401cbc83a$d3a25280$0301010a@asusp4t533"
type="cite">
The
PDP implements a "lowest policy wins" algorithm. Using the path URI
resource-id profile means the PDP has the full information, on a
request for 'd', to determine that (1) both Policy 1 and Policy 2 apply
and (2) Policy 2 takes precedence (the deny overrides the allow) as it
is at a lower level in the context of that particular hierarchy.
This sounds like a
worthy objective, and one could possibly use the existing
policy-combining algorithms to achieve it or maybe not. As you indicate
ahead,
you are leaving that TBD for now. Possibly a new combining algorithm
would
be in order, which is interesting, but seems possibly as a separate
discussion
issue.
(I
am leaving aside the question of what algorithm should be used to
combine policies across multiple hierarchies!)
I think I
agree based on my previous comment, but now you have introduced
a phrase "combine policies across multiple hierarchies". I agree
multiple hierarchies
is an interesting discussion area, but I thought the example was for a
"single hierarchy"
as stated above, which I will continue to assume.
Agreed. This is a separate discussion, let's
focus on a single hierarchy.
Yes, if the rules for a
single hierarchy (single root, single parent) can get nailed down
unambiguously, at least for some problem domain, then more complex
problems can
be systematically introduced and analyzed on that foundation.
006401cbc83a$d3a25280$0301010a@asusp4t533"
type="cite">
The
distinction I was assuming existed between the URI path and the
parent/ancestor attributes was the loss of some information of
structure of the hierarchy in the latter case.
Again, if we are
talking about a single hierarchy, as in the problem statement above, I
am
not sure why you are making this statement., because in the particular
case of a single root,
single parent hierarchy, there is, in fact, no information loss. For
example, in the case given:
- parent of d is c
- ancestors of d are a, b, c
- ancestor or self of d are a, b, c, d
From this
collection of AttributeValues, the policy engine with access to the
node hierarchy
can determine the URI by looking at the parent nodes of c and b, so the
information is not
really "lost" in this case. i.e. one can unambiguously state that a
policy applied to the descendants
of "a" is intended to apply to "d".
I'm still
not clear on this - particularly the phrase "the policy engine with
access to the node hierarchy". I'm making the assumption that the
policy engine has access to the node hierarchy by having been given
these hierarchical attributes (either directly through the context
handler, or through a request for those attributes). Maybe this
assumption is not correct?
Right, in general, this
is a big assumption to make. There are a number of
possibilities, each of which seems to be dependent on situation-specific
characteristics.
- One case is the URI scheme. If a URI is passed in then it
inherently
contains all the parent and ancestor information. But to use the
ancestor
scheme, one has to have a way to parse the URI to produce the list
of ancestors. The Policy could declare an attribute for the list, and if
it is not there, then maybe an attribute finder could be invoked which
would pull the URI from the RequestContext and parse it and return
the parsed components as the value of this "missing attribute". Note:
this only works because all the info is already provided.
- Case 2: might be that some list of ancestors is passed in
plus the resource-id.
The question might arise, how do we know this list is accurate?
Presumably,
it must have come from the hierarchy itself, or some authoritative
representation
of the hierarchy. For example, the PEP could "go to the authority" and
request
the list of ancestors for a specific resource-id.
- Case 3: no URI, no list provided: again we could do a missing
attribute,
however, in this case, the attribute finder would need to go find the
authority
and get the list and return it.
- Case 4: the navigation case you suggested earlier: somehow
the appl is
keeping track of the navigation path, and when the PEP is invoked,
either the node list or the parseable path can be available to the PEP
to do what it chooses.
Bottom line: it is easy
to "talk about" a list of ancestors, but it may not be obvious
how this list is obtained, and depending on what's involved, it may or
may not
influence a possible choice between the URI scheme vs the ancestor
scheme.
006401cbc83a$d3a25280$0301010a@asusp4t533"
type="cite">
If my assumption is correct then the given set
of parent/ancestor attributes could also result if the hierarchy was
/b/a/c/d - so if policies were present that specied "all descendents of
b" and "all descendents of a" then it would not be possible to
determine which was the "lower" policy and therefore which takes
precedence?
I'm not sure I follow
the logic: in the case given
descendants of "a" are "c" and "d",
and descendants of "b" are "a", "c", "d".
So, "a" is lower, since its list is a subset "b"'s list.
So, it seems to me that with a single root single parent hierarchy, no
info is
lost whether you use the URI or the ancestor scheme. I think the issue
only arises when multiple hierarchies are combined in a DAG.
In XACML 2.0, this was the only choice one was presented with, at least
as I read the specs, i.e. you could choose the DAG or the URI scheme.
Upon detailed analysis of the DAG, it became well understood that
information
was lost. Then there was subsequent discussion whether that mattered or
not.
The XACML 3.0 spec attempts to resolve this issue by providing both a
representation where the info is lost for applications where that is
ok, and
a representation (forest) where information is retained (i.e. where a
node
has a list of parents, and each parent is "tagged" as to which
single-rooted,
single-parent hierarchy it belongs to).
The algorithm provides for both including a mix of a forest and
multiple DAGs,
if I remember right.
006401cbc83a$d3a25280$0301010a@asusp4t533"
type="cite">
It is when you get to "hierarchies" with multiple roots, as in the case
of a DAG, that such
information as the "descendants of a" becomes uncertain.
Again, this is a separate discussion area, that diverges from the
original problem statement.
So
focussing now on a single hierarchy example
/a/b/c/d
What I had assumed:
A request for "d" would result in the
following path URI (as resource-id attribute), eg:
Yes, I agree.
Except what do you mean "would result"? Possibly you are referring
to the navigation client that is collecting specific data as it goes,
which has collected
a,b,c on route to d, and can reconstruct the URI based on this.
I'm assuming here that the context handler, with
access to the full hierarchy information, would construct a
hierarchical URI resource-id file:///a/b/c/d, when the incoming request for d. So that's the case using
the hierarchical resource-id URI, rather than using parent/ancestor
attributes.
I agree, the context
handler is the place where a lot of these questions can be
answered in a straight-forward manner. The tricky part, imo, is that
the CH itself,
is an ambiguity wrt the spec, and since a lot of the assumptions one
needs to make
about the hierarchical profile depend on the characteristics of the
specific CH.
This does not mean, policies cannot be written, but w/o knowing
something about
the CH, it is difficult to say whether one specific strategy is
superior to the other.
006401cbc83a$d3a25280$0301010a@asusp4t533"
type="cite">
And the following
ancestor/parent attribute values
parent-id=bag{c}
ancestor-id=bag{a,b,c}
Agree.
Thus if using the
ancestor/parent attribute method, the PDP doesn't have the information
to determine which of the two policies takes precedence (both a and b
are ancestors, but it's not expressed that a is an ancestor of b).
(That's not to say it couldn't go elsewhere to gain information on the
hierarchical structure of the resources and therefore the policy
precedence - perhaps that should be the approach rather than
determining the hierarchy from the request context resource-id
attributes?)
Right, based
on the attributes alone, without access to the hierarchy, one can
probably
not determine whether Policy 1 or Policy 2 takes precedence.
Agreed, this is what I meant by loss of
information about the full hierarchy when using parent/ancestor
attributes as opposed to a resource-id URI with a path specifying the
hierarcy. Again the key phrase is "without access to the hierarchy",
which relates to my earlier assumption.
However, one could probably say the following:
- Policy 1 applies because using some algorithm, one can
presumably determine that
the bag{a,b,c} "matches" the path "/a/.*".
- If Policy 2 were stated as "/a/b/.*" then that would also
"match" bag{a,b,c}, then
maybe one could infer that Policy 2 took precedence because matching a
and b
is more nodes than just matching a, so that might imply "deeper in
hierarchy".
Agreed.
(Personally, imo, this begs the question "Why would anyone
want to do things this way?",
but that is also a divergence and a digression, which could be
discussed separately.
Note: I recognize that there are social networking and semantic web use
cases where
this type of processing might be useful, but I am not sure whether
applying policy
for access control is a workable use case, but again, that's another
discussion.)
What I think you are suggesting is that the
parent-id and ancestor-id values are values that themselves express
their own position in the hierarchy, ie
parent-id=bag{/a/b/c}
ancestor-id=bag{/a,/a/b,/a/b/c}
Yes, this is
my preferred way of looking at this problem. If you have known
hierarchical paths, then I see no reason not to use those as
resource-id's which
enables one to use the URI mechanisms. However, there may be cases where
the URI presents too much overhead of processing, and maybe ancestors
perform
better, but "who knows", to me that is a performance optimization based
on a
specific hierarchy with known characteristics.
I can see what you are saying here - that if
there are known hierarchical paths then these values can be used as
parent/ancestor values. As you state it is a "who knows", but it
"feels" to me that this is doing the job of using the URI mechanism.
Although the advantage I see in using parent/ancestor attributes in
this way is that it avoids needing to match based on regex.
Specifically if one was using a PolicyFinderModule (in the Sun XACML
implementation) then indexing policies (for discovery by the
PolicyFinderModule) based on these parent/ancestor values in the
policies would be easier than indexing based on some decomposition of
regex values of resource-ids (unless one explicitly limited the
allowable range of regex to simple wildcard matches).
I think in some sense
this is getting to the heart of the matter. imo, if there is a fixed
hierarchy, typically where governance is done over subtrees, then the
regex approach
is probably desirable. For example in a file system where all the files
for one department
are within a specific subtree, then one could simply specify the path
to that subtree and
feel confident that the controls being applied were what's needed. In
this case, I think
representing this path as a collection of ancestors pretty much
obscures what the policy
is doing, so while it might work technically, it would seem to have
little practical value.
In the navigation case, where the list of ancestors is a function of
how you got to
the node, i.e. you are effectively defining your own hierarchy on the
fly, then there
is no way to know in advance what paths need to be protected, since
there can be
any number of paths to a specific node. I think this implicitly places
you in the
multiple hierarchy problem space, and if you don't care what particular
path, but
do care about the ancestors, then a DAG might be the right model, in
which case
one would probably look to the ancestor method as being strategically
advantageous.
The intent of the XACML 3.0 profile is to allow designers to freely
choose the
scheme to apply to different aspects of their problem domain. I suspect
the notion
of "resource-type" would play a significant role in the sense that a
specific resource-type
might be best represented by a URI, or a DAG, or a forest (which is a
superset of the
DAG and URI schemes). It is also conceivable, I expect, that even a
single resource-type
might have aspects of its use in an organization that would suggest
that it have independent
representations including any or all of the 3 schemes.
Hopefully this discussion can help development of a structural
framework for analyzing
specific problem domains, and that it can provide structure for
partitioning a complex
problem into a more manageable small set of relatively simpler problems.
Rich
006401cbc83a$d3a25280$0301010a@asusp4t533"
type="cite">
So in this case the structure is not lost
(and I see the equivalence to the path URI method).
Agree.
I need to go through the pseudo-code
algorithm in some more detail, particularly to understand what the
"output" would look like in terms of part of a XACML request context
(ie the resulting XML).
There are parts of the 3.0 spec that do
seem to suggest that the parent/ancestor method could convey less
information than the path URI approach, particularly
2.3: In this
approach, considerable information is discarded. It is not possible to
determine how many hierarchies there are or which ancestors are in
which hierarchies or the relative position of ancestors other than
immediate parents.
Right, this
statement was included to alert users of the nature of the "ancestor"
or DAG method,
which is described in more detail in sections 1.1 and 1.1.1 of the 3.0
profile.
The specific aspect that needs to be preserved is the specific parent
and specific hierarchy
if one is to retain "chain of control" information.
For
example if the CEO of a company applied a policy to all employees,
it is possible that such a policy could be overridden by someone in the
mailroom
who was captain of the company bowling team of which the CEO was a
member.
This has to do with the "tranistive" property of hierarchical
relations, discussion of
which can be found in semantic web references. With single root single
parent
hierarchies this problem can be avoided, and as explained at the end of
section 1.1.1,
retaining the hierarchical identity produces a superset of the DAG,
which also does
not have the cycling problems of DAGs.
None of this was apparent, in the XACML 2.0 version of the hierarchical
profile,
which is the reason for much of the added detail of the 3.0 version,
which still
preserves all the functional capabilities of 2.0, but just
distinguishes the cases.
4.1 The representations of the identities of these parents,
ancestors, or self will not necessarily indicate the path from the root
of the hierarchy to the respective parent, ancestor, or self unless the
representation recommended in Section 3.2: Nodes in a resource that is
not an XML document is used. [ie the path URI method]
Right, this is
included as well to alert users of the choices inherent
in the approach that is used.
From 4.1 it
would seem that the values used for the ancestor/parent attribute
values could in fact be resource-id values that use the path URI
method. So no information is lost.
Yes, if you use
URIs as the node-names you are effectively retain the information,
but in a redundant manner.
However this
seems to be at odds with the assertion in 2.3 (although noting that the
attribute values "... may be identified using identifiers of any XACML
datatype ..."; so there is nothing to preclude usage of path URIs for
these... in which case no information is lost.
This is a special
case of the ancestor method, which allows you to capitalize
on the URI properties from the node names. But it is not a "general
solution"
because it is dependent on a particular node-naming strategy.
Furthermore,
in order to use the retained info, one must "crack" the node-names
which is
outside the specified scope of the ancestor method.
So overall I
am left a little confused as to the intent of the specification in this
respect. Using path URIs for ancestor and parent attribute values
seems like a merging of the two approaches.
The point of the
comment in 4.1 is not to recommend using URIs as node-names
in the ancestor method, it is to point out that if one wanted to use
the ancestor
method, and also if one became "concerned" about the specific paths to
the nodes,
then this would be a "way out" of the problem. However, once one
realized that
the path information was of significant value to a particular use case,
then one would
probably want to consider re-evaluating the original strategy of using
the ancestor
method.
Again, this is an area the could become a whole discussion topic all by
itself.
Though I
guess the datatypes of the ancestor and parent attributes don't have to
be path URIs in terms of the spec, but could be some other datatype
that expresses the hierarchical path.
Agree. There is a
lot of flexibility inherent in the Hierarchical Profile because it
covers
anything that has mathematical hierarchical properties. The original
spec focussed on
the more general multi-root, multi-parent case, which proved to be
problematic
for chain of control use cases, in particular, because the "chain of
control" was tossed
out in the ancestor method, and one was left with only a fairly brittle
URI method.
The purpose of this spec is to unify all these use cases under one
broad umbrella,
where the specifics of each subcase can be identified by specific
properties of
the hierarchy or hierarchies in the application arena that need to be
considered.
The algorithm in 3.3.1 is intended to formalize this unification and
show how all
these methods can logically and peacefully coexist in a fairly
straight-forward
framework for representing the problem space.
Hi Steve,
Generally, I think your question needs to get more specific, because
of the inherent flexibility of the URI/hierarchical mechanism, imo, one
needs to understand what one is trying to achieve in order to
meaningfully
define an optimum strategy. I will take an initiial shot w responses in
line.
Thanks,
Rich
Steve Bayliss wrote:
Hi Rich
Many thanks for this, this greatly clarifies my
understanding.
To clarify one thing, "Scenario 1" may not be
two user applications - it could be a single user application which
presents a number of different "browsing paths" to resources. But I
don't think that changes anything, so long as the application's PEP
constructs the browsing path the user navigated through.
Agree. The
important thing is the path itself, not which module navigated the path.
One further quesion (at this stage!):
Would it be possible to
1) in the Request Context, use the hierarchical
URI profile; and at the same time
2) in policies use the parent/ancestor attribute
profile?
Personally, I
do not see a significant distinction between the "URI profile" and the
"parent/ancestor attribute profile", which I see as effectively being
functionally
equivalent, with the distinction that the URI is a special case of the
parent/ancestor attribute,
where the special case is that the URI has a defined syntax.
So, depending on how one approaches the problem, or specifically how
one goes about
"naming" the resources, steps 1 and 2 as you describe may not be
significantly distinct.
For example, if you start with a collection of resources, and each has
a unique identifier,
such as a serial number, or a social security number, there is only a
"1-level" hierarchy,
the collection, itself, is the parent and each individual resource is a
child of that parent,
with no inherent relation to any of the other members of the collection.
The next step is maybe you want to "structure" the membership of the
collection in
some manner so that the members have relations to each other. At this
point it is
necessary to define the "relation" and how the "relation" is
established between
the members. Once this relation is defined, then it makes sense to
start considering
how policies are applied to the collection.
Let's assume you organize them in a single rooted hierarchy such that
each member
has a single parent and all members have some parent. Now you have the
equivalent
of a file directory, where each file has a name and it is also a member
of a directory
in a hierarchy of directories.
Given this scenario, then what is your question 1 and 2? I would assume
I would
pass the dir-path/filename as the resource-id. The list of ancestors is
nothing more
than a succession of truncated dir-paths. So is the question whether to
pass
"/a/b/filename" as resource-id, vs "/filename" as resource-id plus
"/a/b" and "/a"
as ancestor attributes? Since the latter can be derived from the
former, what is
the difference?
Is the parsing of "/a/b/filename" vs "/a/b/.*" more expensive than
doing bag operations
of ancestors? I honestly have no idea. It may depend on the
implementation of
the engine. It may depend on how large the collections are that are
being processed.
I would look to the engine supplier to provide guidance on how to use
these capabilities,
and possibly suggest situations where one or the other would be optimum.
Personally, I think this is a level of detail that policy designers
should not be concerned
about unless there is some macro characteristic that they can relate to
which will guide
the choice.
I'm thinking here that from a perspective of
indexing/finding policies, this would be much easier to do based on
parent/ancestor attributes rather than for instance decomposing a
resource-id regular expresssion (which may specify a range of
possibilities, such as a path /a/b/.*, specifying "all resources under
b when b is under a", and ".*/b/.*" specifying "all resources under b
irrespective of b's parentage").
The comments
above should address, but not necessarily fully answer the "much
easier" question you are thinking about.
However it may be useful to still include the
resource-id in the request as this specifies the complete ancestor
lineage, which could be useful for instance in a custom policy
combining algorithm which had some rules about policy precedence based
on the position in the hierarchy - for example child policies have a
precedence over parent policies.
Again,
depending on what you are trying to accomplish, the importance of which
hierarchy a node's parent belongs
to may or may not be important. As the algorithm in XACML 3.0 shows,
one actually has to go out of one's way
to effectively remove the specific hierarchy identifier of each parent,
at least, if you represent the original hierarchy
by a single column in the n+1 columns of n hierarchies and 1
resource-id as in the algorithm. And as indicated in
the algorithm, if you start with a DAG, then you must have m columns,
where m is the max number of parents any
single member of the DAG has, and if a new parent is added to an
m-parent node, then just add a new column
to the table and increase the DAG width to m+1.
Regards
Steve
Hi Steve,
I find your explanation much easier to understand now. It sounds like:
- there is one underlying "physical" resource, "c",
that can be obtained either
- by application "A" by navigating via /a/b/c
- or by application "X" by navigating via
/x/y/c
- basically "A" presents user with /a/b/c,
and "X" presents user with /x/y/c
where /a/b and /x/y are "steps" the user takes thru the appls
I think the
following Policy elements placed in a Target would make the Policy
applicable to any resource prefixed with "file:///a/b"
<ResourceMatch
MatchId="urn:oasis:names:tc:xacml:1.0:function:regexp-string-match">
<AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string"
>file:///a/b/.*</AttributeValue>
<ResourceAttributeDesignator
AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id"
DataType="http://www.w3.org/2001/XMLSchema#string" />
</ResourceMatch>
A similar
match statement could be prepared with "file:///x/y/.*" to
match any request
prefixed with "file:///x/y".
These match statements can be combined within a Resource statement to
create an "AND",
or combined within 2 separate Resource statements to create an "OR".
Also, I have added a prefix "file://" in order to make
these proper URIs, but will not
use it below. The point is while the example does not use full URI
syntax, there is no
reason why it can't, and we will just assume it is shortened for
convenience.
I think the rest of my comments can now go in line.
Thanks,
Rich
Steve Bayliss wrote:
Hi
Rich
Many
thanks for your response. I realise my questions are not clear, I was
struggling myself with how to phrase them, and my terminology is rather
loose in places. Perhaps the best approach is for me to step back and
outline two distinct scenarios; particularly to address what I was
trying to express with "resources-in-path-context".
In
both scenarios there is a resource c that is present in two hierarchies:
/a/b/c
/x/y/c
And
in both scenarios, there is a user application that allows a user to
browse through the hierarchies to locate the resource. The user could
browse by starting at the top of either hierarchy (a or x) to locate
the resource c.
Scenario
1:
The
user browses starting at "x", and locates the resource under the
hierarchy /x/y/c
Is
it possible to write a policy that applies based on which hierarchy the
user navigated to locate the resource, ie a policy that applies to
their browsing path of /x/y/c, so that the policy would apply if they
had browsed through x->y->c but would not apply if they had
browed to the resource through a->b->c?
The first
(and only) ResourceMatch element above I think is what such a Policy
would need
to contain to match the "/a/b/c" path.
(This
is what I was attempting to express with "resource-path-in-context",
the context being the route a user took to locate the resource.)
That is
much more clear now, thanks.
The
application's PEP would be able to supply the browsing path in the
request context.
Agree, the
general assumption is that the PEP is able to collect information about
the
request and transform that information to an appropriate set of
attributes.
(If additional attributes are required they can be obtained via the
ContextHandler or
by using the MissingAttributesDetail technique.)
Is
this something that the Hierarchical Resource Profile is suited to?
I believe
so. It appears to me to be a perfect fit.
Or
should this "navigation path" should be a separate attribute outside of
those specified in the Hierarchical Resource Profile?
I don't
think it is necessary for an additional "navigation path" attribute. I
think the navigation
path makes for a perfectly good resource-id.
I
can see how a policy could be written using a "navigation path"
attribute, my question is whether the scenario is relevant to/better
implemented with the Hierarchical Resource Profile, using the
resource-id attribute to represent the navigational path used by the
user.
The only
distinction I can think of right now is that a "navigation path" in the
sense of tracing everything the user
has been doing may have redundant info. However if the "navigation
path" removes backsteps, then I think it
is effectively equivalent to the URI path, which is a legitimate
resource-id value.
Am I missing something here?
Scenario
2:
The
user browses to the resource c. It is irrelevant from a policy
perspective of how they navigated to the resource.
Presumably,
within this particular example there are only the two paths available.
In any event, even if there
were more paths, the profile should work just as well.
Two
separate policies are to be written, one for all resources that are
descendents of "a", and another for all resources that are descendents
of "x". Both policies should be applied when the user requests
resource "c".
This sounds
like it could be done with an "AND" of the two ResourceMatch elements
(one for "/a.*",
and the other for "/x.*") within a single Resource element.
It
seems that this is suited to the Hierarchical Resource Profile.
Agree, but
also think scenario 1 is just as applicable.
It
would be the responsibility of the Context Handler to provide two
hierarchical resource-id attributes.
The
algorithm in HRP XACML 3.0 section 3.3.1 shows how resource-ids can be
constructed given a list
of unique resource identifiers (can be a simple integer starting from
zero and incremented for each resource
that is added to the collection). This "list" can be regarded as
"column 0" of the "resource map".
For each hierarchy applied to the collection an additional column is
created for the map. Within the column,
the resource identifier of the parent of the current resource is
assigned to create the hierarchy.
The context handler can use this map to create all the resource-ids
needed for the resource.
Policies
could be then written using (URI regex match) /a/.* and /b/.* (or
indeed separate rules as you suggest). Alternatively the "ancestor
scheme" could be used as you suggest, in which case the Context Handler
would provide the resource-parent, resource-ancestor,
resource-ancestor-or-self attributes (and the policy would be written
in terms of these rather than a resource-id regex match.
I agree the
"ancestor scheme" can be used here. Personally, I think the URI scheme
is sufficient for
the use cases I have considered to date. I think it is just a matter of
how the information is viewed
as to which approach is preferable.
With the DAGs, I think the URI would not work very well, if at all
since there are no definitive
unambiguous URIs that can be constructed. So, if resources are viewed
as DAG, then should
use ancestor approach. Same algorithm in 3.3.1 is applicable.
Further
comments:
To help
my understanding of the difference between the URI scheme and the
"ancestor scheme":
- If the URI scheme is used, the resource-id
attribute is a URI specifying the hierarchical path to the resource.
Multiple resource-id attributes would be present if the resource is to
be found under more than one path.
Yes, that
is correct, and it is allowed as specified in XACML 2.0 section 6.3
lines 2947-2954,
and XACML 3.0 generally Attributes can contain multiple
AttributeValues. It probably would
be more correct to say something like "the Attribute with AttributeId =
"...resource-id" generally
can contain multiple AttributeValue elements with different values".
- If the "ancestor scheme" is used, the
resource-id does not have to be a URI, and does not have to contain any
information about which hierarchies the resource belongs to.
Agree.
- Instead resource-parent, resource-ancestor
and resource-ancestor-or-self attributes are used to represent this
information
Yes,
although, this probably is only relevant to DAGs, and is superfluous
for any number of single parent hierarchies.
The
"relaxation" I thought I had identified for the resource-id in the URI
scheme was due to the 2.0 document stating (section 2.2) that "The
identity of a node in a hierarchcial resource ... SHALL be represented
as a URI ...", whereas the 3.0 document (again section 2.2) has "The
identity of a node in a hierarchical resource ... MAY be represented as
a URI...". However on re-reading the subsequent text in section 2.2 of
the 3.0 spec, it now seems clear to me that the resource-id must indeed
be a URI.
I
hope this goes someway to clarify the questions I was attempting to
pose in my original post.
Yes, I
think we are in agreement, assuming my comments are consistent with
what your
statements are intended to mean.
Rich
Thanks
Steve
Hi Steve,
Not sure if I fully understand your questions, but will try to respond
inline. Note:
my first few responses are attempting to understand your question, the
later
responses are probably closer to being in the answer space of your
questions.
I do not expect that this will fully answer your questions but
hopefully it will
move the ball down the field a bit.
Thanks,
Rich
Steve Bayliss wrote:
Hi
I'm
seeking to clarify my understanding / the intent of the Hierarchical
Resource Profile. Primarily XACML 2.0, but I understand that 3.0
mainly refines and adds clarity, so anything from 3.0 that's relevant
in this respect I am interested in.
I'm
iinterested in the hierarchical path representations of non-XML nodes,
ie 2.0 spec lines 190 et seq.
I am
having trouble parsing the following paragraph. I will deal with each
segment in sequence:
My
question is if the Hierarchical Resource Profile deals (is intented to
deal with) with resources-in-path-context.
The
sentence above appears to be asking the question:
Does the HRP deal with "resources-in-path-context"?
If that is the question, then what does the term
"resources-in-path-context" mean? For example,
XACML Policy deals with information in the RequestContext. Is this what
you are referring to?
That
is, if a resource belongs to multiple hierarchies/has multiple
parents/ancestors, can a policy be written for this
resource-in-path-context.
The
answer to the first part of the question is "Yes, a policy can be
written for it.".
As above the term "resource-in-path-context" is undefined, at least to
me, so I do not understand the last part of the question.
(to
put it another way, policies dealing with resources when being accessed
as part of a particular collection having a specific policy, rather
than a resource being accessed, and because it is a member of - rather
than being accessed in the context of being a member of - having a
specific policy).
I do
not understand the above statement at all. The first phrase appears to
use the term policy redundantly:
"policies
(dealing with resources when being accessed as part of a particular
collection) having a specific policy"
The
remainder I find in more difficult to parse. My point here is not to
criticize your sentence structure, but to
try to understand what the point is you are trying to make, and
possibly by explaining why I find it confusing,
will help to establish a terminology where the issues can be addressed.
That being said, the last phrase appears
to me to be saying:
There is some kind of distinction between:
- a resource being accessed because it is a
member of (a specific collection?)
- a resource being accessed in the context of
being a member of (a specific collection?)
Is
this the point, that the above two bullets are distinct in some manner?
Example
paths for a resource "c" (ignoring the fact these aren't URIs, I gather
this restriction was relaxed in 3.0)
/a/b/c
/x/y/c
Ok,
assuming the above URI pathname portions plus leading "/" are provided
as resource-ids (where multiple resource-ids
for the same resource are allowed as specified in section 6.3), the two
provided above seem like a reasonable pair of example paths.
Note: I do not believe there was intentional "relaxation" in 3.0. Note
there are 2 non-XML node identification schemes in the HRP:
- The URI scheme: section 2.2 (of both XACML
2.0 and 3.0)
- The "ancestor scheme": section 3.2 of XACML
2.0, and section 2.3 of XACML 3.0.
If the
URI scheme is used the resource-id must be a URI. If the ancestor
scheme is used, then that is not a necessary condition.
According
to the spec, a request for resource c should result in two separate
resources in the request context, a resource-id with value /a/b/c and a
resource-id with value /x/y/z - is that correct?
There
is only one resource, but it will have 2 resource-id's, so the answer
to above is "no" to the first part and "yes" to the 2nd part.
And
two different policies, one specifying (as URI regex match) /a/b/.* and
the other /x/y/.* would both match?
There
is no requirement for two Policies, as opposed to say two Rules, but
yes, it is likely that both the regex's you
propose would appear.
Is
it also permissible for the incoming request to the PEP to specify the
full path, for instance to state that resource c is being accessed in
the context of collection b which is a member of collection a? Thus
only the first policy would match?
It is
ok to specify only one path in the request, but it depends on the
Policy structure whether that would
be accepted or not. i.e. does the Policy state:
- ( /a/b/.* AND /x/y/.* ), or
- ( /a/b/.* OR /x/y/.* )
That
is, because a full resource-id is already present then the engine
shouldn't go away and build the paths.
Sorry,
I am not sure what you are trying to say here. There is no requirement
on the engine to use any
specific method for processing the above scenario.
And
is it conceivable that both forms could exist together?
I
assume so, however, it sounds like you may have a concern that for some
reason both forms should not
co-exist. Is that a concern?
For
instance, if there was a resource attribute of object-ID, and it is
defined that resource-id is formed of the full path to object-ID then
we could have
Request
one:
specifies
resource object-id = "c"
result:
engine builds all applicable paths to "c" - ie two separate resources
with their own resource-id (so both above policies match)
I'm
afraid you are going to have to be more specific here about what
information is being processed.
My assumption is that all required info is in the request context. If
all that is there is "c" then how does one
derive full paths?
Request
two:
specifies
resource-in-path-context, specifying resource-id directly as "/a/b/c"
result:
engine does not build resource-id and only the first policy matches
Correct,
but see comment above. It depends if Policy requires resource to be
member
of both hierarchies or only one hierarchy.
Any
guidance on this appreciated. My reading so far of the specs suggests
that the Hierarchical Resource Profile isn't intended for
resource-in-path-context (or resource-accessed-as-member-of-collection).
Again,
I am not sure what the term "resource-in-path-context"
precisely means or the alternate phrase:
"resource-accessed-as-member-of-collection",
at least in the context of the HRP. The HRP is intended
to represent single nodes in a hierarchy, and to also consider using
the Multiple Resource Profile
for defined "scopes" of nodes within a hierarchy.
Thanks
Steve
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