Administrative Policies in XACML

Erik Rissanen

mirty@sics.seSwedish Institute of Computer Science

Delegation

• Delegation is a mechanism for administration

• With delegation we mean the granting of rights for the purpose of administering access control.

• We do not mean proxy delegation between tiers in an application stack.

The need of administrative rights

• XACML has no way to specify who may change policy– Application writers need to reinvent the wheel

• Models for administrative rights are needed in large systems– Many administrators are needed

• Centralized administration can be a bottle neck• Centralized administration can be a security risk

– Missunderstandings or too much power

Our approach

• We have made a simple implementation of administrative rights in XACML.

• We did not modify the core of XACML– Easier implementation and less intrusive– But, some things are not as elegant as they could be.

• We participate in the ”real” work in the XACML TC

• Note that the examples here are simplified!– Important technical details are left out

Administrative rights in XACML

• Two sources of policies– Trusted root policies– Policies with an ”external” issuer

• Two kinds of policies– Access rights– Administrative rights

Regular XACML Right

<Policy>

<Target>

<Subject>Alice<Subject>

<Resource>Printing</Resource>

<Action>Access</Action>

</Target>

<Rule Effect=”Permit”>

<Target><AnyTarget/></Target>

</Rule>

</Policy>

Request and response<Request> <Subject>Alice</Subject> <Resource>Printing</Resource> <Action>Access</Action></Request>

<Response> <Decision>Permit</Decision></Response>

Delegated access right

• Looks like a regular XACML access right– Except it contains a condition that matches

only access requests (in contrast to administrative requests)

• Is in some way associated with an issuer– A digital signature for instance

• Is rewritten to contain an obligation to verify the right to issue the policy

Delegated access right<Policy> <Target> <Subject>Alice<Subject> <Resource>Printing</Resource> <Action>Access</Action> </Target> <Rule Effect=”Permit”> <Target><AnyTarget/></Target> <Condition FunctionId="delegationMatch"> <AttributeValue DataType="delegationConstraint"/> <EnvironmentAttributeDesignator AttributeId="delegationChain"/> </Condition> </Rule> <Obligation ObligationId="authorize-issuer” FulfillOn="Permit"> <AttributeAssignment AttributeId="subject-id” >bob</AttributeAssignment> </Obligation></Policy>

Bob is the issuer

Empty delegation constraint indicates

access permission

Request and response<Request> <Subject>Alice</Subject> <Resource>Printing</Resource> <Action>Access</Action> <Environment> <Attribute AttributeId="delegationChain"> <AttributeValue/> </Attribute> </Environment></Request>

<Response> <Decision>Permit</Decision> <Obligation ObligationId="authorize-issuer"> <AttributeAssignment AttributeId="subject-id“ >Bob</AttributeAssignment> </Obligation></Response>

Empty delegation chain indicates

access request

”The decision is not valid unless

the authority of Bob is verified.”

Simple Administrative Right

• The right to create access rights

• Two parts– Access permission

• The scope of the access rights that are allowed in the end

– Administrative subject• This is the person/people who may create policies• In the form of a condition that will match an

administrative access request

Example Administrative Right<Policy>

<Target>

<Subject>Employee<Subject>

<Resource>Printing</Resource>

<Action>Access</Action>

</Target>

<Rule Effect=”Permit”>

<Target><AnyTarget/></Target>

<Condition FunctionId="delegationMatch">

<AttributeValue DataType="delegationConstraint">

<Step>Bob</Step>

</AttributeValue>

<EnvironmentAttributeDesignator AttributeId="delegationChain"/>

</Condition>

</Rule>

</Policy>

Non-empty delegation constraint

indicates administrative right.

(For Bob.)

Administrative Request<Request> <Subject>Alice</Subject> <Resource>Printing</Resource> <Action>Access</Action> <Environment> <Attribute AttributeId="delegationChain"> <AttributeValue> <Step>Bob<Step> <AttributeValue> </Attribute> </Environment></Request>

<Response> <Decision>Permit</Decision></Response>

Non-empty delegation chain

indicates an administrative request.

”Verify the authority of Bob to grant

this access.”

General Administrative Right

• The right to create policies• Two parts

– Access permission• The scope of the access permissions that are

allowed in the end

– Delegation constraint• Specifies all permitted chains of delegation in the

form of a pattern that is compared with the chain of delegation in an administrative request

• Depending on the condition, it may allow the creation of other administrative rights

General Administrative Rights

Root: S1 S2 S3 S4 AP1

Issuer1: S5 S6 S7 AP2

Issuer2: S8 S9 AP3

Issuer3: S10 AP4

Issuer4: AP5

Access request: AR

Policy processing

• The PDP owner can insert any policy into the PDP– These root policies specify which other policies may

be created and by whom• External policies are rewritten to contain the

verify issuer obligation• The rewritten policies are collected into the PDP

– We use a single policy set in our application.• The PDP owner needs to define the policy

combining algorithm– We use permit-overrides.

Request processing

• Starts with an almost regular XACML access request:– Except an extra environment condition with an empty delegation

chain

• Is evaluated against the policies– Special combining algorithm collects all the obligations of

delegated access permissions

• The context handler has to repeat the request for each obligation until a permit is received– The issuer is added to the environment delegation chain. This

indicates an administrative request.– For repeated requests the chain grows longer with each new

issuer.

Optimization of evaluation

• The current approach requires repeated access requests to verify administrative authority

• If rights would be directly comparable, the verification could be done ahead of the access.– No need for repeated requests.– But also not possible to combine rights from multiple

sources into a single policy.– Also would probably limit the form of policies

significantly.

Combining algorithms

• Any combining algorithm can be used within a delegated policy

• When policies from multiple sources are collected, the ”PDP owner” must specify which policy combining algorithm to use

Policy Sets

• Delegated policies could be Policy sets– Rewritten in the same way– However, Policies are enough for us

• I haven’t thought about policy references yet

Deny

• We do not use deny effects in our application

• Deny should work

• However,– Some policy combining algorithms may not

make sense in a delegated setting.• For instance: The order of policies that are

externally issued and collected into a policy set may be unpredictable.

Access control data in our application

• Policy assertions

• Attribute assertions

• Attribute definition assertions

• Revocations

Distribution of data

• Data is distributed in a P2P-network, DKS• A PDP subscribes to all access control

data it needs.• The PDP starts from a resource id

– Fetches the resource attributes– Fetches the policies on those attributes– Fetches the issuer attributes

• End user attributes are fetched from the network or uploaded by the user

Architecture

Network

DKS-node

DKS-node

PDPResource/

PEP

Policy

Attribute assertion

Revocation

Access control data

PAP

Main open issues

• Difficult for people to understand dependencies?– Descriptive text in policies?– Limit policies to a comparable form?– Perhaps application semantics are enough?

• Replacement of administrators– When an administrative right is removed from an administrator,

other policies fall as well. This may not be desirable.– Issue instants of policies are perhaps not trusted.– History of issuer attributes would be needed.

• Implementation with obligations is not elegant– Work in progress at the XACML TC

• I haven’t thought about all features of XACML yet.

The End

• More information:– http://www.sics.se/isl/pbr/xacml.html