GEOPRIV H. Schulzrinne
Internet-Draft Columbia U.
Expires: January 19, 2006 J. Morris
CDT
H. Tschofenig
J. Cuellar
Siemens
J. Polk
Cisco
J. Rosenberg
DynamicSoft
July 18, 2005
A Document Format for Expressing Privacy Preferences
draft-ietf-geopriv-common-policy-05.txt
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Abstract
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This document defines a framework for authorization policies
controling access to application specific data. This framework
combines common location- and presence-specific authorization
aspects. An XML schema specifies the language in which common policy
rules are represented. The common policy framework can be extended
to other application domains.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Modes of Operation . . . . . . . . . . . . . . . . . . . . . 6
3.1 Passive Request-Response - PS as Server (Responder) . . . 6
3.2 Active Request-Response - PS as Client (Initiator) . . . . 6
3.3 Event Notification . . . . . . . . . . . . . . . . . . . . 6
4. Goals and Assumptions . . . . . . . . . . . . . . . . . . . 8
5. Non-Goals . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Basic Data Model and Processing . . . . . . . . . . . . . . 11
6.1 Identification of Rules . . . . . . . . . . . . . . . . . 12
6.2 Extensions . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Conditions . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1 Identity . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.2 Sphere . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3 Validity . . . . . . . . . . . . . . . . . . . . . . . . . 18
8. Actions . . . . . . . . . . . . . . . . . . . . . . . . . . 20
9. Transformations . . . . . . . . . . . . . . . . . . . . . . 21
10. Procedure for Combining Permissions . . . . . . . . . . . . 22
10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . 22
10.2 Algorithm . . . . . . . . . . . . . . . . . . . . . . . 22
10.3 Example . . . . . . . . . . . . . . . . . . . . . . . . 23
11. Meta Policies . . . . . . . . . . . . . . . . . . . . . . . 26
12. Example . . . . . . . . . . . . . . . . . . . . . . . . . . 27
13. XML Schema Definition . . . . . . . . . . . . . . . . . . . 28
14. Security Considerations . . . . . . . . . . . . . . . . . . 32
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . 33
15.1 Common Policy Namespace Registration . . . . . . . . . . 33
15.2 Content-type registration for
'application/auth-policy+xml' . . . . . . . . . . . . . 33
15.3 Common Policy Schema Registration . . . . . . . . . . . 35
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 36
16.1 Normative References . . . . . . . . . . . . . . . . . . 36
16.2 Informative References . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 37
A. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 39
B. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 40
Intellectual Property and Copyright Statements . . . . . . . 41
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1. Introduction
This document defines a framework for creating authorization policies
for access to application specific data. This framework is the
result of combining the common aspects of single authorization
systems that more specifically control access to presence and
location information and that previously had been developed
separately. The benefit of combining these two authorization systems
is two-fold. First, it allows to build a system which enhances the
value of presences with location information in a natural way and
reuses the same underlying authorization mechanism. Second, it
encourages a more generic authorization framework with mechanisms for
extensibility. The applicability of the framework specified in this
document is not limited to policies controling access to presence and
location information data, but can be extended to other application
domains.
The general framework defined in this document is intended to be
accompanied and enhanced by application-specific policies specified
elsewhere. The common policy framework described here is enhanced by
domain-speific policy documents, including presence [5] and
location[6]. This relationship is shown inFigure 1.
+-----------------+
| |
| Common |
| Policy |
| |
+---+---------+---+
/|\ /|\
| |
+-------------------+ | | +-------------------+
| | | enhance | | |
| Location-specific | | | | Presence-specific |
| Policy |----+ +----| Policy |
| | | |
+-------------------+ +-------------------+
Figure 1: Common Policy Enhancements
This document starts with an introduction to the terminology in
Section 2, an illustration of basic modes of operation in Section 3,
a description of goals (see Section 4) and non-goals (see Section 5)
of the authorization policy framework, followed by the data model in
Section 6. The structure of a rule, namely conditions, actions and
transformations, are described in Section 7, in Section 8 and in
Section 9. The procedure for combining permissions is explained in
Section 10 and used when more than one rule fires. A short
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description of meta policies is given in Section 11. An example is
provided in Section 12. The XML schema will be discussed in
Section 13. IANA considerations in Section 15 follow security
considerations Section 14.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT","RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [1].
This document introduces the following terms:
PT - Presentity / Target: The PT is the entity about whom information
has been requested.
RM - Rule Maker: RM is an entity which creates the authorization
rules which restrict access to data items.
PS - (Authorization) Policy Server: This entity has access to both
the authorization policies and to the data items. In location-
specific applications, the entity PS is labeled as location server
(LS).
WR - Watcher / Recipient: This entity requests access to data items
of the PT. An access operation might be either be a read, write
or any other operation. In case of access to location information
it might be a read operation.
An 'authorization policy' is given by a 'rule set'. A 'rule set'
contains an unordered list of 'rules'. A 'rule' has a 'conditions',
an 'actions' and a 'transformations' part.
The term 'permission' indicates the action and transformation
components of a 'rule'.
The terms 'authorization policy', 'policy' and 'rule set' are used
interchangeably.
The terms 'authorization policy rule', 'policy rule' and 'rule' are
used interchangeable.
The term 'using protocol' is defined in [7]. It refers to the
protocol which is used to request access to and to return privacy
sensitive data items.
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3. Modes of Operation
The abstract sequence of operations can roughly be described as
follows. The PS receives a query for data items for a particular PT,
via the using protocol. The using protocol provides the identity of
the requestor (or more precisely the authentication protocol), either
at the time of the query or at the subscription time. The
authenticated identity of the WR, together with other information
provided by the using protocol or generally available to the server,
is then used for searching through the rule set. All matching rules
are combined according to a permission combining algorithm described
in Section 10. The combined rules are applied to the application
data, resulting in the application of privacy based on the
transformation policies. The resulting application data is returned
to the WR.
Three different modes of operation can be distinguished:
3.1 Passive Request-Response - PS as Server (Responder)
In a passive request-response mode, the WR queries the PS for data
items about the PT. Examples of protocols following this mode of
operation include HTTP, FTP, LDAP, finger or various RPC protocols,
including Sun RPC, DCE, DCOM, Corba and SOAP. The PS uses the
ruleset to determine whether the WR is authorized to access the PTs
information, refusing the request if necessary. Furthermore, the PS
might filter information by removing elements or by reducing the
resolution of elements.
3.2 Active Request-Response - PS as Client (Initiator)
Alternatively, the PS may contact the WR and convey data items.
Examples include HTTP, SIP session setup (INVITE request), H.323
session setup or SMTP.
3.3 Event Notification
Event notification adds a subscription phase to the "PS as client"
mode of operation. A watcher or subscriber asks to be added to the
notification list for a particular presentity or event. When the
presentity changes state or the event occurs, the PS sends a message
to the WR containing the updated state. (Presence is a special case
of event notification; thus, we often use the term interchangeably.)
In addition, the subscriber may itself add a filter to the
subscription, limiting the rate or content of the notifications. If
an event, after filtering by the rulemaker-provided rules and by the
subscriber-provided rules, only produces the same notification
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content that was sent previously, no event notification is sent.
A single PS may authorize access to data items in more than one mode.
Rather than having different rule sets for different modes all three
modes are supported with a one rule set schema. Specific instances
of the rule set can omit elements that are only applicable to the
subscription model.
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4. Goals and Assumptions
Below, we summarize our design goals and constraints.
Table representation:
Each rule must be representable as a row in a relational database.
This design goal should allow efficient policy rule implementation
by utilizing standard database optimization techniques.
Permit only:
Rules only provide permissions rather than denying them. Allowing
both 'permit' and 'deny' actions would require some rule ordering
which had implications on the update operations executed on these
rules. Additionally it would make distributed rule sets more
complicated. Hence, only 'permit' actions are allowed which
result in more efficient rule processing. This also implies that
rule ordering is not important. Consequently, to make a policy
decision requires processing all policy rules.
Additive permissions:
A query for access to data items is matched against the rules in
the rule database. If several rules match, then the overall
permissions granted to the WR are the union of those permissions.
A more detailed discussion is provided inSection 10.
Upgradeable:
It should be possible to add additional rules later, without
breaking PSs that have not been upgraded. Any such upgrades must
not degrade privacy constraints, but PSs not yet upgraded may
reveal less information than the rulemaker would have chosen.
Versioning support:
In addition to the previous goal, a RM should be able to determine
which types of rules are supported by the PS. The mechanism used
to determine the capability of a PS is outside the scope of this
specification.
Protocol-independent:
The rule set supports constraints on both notifications or queries
as well as subscriptions for event-based systems such as presence
systems.
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No false assurance:
It appears more dangerous to give the user the impression that the
system will prevent disclosure automatically, but fail to do so
with a significant probability of operator error or
misunderstanding, than to force the user to explicitly invoke
simpler rules. For example, rules based on weekday and time-of-
day ranges seem particularly subject to misinterpretation and
false assumptions on part of the RM. (For example, a non-
technical RM would probably assume that the rules are based on the
timezone of his current location, which may not be known to other
components of the system.)
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5. Non-Goals
We explicitly decided that a number of possibly worthwhile
capabilities are beyond the scope of this first version. Future
versions may include these capabilities, using the extension
mechanism described in this document. Non-goals include:
No external references:
Attributes within specific rules cannot refer to external rule
sets, databases, directories or other network elements. Any such
external reference would make simple database implementation
difficult and hence they are not supported in this version.
No regular expression or wildcard matching:
Conditions are matched on equality or 'greater-than'-style
comparisons, not regular expressions, partial matches such as the
SQL LIKE operator (e.g., LIKE "%foo%") or glob-style matches
("*@example.com"). Most of these are better expressed as explicit
elements.
No all-except conditions:
It is not possible to express exclusion conditions based on
identities such as "everybody except Alice". However, this
restriction does not prevent all forms of blacklisting. It is
still possible to express an authorization rule like 'I allow
access to my location information for everyone of domain
example.com except for John'. See the example in Section 7.1
describing how exceptions can be made to work. The reason for
this choice is the ease with which identities can be manufactured,
and the implication that all-except types of rules are easily
subverted.
No repeat times:
Repeat times are difficult to make work correctly, due to the
different time zones that PT, WR, PS and RM may occupy. It
appears that suggestions for including time intervals are often
based on supporting work/non-work distinctions, which
unfortunately are difficult to capture by time alone.
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6. Basic Data Model and Processing
A rule set (or synonymously, a policy) consists of zero or more
rules. The ordering of these rules is irrelevant. The rule set can
be stored at the PS and conveyed from RM to PS as a single document,
in subsets or as individual rules. A rule consists of three parts -
conditions (see Section 7), actions (see Section 8), and
transformations (see Section 9).
The conditions part is a set of expressions, each of which evaluates
to either TRUE or FALSE, i.e. each of which is equipped with a value
of either TRUE or FALSE by the PS. When a WR asks for information
about a PT, the PS goes through each rule in the rule set. For each
rule, it evaluates the expressions in the conditions part. If all of
the expressions evaluate to TRUE, then the rule is applicable to this
request. Generally, each expression specifies a condition based on
some variable that is associated with the context of the request.
These variables can include the identity of the WR, the domain of the
WR, the time of day, or even external variables, such as the
temperature or the mood of the PT.
Assuming that the rule is applicable to the request, the actions and
transformations (commonly referred to as permissions) in the rule
specify how the PS is supposed to handle this request. If the
request is to view the location of the PT, or to view its presence,
the typical action is "permit", which allows the request to proceed.
Assuming the action allows the request to proceed, the
transformations part of the rule specifies how the information about
the PT - their location information, their presence, etc. - is
modified before being presented to the WR. These transformations are
in the form of positive permissions. That is, they always specify a
piece of information which is allowed to be seen by the WR. When a
PS processes a request, it takes the transformations specified across
all rules that match, and creates the union of them. The means for
computing this union depend on the data type - Integer, Boolean, Set,
or the Undef data type - and are described in more detail in
Section 10. The resulting union effectively represents a "mask" - it
defines what information is exposed to the WR. This mask is applied
to the actual location or presence data for the PT, and the data
which is permitted by the mask is shown to the WR. If the WR request
a subset of information only (such as city-level civil location data
only, instead of the full civil location information), the
information delivered to the WR SHOULD be the intersection of the
permissions granted to the WR and the data requested by the WR.
In accordance to this document, rules are encoded in XML. To this
end, Section 13 contains an XML schema defining the Common Policy
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Markup Language. This, however, is purely an exchange format between
RM and PS. The format does not imply that the RM or the PS use this
format internally, e.g., in matching a query with the policy rules.
The rules are designed so that a PS may translate the rules into a
relational database table, with each rule represented by one row in
the database. The database representation is by no means mandatory;
we will use it as a convenient and widely-understood example of an
internal representation. The database model has the advantage that
operations on rows have tightly defined meanings. In addition, it
appears plausible that larger-scale implementations will employ a
backend database to store and query rules, as they can then benefit
from existing optimized indexing, access control, scaling and
integrity constraint mechanisms. Smaller-scale implementations may
well choose different implementations, e.g., a simple traversal of
the set of rules.
6.1 Identification of Rules
Each rule is equipped with a parameter that identifies the rule.
This rule identifier is an opaque token chosen by the RM. A RM MUST
NOT use the same identifier for two rules that are available to the
PS at the same time for a given PT.
6.2 Extensions
The authorization policy framework defined in this document is meant
to be extensible towards specific application domains. Such an
extension is accomplished by defining conditions, actions and
transformations that are specific to the desired application domain.
Each extension MUST define its own namespace.
Extensions cannot change the schema defined in this document, and
this schema is not expected to change excepting a revision to this
specification. and that no versioning procedures for this schema or
namespace are therfore provided.
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7. Conditions
The access to data items needs to be matched with the rule set stored
at the PS. Each instance of a request has different attributes
(e.g., the identity of the requestor) which are used for
authorization. A rule in a rule set might have a number of
conditions which need to be met before executing the remaining parts
of a rule (i.e., actions and transformations). Details about rule
matching are described in Section 10. This document specifies only a
few conditions (namely identity, sphere, and validity). Other
conditions are left for extensions of this document.
7.1 Identity
The authorization policy framework specified in this document
supports the usage of identities as input to access authorization
decision processes. This document, however, abstracts from the
particularities of concrete authentication mechanisms employed by
different using protocols and is therefore unable to specify
explicitly the details of identity relevant information. Hence, we
only assume that an identity has a structure with a username and a
domain part. Documents that enhance the schema defined in this
document should, if possible, describe how a particular using
protocol is able to provide identity information in a meaningful way.
Such an enhancement needs to map the identity used by the
authentication protocol employed in the using protocol to an identity
used in the authorization policy. It is necessary to clearly define
a mapping between the authenticated identity of the user and the
identities used in the authorization policies. This mapping needs to
consider the large number of possible identities used in various
authentication protocols.
In this document we distinguish between three different identities:
Authenticated Identities:
The WR was authenticated by the PS. The authenticated identity is
used as input to the rule matching procedure.
Unauthenticated Identities:
The WR was not authenticated by the PS and thus any identity
provided by the WR might be spoofed.
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Asserted Identities:
The WR was not directly authenticated by the PS. Instead the WR
was authenticated by an asserting party that provides an assurance
to the PS about the authentication of the WR. It is assumed that
the PS and the asserting party have some relationship with each
other and that there is a certain degree of trust that the
asserting party does not misbehave (such as lying about a non-
existing authentication of the WR and by making an incorrect
assertion). An example of this approach can be found in [8].
These three identities are used for the rule matching algorithms
whereby no differentiation is made between an authenicated and an
asserted identity in this document. However, there is another
identity, the anonymous identity, that plays a role in this context.
Most using protocols also designate an identifier that denotes an
'anonymous user'. The WR, as such, is not authenticated by the PS
nor does an assertion exist that provides the PS with information
about the authenticated identity. In order to provide an
authorization decision non-identity based (or trait-based
authorization) might be applicable. This might be accomplished by an
assertion provided by a third party that hides the identity but
offers attributes about the WR that are used as the foundation for an
access control decision. Ensuring authorization for anonymous users
is outside the scope of this document.
The following guidelines apply when performing rule matching (related
to the identity element in the conditions part of a rule):
1. A rule with no conditions matches any subscription irrelevant
whether an authenticated or an unauthenticated was used by the WR
to access resources.
2. A rule with an <identity> element assumes that there are either
<id> or <domain> elements as child elements. Unauthenticated
identities never match the values of the <id> element(s) or the
<domain> element(s).
1. If the <identity> element contains one or multiple <id>
element(s) then the authenticated identities (as provided by
the security protocol used by the using protocol) MUST be
used for an equality match.
2. If the <identity> element contains the <domain> element
consisting of one or more <except> element(s) then the domain
part of the authenticated identity MUST be used for an
equality match. This allows implementing a simple blacklist
mechanism. The <except> element contains the identity
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without the domain part (i.e., the username part) since it
equals the domain of the <domain> element. Only
authenticated identities MUST be matched with the username
part of the values of the <except> element(s).
3. The <any-identity> condition that matches authenticated and
unauthenticated identities. The <any-identity> MAY contain one
or multiple <except-domain> elements and MAY contain one or
multiple <domain> elements.
1. If the <any-identity> element does not contain child elements
then the rule matches with any authenticated or
unauthenticated identity.
2. If the <any-identity> element contains one or multiple
<domain> elements then the domain part of the authenticated
or unauthenticated identity MUST be matched against the
values of the <domain> element(s).
3. If the <any-identity> element contains one or multiple
<except-domain> element(s) then the domain part of the
authenticated or unauthenticated identity MUST be matched
against the values of the <except-domain> element(s).
Regarding string comparison the following rules apply:
1. The <id> elements of the <identity> condition part of the rule
matches, if the identity of the WR matches, based on case
sensitive string comparison, the user part of the <id>, and the
domain part of the of the <id>, based on case insensitive string
comparison.
2. The identity of the WR matches a <domain> (either listed in the
<identity> or the <any-identity> element) when the domain part of
the identity matches, based on case insensitive string
comparison, the value of the <domain> element, and the user part
matches none of the <except> element values, based on case
sensitive string comparison. The values of a <domain> element
MUST be matched with the domain part of the provided identity
using an equality match. No wildcarding is provided.
Next, we list a few examples. The following example illustrates
conditions based on an identity.
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Authenticated Identities
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r2">
<conditions>
<identity>
<id entity="alice@example.com"/>
<id entity="bob@example.com"/>
</identity>
</conditions>
</rule>
</ruleset>
Figure 2: Matching authenticated identities
The example shown in Figure 2 shows a rule that matches if the
authenticated identity of the WR is either alice@example.com or
bob@example.com.
Exceptions within the Identity Element:
The example in Figure 3 shows how exceptions are implemented. For
the given example the rule matches if the authenticated identity
of the WR is from the example.com domain but the user identity of
the WR is neither joe@example.com, toni@example.com nor
mike@example.com.
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r2">
<conditions>
<identity>
<domain domain="example.com"/>
<except entity="joe"/>
<except entity="tony"/>
<except entity="mike"/>
</identity>
</conditions>
</rule>
</ruleset>
Figure 3: Using the Domain/Except Elements
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If a WR with the identity john@foo.example.com requests access to
a resource the above-shown rule is not applied since the case
insensitive equality match between the domain part of the WR
identity (namely 'foo.example.com') does not match the value in
the domain part of the rule (namely 'example.com').
Any Identity
The following example matches any authenticated and any
unauthenticated identity.
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r2">
<conditions>
<any-identity/>
</conditions>
</rule>
</ruleset>
Figure 4: The Any-Identity Element
The following rule fires for any authenticated and any
unauthenticated identity except for WR's from the example.com and
the foo.com domain. The rule would therefore fire for
joe@foo.bar.com but not for alice@foo.com.
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r2">
<conditions>
<any-identity>
<except-domain domain="example.com"/>
<except-domain domain="foo.com"/>
</any-identity>
</conditions>
</rule>
</ruleset>
Figure 5: Any-Identity combined with Except-Domain Element
7.2 Sphere
The <sphere> element belongs to the group of condition elements. It
can be used to indicate a state (e.g., 'work', 'home', 'meeting',
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'travel') the PT is currently in. A sphere condition matches only if
the PT is currently in the state indicated. The state may be
conveyed by manual configuration or by some protocol. For example,
RPID [9] provides the ability to inform the PS of its current sphere.
The application domain needs to describe in more detail how the
sphere state is determined. Switching from one sphere to another
causes to switch between different modes of visibility. As a result
different subsets of rules might be applicable. An example of a rule
fragment is shown below:
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r2">
<conditions>
<sphere value="work"/>
<identity>
<id entity="andrew@example.com"/>
</identity>
</conditions>
<actions/>
<transformations/>
</rule>
<rule id="y6y55r2">
<conditions>
<sphere value="home"/>
<identity>
<id entity="allison@example.com"/>
</identity>
</conditions>
<actions/>
<transformations/>
</rule>
</ruleset>
The rule example above illustrates that the rule with the entity
andrew@example.com matches if the sphere is been set to 'work'. In
the second rule with the entity allison@example.com matches if the
sphere is set to 'home'.
7.3 Validity
The <validity> element is the third condition element specified in
this document. It expresses the rule validity period by two
attributes, a starting and a ending time. Times are expressed in XML
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dateTime format.A rule maker might not have always access to the PS
to invalidate some rules which grant permissions. Hence this
mechanisms allows to invalidate granted permissions automatically
without further interaction between the rule maker and the PS. The
PS does not remove the rules instead the rule maker has to clean them
up.
An example of a rule fragment is shown below:
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r3">
<conditions>
<validity>
<from>2003-08-15T10:20:00.000-05:00</from>
<until>2003-09-15T10:20:00.000-05:00</until>
</validity>
</conditions>
<actions/>
<transformations/>
</rule>
</ruleset>
The <identity>, the <sphere> and the <validity> element MUST NOT
appear more than once in the conditions part of a single rule. The
<id> element on the other hand may appear more than once as described
in this section.
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8. Actions
While conditions are the 'if'-part of rules, actions and
transformations build the 'then'-part of them. The actions and
transformations parts of a rule determine which operations the PS
MUST execute after having received from a WR a data access request
that matches all conditions of this rule. Actions and
transformations only permit certain operations; there is no 'deny'
functionality. Transformations exclusively specify PS-side
operations that lead to a modification of the data items requested by
the WR. Regarding location data items, for instance, a
transformation could force the PS to lower the precision of the
location information which is returned to the WR.
Actions, on the other hand, specify all remaining types of operations
the PS is obliged to execute, i.e., all operations that are not of
transformation type. Actions are defined by application specific
usages of this framework. The reader is referred to the
corresponding extensions to see examples of such elements.
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9. Transformations
Two sub-parts follow the conditions part of a rule: transformations
and actions. As defined in Section 8, transformations specify
operations that the PS MUST execute and that modify the result which
is returned to the WR. This functionality is particularly helpful in
reducing the granularity of information provided to the WR, as for
example required for location privacy. Transformations are defined
by application specific usages of this framework.
A simple transformation example is provided in Section 10.
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10. Procedure for Combining Permissions
10.1 Introduction
This section describes the mechanism to evaluate the final result of
a rule evaluation. The result is reflected in the action and
transformation part of a rule. This procedure is sometimes referred
as conflict resolution.
We use the following terminology (which in parts has already been
introduced in previous sections): The term 'permission' stands for an
action or a transformation. The notion 'attribute' terms a
condition, an action, or a transformation. An attribute has a name,
has a certain data type. A value may be assigned to an attribute or
it may be undefined, in case it does not have a value associated with
the attribute. For example, the name of the <sphere> attribute
discussed in Section 7 is 'sphere', its data type is 'string', and
its value may be set to 'home'. To evaluate a condition means to
associate either TRUE or FALSE to the condition. Please note that
the <identity> element is a condition whereas the <id> element is a
parameter of that condition. A rule matches if all conditions
contained in the conditions part of a rule evaluate to TRUE.
When the PS receives a request for access to privacy-sensitive data
then it needs to be matched against a rule set. The conditions part
of each individual rule is evaluated and as a result one or more
rules might match. If only a single rule matches then the result is
determined by executing the actions and the transformations part
following the conditions part of a rule. However, it can also be the
case that two or more matching rules contain a permission of the same
name (e.g., two rules contain a permission named 'precision of
geospatial location information'), but do not specify the same value
for that permission (e.g., the two rule might specify values of '10
km' and '200 km', respectively, for the permission named 'precision
of geospatial location information'). This section describes the
procedure for combining permissions in such cases.
10.2 Algorithm
The combining rules are simple and depend on the data types of the
values of permissions: Let P be a policy. Let M be the subset of P
consisting of rules r in P that match with respect to a given
request. Let n be a name of a permission contained in a rule r in M,
and let M(n) be the subset of M consisting of rules r in M that have
a permission of name n. For each rule r in M(n), let v(r,n) and
d(r,n) be the value and the data type, respectively, of the attribute
of r with name n. Finally, let V(n) be the combined value of all the
permissions values v(r,n), r in M(n). The combining rules that lead
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to the resulting value V(n) are the following:
CR 1: If d(r,n)=Boolean for all r in M(n), then V(n) is given as
follows: If there is a r in M(n) with v(r,n)=TRUE, then V(n)=TRUE.
Otherwise, V(n)=FALSE.
CR 2: If d(r,n)=Integer for all r in M(n), then V(n) is given as
follows: If v(r,n)=undefined for all r in M(n), then V(n) is not
specified by this specification. Otherwise, V(n)=max{v(r,n) | r
in M(n)}.
CR 3: If d(r,n)=Set for all r in M(n), then V(n) is given as
follows: V(n)=union of all v(r,n), the union to be computed over all
r in M(n) with v(r,n)!=undefined.
The combining operation will result in the largest value for an
Integral type, the Or operation for boolean, and union for set.
As a result, applications should define values such that, for
integers, the lowest value corresponds to the most privacy, for
booleans, false corresponds to the most privacy, and for sets, the
empty set corresponds to the most privacy. More
10.3 Example
In the following example we illustrate the process of combining
permissions. We will consider three conditions for our purpose,
namely those of name identity, sphere, and validity. For editorial
reasons the rule set in this example is represented in a table.
Furthermore, the domain part of the identity of the WR is omitted.
For actions we use two permissions with names X and Y. The values of
X and Y are of data types Boolean and Integer, respectively.
Permission X might, for example, represent the <sub-handling> action.
For transformations we use the attribute with the name Z whose value
can be set either to '+'(or 1), 'o' (or 2) or '-' (or 3). Permission
Z allows us to show the granularity reduction whereby a value of '+'
shows the corresponding information unrestricted and '-' shows
nothing. This permission might be related to location information or
other presence attributes like mood. Internally we use the data type
Integer for computing the permission of this attribute.
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Conditions Actions/Transformations
+---------------------------------+---------------------+
| Id WR-ID sphere from until | X Y Z |
+---------------------------------+---------------------+
| 1 bob home A1 A2 | TRUE 10 o |
| 2 alice work A1 A2 | FALSE 5 + |
| 3 bob work A1 A2 | TRUE 3 - |
| 4 tom work A1 A2 | TRUE 5 + |
| 5 bob work A1 A3 | undef 12 o |
| 6 bob work B1 B2 | FALSE 10 - |
+---------------------------------+---------------------+
Again for editorial reasons, we use the following abbreviations for
the two <validity> attributes 'from' and 'until':
A1=2003-12-24T17:00:00+01:00
A2=2003-12-24T21:00:00+01:00
A3=2003-12-24T23:30:00+01:00
B1=2003-12-22T17:00:00+01:00
B2=2003-12-23T17:00:00+01:00
Note that B1 < B2 < A1 < A2 < A3.
The entity 'bob' acts as a WR and requests data items. The policy P
consists of the six rules shown in the table and identified by the
values 1 to 6 in the 'Id' column. The PS receives the query at 2003-
12-24T17:15:00+01:00 which falls between A1 and A2. The value of the
attribute with name 'sphere' indicating the state the PT is currently
in is set to 'work'.
Rule 1 does not match since the sphere condition does not match.
Rule 2 does not match as the identity of the WR (here 'alice') does
not equal 'bob'. Rule 3 matches since all conditions evaluate to
TRUE. Rule 4 does not match as the identity of the WR (here 'tom')
does not equal 'bob'. Rule 5 matches. Rule 6 does not match since
the rule is not valid anymore. Therefore, the set M of matching
rules consists of the rules 3 and 5. These two rules are used to
compute the combined permission V(X), V(Y), and V(Z) for each of the
permissions X, Y, and Z:
Actions/Transformations
+-----+-----------------------+
| Id | X Y Z |
+-----+-----------------------+
| 3 | TRUE 3 - |
| 5 | undef 12 o |
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+-----+-----------------------+
The results of the permission combining algorithm is shown below.
The combined value V(X) regarding the permission with name X equals
TRUE according to the first combining rule listed above. The maximum
of 3 and 12 is 12, so that V(Y)=12. For the attribute Z in this
example the maximum between 'o' and '-' (i.e., between 2 and 3) is
'-'.
Actions/Transformations
+-----+-----------------------+
| Id | X Y Z |
+-----+-----------------------+
| 5 | TRUE 12 - |
+-----+-----------------------+
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11. Meta Policies
Meta policies authorize a rulemaker to insert, update or delete a
particular rule or an entire rule set. Some authorization policies
are required to prevent unauthorized modification of rule sets. Meta
policies are outside the scope of this document.
A simple implementation could restrict access to the rule set only to
the PT but more sophisticated mechanisms could be useful. As an
example of such policies one could think of parents configuring the
policies for their children.
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12. Example
This section gives an example of an XML document valid with respect
to the XML schema defined in Section 13. Semantically richer
examples can be found in documents which extend this schema with
application domain specific data (e.g., location or presence
information).
Below a rule is shown with a condition that matches for a given
authenticated identity (bob@example.com) and within a given time
period. Additionally, the rule matches only if the target has set
its sphere to 'work'.
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r1">
<conditions>
<identity>
<id entity="bob@example.com"/>
</identity>
<sphere value="work"/>
<validity>
<from>2003-12-24T17:00:00+01:00</from>
<until>2003-12-24T19:00:00+01:00</until>
</validity>
</conditions>
<actions/>
</rule>
</ruleset>
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13. XML Schema Definition
This section provides the XML schema definition for the common policy
markup language described in this document.
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema
targetNamespace="urn:ietf:params:xml:ns:common-policy"
xmlns:cp="urn:ietf:params:xml:ns:common-policy"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<!-- Rule Set -->
<xs:element name="ruleset">
<xs:complexType>
<xs:sequence>
<xs:element name="rule" type="cp:ruleType"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<!-- Rule -->
<xs:complexType name="ruleType">
<xs:sequence>
<!-- Conditions -->
<xs:element name="conditions" minOccurs="0">
<xs:complexType>
<xs:sequence>
<xs:element name="validity" minOccurs="0">
<xs:complexType>
<xs:all>
<xs:element name="from" type="xs:dateTime"/>
<xs:element name="to" type="xs:dateTime"/>
</xs:all>
</xs:complexType>
</xs:element>
<xs:element name="identity" minOccurs="0">
<xs:complexType>
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<xs:choice>
<xs:element name="id" maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="entity"
type="xs:string" use="required"/>
</xs:complexType>
</xs:element>
<xs:sequence>
<xs:element name="domain"
minOccurs="0" maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="domain"
type="xs:string" use="required"/>
</xs:complexType>
</xs:element>
<xs:element name="except"
minOccurs="0" maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="entity"
type="xs:string" use="required"/>
</xs:complexType>
</xs:element>
</xs:sequence>
<xs:element name="any-identity">
<xs:complexType>
<xs:sequence>
<xs:element name="domain"
minOccurs="0" maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="domain"
type="xs:string" use="required"/>
</xs:complexType>
</xs:element>
<xs:element name="except-domain"
minOccurs="0" maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="domain"
type="xs:string" use="required"/>
</xs:complexType>
</xs:element>
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</xs:sequence>
</xs:complexType>
</xs:element>
</xs:choice>
</xs:complexType>
</xs:element>
<xs:element name="sphere"
minOccurs="0" maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="value"
type="xs:string" use="required"/>
</xs:complexType>
</xs:element>
<xs:any namespace="##other" processContents="lax"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<!-- Actions -->
<xs:element name="actions" minOccurs="0">
<xs:complexType>
<xs:sequence>
<xs:any namespace="##other" processContents="lax"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<!-- Transformations -->
<xs:element name="transformations" minOccurs="0">
<xs:complexType>
<xs:sequence>
<xs:any namespace="##other" processContents="lax"
minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
<xs:attribute name="id" type="xs:string" use="required"/>
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</xs:complexType>
</xs:schema>
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14. Security Considerations
This document describes a framework for authorization policy rules.
This framework is intended to be enhanced elsewhere towards
application domain specific data. Security considerations are to a
great extent application data dependent, and therefore need to be
covered by documents that extend the framework defined in this
specification. However, new action and transformation permissions
along with their allowed values must be defined in a way so that the
usage of the permissions combining rules of Section 10 does not lower
the level of privacy protection. See Section 10 for more details on
this privacy issue.
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15. IANA Considerations
This section registers a new XML namespace, a new XML schema and a
new MIME-type. This section registers a new XML namespace per the
procedures in [2].
15.1 Common Policy Namespace Registration
URI: urn:ietf:params:xml:ns:common-policy
Registrant Contact: IETF Geopriv Working Group, Henning Schulzrinne
(hgs+geopriv@cs.columbia.edu).
XML:
BEGIN
<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
"http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="content-type"
content="text/html;charset=iso-8859-1"/>
<title>Common Policy Namespace</title>
</head>
<body>
<h1>Namespace for Common Authorization Policies</h1>
<h2>urn:ietf:params:xml:ns:common-policy</h2>
<p>See <a href="[URL of published RFC]">RFCXXXX
[NOTE TO IANA/RFC-EDITOR:
Please replace XXXX with the RFC number of this
specification.]</a>.</p>
</body>
</html>
END
15.2 Content-type registration for 'application/auth-policy+xml'
This specification requests the registration of a new MIME type
according to the procedures of RFC 2048 [3] and guidelines in RFC
3023 [4].
MIME media type name: application
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MIME subtype name: auth-policy+xml
Mandatory parameters: none
Optional parameters: charset
Indicates the character encoding of enclosed XML. Default is
UTF-8.
Encoding considerations:
Uses XML, which can employ 8-bit characters, depending on the
character encoding used. See RFC 3023 [4], Section 3.2.
Security considerations:
This content type is designed to carry authorization policies.
Appropriate precautions should be adopted to limit disclosure of
this information. Please refer to RFCXXXX [NOTE TO IANA/
RFC-EDITOR: Please replace XXXX with the RFC number of this
specification.] security considerations section for more
information.
Interoperability considerations: none
Published specification: RFCXXXX [NOTE TO IANA/RFC-EDITOR: Please
replace XXXX with the RFC number of this specification.] this
document
Applications which use this media type:
Presence- and location-based systems
Additional information:
Magic Number: None
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File Extension: .xml
Macintosh file type code: 'TEXT'
Personal and email address for further information: Hannes
Tschofenig, Hannes.Tschofenig@siemens.com
Intended usage: LIMITED USE
Author/Change controller:
This specification is a work item of the IETF GEOPRIV working
group, with mailing list address <geopriv@ietf.org>.
15.3 Common Policy Schema Registration
URI: urn:ietf:params:xml:schema:common-policy
Registrant Contact: IETF Geopriv Working Group, Henning Schulzrinne
(hgs+geopriv@cs.columbia.edu).
XML: The XML schema to be registered is contained in Section 13. Its
first line is
<?xml version="1.0" encoding="UTF-8"?>
and its last line is
</xs:schema>
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16. References
16.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", March 1997.
[2] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[3] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures",
BCP 13, RFC 2048, November 1996.
[4] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types",
RFC 3023, January 2001.
16.2 Informative References
[5] Rosenberg, J., "Presence Authorization Rules",
draft-ietf-simple-presence-rules-02 (work in progress),
February 2005.
[6] Schulzrinne, H., "A Document Format for Expressing Privacy
Preferences for Location Information",
draft-ietf-geopriv-policy-05 (work in progress), November 2004.
[7] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and J.
Polk, "Geopriv Requirements", RFC 3693, February 2004.
[8] Jennings, C., Peterson, J., and M. Watson, "Private Extensions
to the Session Initiation Protocol (SIP) for Asserted Identity
within Trusted Networks", RFC 3325, November 2002.
[9] Schulzrinne, H., "RPID: Rich Presence Extensions to the Presence
Information Data Format (PIDF)", draft-ietf-simple-rpid-07 (work
in progress), June 2005.
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Authors' Addresses
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
USA
Phone: +1 212 939 7042
Email: schulzrinne@cs.columbia.edu
URI: http://www.cs.columbia.edu/~hgs
John B. Morris, Jr.
Center for Democracy and Technology
1634 I Street NW, Suite 1100
Washington, DC 20006
USA
Email: jmorris@cdt.org
URI: http://www.cdt.org
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@siemens.com
URI: http://www.tschofenig.com
Jorge R. Cuellar
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Jorge.Cuellar@siemens.com
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James Polk
Cisco
2200 East President George Bush Turnpike
Richardson, Texas 75082
USA
Email: jmpolk@cisco.com
Jonathan Rosenberg
DynamicSoft
600 Lanidex Plaza
Parsippany, New York 07054
USA
Email: jdrosen@dynamicsoft.com
URI: http://www.jdrosen.net
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Appendix A. Contributors
We would like to thank Christian Guenther for his help with this
document.
Christian Guenther
Siemens AG
Corporate Technology
81730 Munich
Email: christian.guenther@siemens.com
Germany
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Appendix B. Acknowledgments
This document is partially based on the discussions within the IETF
GEOPRIV working group. Discussions at the Geopriv Interim Meeting
2003 in Washington, D.C., helped the working group to make progress
on the authorization policies based on the discussions among the
participants.
We particularly want to thank Allison Mankin <mankin@psg.com>,
Randall Gellens <rg+ietf@qualcomm.com>, Andrew Newton
<anewton@ecotroph.net>, Ted Hardie <hardie@qualcomm.com>, Jon
Peterson <jon.peterson@neustar.biz> for discussing a number of
details with us. They helped us to improve the quality of this
document.
Furthermore, we would like to thank the IETF SIMPLE working group for
their discussions of J. Rosenberg's draft on XCAP authorization
policies. We thank Stefan Berg, Christian Schmidt, Markus Isomaki,
Aki Niemi and Eva Maria Leppanen for their comments.
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Copyright (C) The Internet Society (2005). This document is subject
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except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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