RuleML2015: Towards Formal Semantics for ODRL Policies

Towards Formal Semantics for ODRL Policies
Simon Steyskal and Axel Polleres
web: http://steyskal.info
mail: simon.steyskal@wu.ac.at
twitter: @simonsteys

Agenda
1. Motivation
 Policy-driven Data Markets
 Compliance Checking in BPM
 Requirements for Policy Language
2. Formulating Expressive Policies using ODRL
 Open Digital Rights Language (ODRL)
 Policy Examples
 Implicit/Explicit Dependencies among ODRL Policies
3. Formal Semantics of ODRL
 General Evaluation Procedure
 Abstract Syntax of ODRL
 Conflict Resolution
PAGE 2

Agenda
1. Motivation
 Policy Examples
PAGE 3

Policy-driven Data Markets
PAGE 4

Compliance Checking in BPM
PAGE 5
http://ssrg.nicta.com.au/projects/bpc
https://ai.wu.ac.at/shape-project/

Requirements for Policy Language
 Expressivity
 It should be possible to model complex policies.
 Such complex policies may include obligations, constraints or
specific conflict resolution strategies.
 Extensibility
 If required, it should be easy to add additional concepts to the
policy language.
 Flexibility
 The policy language should be flexible enough to be used within
different scenarios.
PAGE 6

Agenda
1. Motivation
 Policy Examples
PAGE 7

Open Digital Rights Language (ODRL)
PAGE 8
https://www.w3.org/community/odrl/model/2.1/

@prefix odrl: <http://w3.org/ns/odrl/2/> .
@prefix : <http://www.example.com/> .
:policy1 a odrl:Agreement ;
odrl:permission [
a odrl:Permission;
odrl:assigner :owner;
odrl:assignee :alice;
odrl:action odrl:read;
odrl:target :dataset1;
odrl:constraint [
a odrl:Constraint;
odrl:operator odrl:lteq;
odrl:dateTime "2016-12-31"^^xsd:date
] .
Listing 1
Policy Examples 1/3
Permitting access only in specific time frames

:policy2 a odrl:Set;
odrl:permission [
a odrl:Permission;
odrl:action odrl:reproduce,
odrl:distribute,
odrl:derive;
odrl:duty odrl:attribution,
odrl:attachPolicy,
odrl:shareAlike
] .
odrl:prohibiton odrl:commercialize .
Listing 2
Policy Examples 2/3
Representing license information (CC-BY-NC-SA)
Villata et al. (ESWC 2014)

@prefix gr: <http://purl.org/goodrel/v1#> .
@prefix gn: <http://www.geonames.org/ontology#">.
:policy3 a odrl:Set;
odrl:permission [
a odrl:Permission;
odrl:target :dataset;
odrl:duty [
a odrl:Duty;
odrl:action odrl:pay;
odrl:constraint [
a odrl:Constraint ;
odrl:payAmount 50.00 ;
odrl:operator odrl:eq ;
odrl:unit
<http://cvx.iptc.org/iso4217a:EUR>
] .
Listing 3
Policy Examples 3/3
Combining prohibitions and permissions
odrl:prohibtion [
a odrl:Prohibition;
odrl:action odrl:distribute;
odrl:target :dataset;
odrl:constraint [
a odrl:Constraint;
odrl:operator odrl:eq;
odrl:spatial [
a gn:Feature.
gn:countryCode “AT”
] .
] .
] .

 Policies govern execution of actions over assets.
 Does permission of one action interfere with prohibition
of another action?
 Direct Dependency
 Implicit Dependency
 Explicit Dependency
Dependencies among ODRL Policies
PAGE 12
:ex1 a odrl:Set;
odrl:permission odrl:read.
:ex2 a odrl:Set;
odrl:prohibition odrl:read.
:ex1 a odrl:Set;
odrl:permission odrl:share.
:ex2 a odrl:Set;
odrl:prohibition odrl:distribute.
:ex1 a odrl:Set;
odrl:permission odrl:use.
:ex2 a odrl:Set;
odrl:prohibition odrl:display.
?
?
?

 ODRL explicitly defines a hierarchy among its actions
 e.g. odrl:present is a broader term/action for odrl:display
 Governing execution of a more general action, influences
execution of its narrower ones too.
Explicit Dependencies among ODRL
Policies
PAGE 13
odrl:present
odrl:display odrl:play
odrl:print
skos:broaderTransitive

Implicit Dependencies among ODRL
Policies
PAGE 14
 Other dependencies are only implicitly expressed as part
of the natural language description of ODRL actions.
 e.g. odrl:share
 Prohibition of either odrl:reproduce/odrl:copy or odrl:distribute
would cause a conflict, if odrl:share would be permitted at the
same time.

Agenda
1. Motivation
 Policy Examples
PAGE 15

General Evaluation Procedure
PAGE 16
Data Consumer Data Provider
Request
(party,action,asset)
Check applicable
policies
1
2
1. A query request consists of:
 optional information about requesting party,
 the requested action to be performed, and
 asset the requested action should be performed on.
Evaluation result3
Policy Store

2. A policy is applicable, if at least one of its rules is applicable.
 A rule is applicable, if
 its action, asset, and party (if specified) information match those of the request,
 its constraints hold (if specified), and
 its duties are fulfilled (if specified).
PAGE 17
Request
Policy Store
Check applicable
policies
1
2Evaluation result3

3. Result of a query request evaluation can either be:
 permission – query request is permitted
 prohibition – query request is prohibited
 conditional prohibition – query request is prohibited due to open obligation(s)
 condition permission – query request is permitted since all its obligation(s) are
fulfilled
 not applicable – there is no applicable nor active policy for the query request
PAGE 18
Request
Policy Store
Check applicable
policies
1
2Evaluation result3

Abstract Syntax of ODRL
PAGE 19

Conflict Resolution
 How to deal with conflicting evaluation results?
PAGE 20
 ODRL defines three different conflict resolution
strategies
 perm, prohibit, invalid
odrl:permission [
a odrl:Permission;
odrl:prohibition [
a odrl:Prohibition;

Permission Overrides (perm)
 Semantics
 Whenever there are two rules in conflict with each other, the one
granting permission to execute an action a on a particular asset
overrules the one prohibiting its execution.
PAGE 21

Prohibition Overrides (prohibit)
 Semantics
 Whenever there are two rules in conflict with each other, the one
prohibiting execution of an action a on a particular asset
overrules any permission of a.
PAGE 22

No Conflicts Allowed (invalid)
 Semantics
 Whenever there are two rules in conflict with each other, no
answer can be returned.
 invalid is ODRL’s default conflict resolution strategy.
PAGE 23

Conclusion
 Contributions
 Definition of an abstract syntax for expressing ODRL policies.
 Formalization of a possible interpretation of ODRL policy semantics.
 Discussion of a solution proposal for considering dependencies
among ODRL actions for policy evaluation.
 Future Work
 Introducing the concept of Policy Sets as container for policies which
allows to combine the evaluation results of policies independently of
their respective chosen conflict resolution strategy.
 Formalizing and extending the mapping between ODRL policies and
logic programs, which enables basic, rule-based reasoning
 Addressing the elaborate provision of proofs for constraints and
duties which are currently assumed to be provided by the requester
itself.
PAGE 24

RuleML2015: Towards Formal Semantics for ODRL Policies

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