Internet Engineering Task Force (IETF) H. Schulzrinne, Ed.
Request for Comments: 6772 Columbia University
Category: Standards Track H. Tschofenig, Ed.
ISSN: 2070-1721 Nokia Siemens Networks
J. Cuellar
Siemens
J. Polk
Cisco
J. Morris
M. Thomson
Microsoft
November 2012
January 2013
Geolocation Policy: A Document Format for Expressing Privacy Preferences
for Location Information
Abstract
This document defines an authorization policy language for
controlling access to location information. It extends the Common
Policy authorization framework to provide location-specific access
control. More specifically, this document defines condition elements
specific to location information in order to restrict access to data
based on the current location of the Target.
Furthermore, this document defines two algorithms for reducing the
granularity of returned location information. The first algorithm is
defined for usage with civic location information, whereas the other
one applies to geodetic location information. Both algorithms come
with limitations. There are circumstances where the amount of
location obfuscation provided is less than what is desired. These
algorithms might not be appropriate for all application domains.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6772.
Copyright Notice
Copyright (c) 2012 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Generic Processing . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Structure of Geolocation Authorization Documents . . . . . 6
3.2. Rule Transport . . . . . . . . . . . . . . . . . . . . . . 7
4. Location-Specific Conditions . . . . . . . . . . . . . . . . . 7
4.1. Geodetic Location Condition Profile . . . . . . . . . . . 8
4.2. Civic Location Condition Profile . . . . . . . . . . . . . 8
5. Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Transformations . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Set Retransmission-Allowed . . . . . . . . . . . . . . . . 9
6.2. Set Retention-Expiry . . . . . . . . . . . . . . . . . . . 9
6.3. Set Note-Well . . . . . . . . . . . . . . . . . . . . . . 10
6.4. Keep Ruleset Reference . . . . . . . . . . . . . . . . . . 10
6.5. Provide Location . . . . . . . . . . . . . . . . . . . . . 11
6.5.1. Civic Location Profile . . . . . . . . . . . . . . . . 11
6.5.2. Geodetic Location Profile . . . . . . . . . . . . . . 12
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Rule Example with Civic Location Condition . . . . . . . . 14
7.2. Rule Example with Geodetic Location Condition . . . . . . 15
7.3. Rule Example with Civic and Geodetic Location Condition . 16
7.4. Rule Example with Location-Based Transformations . . . . . 17
7.5. Location Obfuscation Example . . . . . . . . . . . . . . . 19
8. XML Schema for Basic Location Profiles . . . . . . . . . . . . 22
9. XML Schema for Geolocation Policy . . . . . . . . . . . . . . 23
10. XCAP Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.1. Application Unique ID . . . . . . . . . . . . . . . . . . 25
10.2. XML Schema . . . . . . . . . . . . . . . . . . . . . . . . 25
10.3. Default Namespace . . . . . . . . . . . . . . . . . . . . 25
10.4. MIME Media Type . . . . . . . . . . . . . . . . . . . . . 25
10.5. Validation Constraints . . . . . . . . . . . . . . . . . . 25
10.6. Data Semantics . . . . . . . . . . . . . . . . . . . . . . 25
10.7. Naming Conventions . . . . . . . . . . . . . . . . . . . . 25
10.8. Resource Interdependencies . . . . . . . . . . . . . . . . 25
10.9. Authorization Policies . . . . . . . . . . . . . . . . . . 26
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
11.1. Geolocation Policy XML Schema Registration . . . . . . . . 26
11.2. Geolocation Policy Namespace Registration . . . . . . . . 26
11.3. Geolocation Policy Location Profile Registry . . . . . . . 27
11.4. Basic Location Profile XML Schema Registration . . . . . . 27
11.5. Basic Location Profile Namespace Registration . . . . . . 28
11.6. XCAP Application Usage ID . . . . . . . . . . . . . . . . 28
12. Internationalization Considerations . . . . . . . . . . . . . 29
13. Security Considerations . . . . . . . . . . . . . . . . . . . 29
13.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 29
13.2. Obfuscation . . . . . . . . . . . . . . . . . . . . . . . 30
13.3. Algorithm Limitations . . . . . . . . . . . . . . . . . . 31
13.4. Usability . . . . . . . . . . . . . . . . . . . . . . . . 32
13.5. Limitations of Obscuring Locations . . . . . . . . . . . . 32
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
14.1. Normative References . . . . . . . . . . . . . . . . . . . 34
14.2. Informative References . . . . . . . . . . . . . . . . . . 34
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 36
Appendix B. Pseudocode . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction
Location information needs to be protected against unauthorized
access to preserve the privacy of humans. In RFC 6280 [RFC6280], a
protocol-independent model for access to geographic information is
defined. The model includes a Location Generator (LG) that
determines location information, a Location Server (LS) that
authorizes access to location information, a Location Recipient (LR)
that requests and receives location information, and a Rule Maker
(RM) that writes authorization policies. An authorization policy is
a set of rules that regulates an entity's activities with respect to
privacy-sensitive information, such as location information.
The data object containing location information in the context of
this document is referred to as a Location Object (LO). The basic
rule set defined in the Presence Information Data Format Location
Object (PIDF-LO) [RFC4119] can restrict how long the Location
Recipient is allowed to retain the information, and it can prohibit
further distribution. It also contains a reference to an enhanced
rule set and a human-readable privacy policy. The basic rule set
does not protect access to location information. It only conveys the
user's privacy preferences. This document describes an enhanced rule
set that provides richer constraints on the distribution of LOs.
The enhanced rule set allows the entity that uses the rules defined
in this document to restrict the retention and to enforce access
restrictions on location data, including prohibiting any
dissemination to particular individuals, during particular times or
when the Target is located in a specific region. The RM can also
stipulate that only certain parts of the Location Object are to be
distributed to recipients or that the resolution is reduced for parts
of the Location Object.
In the typical sequence of operations, a Location Server receives a
query for location information for a particular Target. The
authenticated identity of the Location Recipient, together with other
information provided with the request or generally available to the
server, is then used for searching through the rule set. If more
than one rule matches the condition element, then the combined
permission is evaluated according to the description in Section 10 of
[RFC4745]. The result of the rule evaluation is applied to the
location information, yielding a possibly modified Location Object
that is delivered to the Location Recipient.
This document does not describe the protocol used to convey location
information from the Location Server to the Location Recipient.
This document extends the Common Policy framework defined in
[RFC4745]. That document provides an abstract framework for
expressing authorization rules. As specified there, each such rule
consists of conditions, actions, and transformations. Conditions
determine under which circumstances the entity executing the rules,
such as a Location Server, is permitted to apply actions and
transformations. In a location information context, transformations
regulate how a Location Server modifies the information elements that
are returned to the requestor by, for example, reducing the
granularity of returned location information.
This document defines two algorithms for reducing the granularity of
returned location information. The first algorithm is defined for
usage with civic location information (see Section 6.5.1) while the
other one applies to geodetic location information (see
Section 6.5.2). Both algorithms come with limitations, i.e., they
provide location obfuscation under certain conditions and may
therefore not be appropriate for all application domains. These
limitations are documented within the Security Consideration section
(see Section 13). The geodetic transformation algorithm in
Section 6.5.2 mitigates privacy risks for both stationary and moving
Targets. However, moving Targets will reveal additional information
to an adversary. To cover applications that have more sophisticated
privacy requirements, additional algorithms may need to be defined.
This document foresees extensions in the form of new algorithms and
therefore defines a registry (see Section 11.3).
The XML schema defined in Section 9 extends the Common Policy schema
by introducing new child elements to the condition and transformation
elements. This document does not define child elements for the
action part of a rule.
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 RFC 2119 [RFC2119].
This document reuses the terminology of RFC 6280 [RFC6280], such as
Location Server (LS), Location Recipient (LR), Rule Maker (RM),
Target, Location Generator (LG), and Location Object (LO). This
document uses the following terminology:
Presentity or Target:
RFC 6280 [RFC6280] uses the term "Target" to identify the object
or person of which location information is required. The presence
model described in RFC 2778 [RFC2778] uses the term "presentity"
to describe the entity that provides presence information to a
presence service. A presentity in a presence system is a Target
in a location information system.
Watcher or Location Recipient:
The receiver of location information is the Location Recipient
(LR) in the terminology of RFC 6280 [RFC6280]. A watcher in a
presence system, i.e., an entity that requests presence
information about a presentity, is a Location Recipient in a
location information system.
Authorization policy:
An authorization policy is given by a rule set. A rule set
contains an unordered list of (policy) rules. Each rule has a
condition, an action, and a transformation component.
Permission:
The term "permission" refers to the action and transformation
components of a rule.
Location Servers:
Entities that evaluate the geolocation authorization policies.
Presence Servers:
The geolocation privacy architecture is, as described in RFC 4079
[RFC4079], aligned with the presence architecture, and a "Presence
Server" is therefore an entity that distributes location
information along with other presence-specific XML data elements.
3. Generic Processing
3.1. Structure of Geolocation Authorization Documents
A geolocation authorization document is an XML document, formatted
according to the schema defined in [RFC4745]. Geolocation
authorization documents inherit the media type of Common Policy
documents, application/auth-policy+xml. As described in [RFC4745],
this document is composed of rules that contain three parts:
conditions, actions, and transformations. Each action or
transformation, which is also called a permission, has the property
of being a positive grant of information to the Location Recipient.
As a result, there is a well-defined mechanism for combining actions
and transformations obtained from several sources. This mechanism is
privacy enabling, since the lack of any action or transformation can
only result in less information being presented to a Location
Recipient.
3.2. Rule Transport
There are two ways the authorization rules described in this document
may be conveyed between different parties:
o RFC 4119 [RFC4119] allows enhanced authorization policies to be
referenced via a Uniform Resource Locator (URL) in the 'ruleset-
reference' element. The 'ruleset-reference' element is part of
the basic rules that always travel with the Location Object.
o Authorization policies might, for example, also be stored at a
Location Server / Presence Server. The Rule Maker therefore needs
to use a protocol to create, modify, and delete the authorization
policies defined in this document. Such a protocol is available
with the Extensible Markup Language (XML) Configuration Access
Protocol (XCAP) [RFC4825].
4. Location-Specific Conditions
This section describes the location-specific conditions of a rule.
The <conditions> element contains zero or more <location-condition>
child element(s). The <conditions> element only evaluates to TRUE if
all child elements evaluate to TRUE; therefore, multiple <location-
condition> elements are not normally useful.
The <location-condition> element MUST contain at least one <location>
child element. The <location-condition> element evaluates to TRUE if
any of its child <location> elements matches the location of the
Target, i.e., <location> elements are combined using a logical OR.
The three attributes of <location> are 'profile', 'xml:lang', and
'label'. The 'profile' indicates the location profile that is
included as child elements in the <location> element. Two location
profiles, geodetic and civic, are defined in Sections 4.1 and 4.2.
Each profile describes under what conditions a <location> element
evaluates to TRUE.
The 'label' attribute allows a human-readable description to be added
to each <location> element. The 'xml:lang' attribute contains a
language tag providing further information for rendering of the
content of the 'label' attribute.
The <location-condition> and the <location> elements provide
extension points. If an extension is not understood by the entity
evaluating the rules, then this rule evaluates to FALSE. This causes
a <conditions> element to evaluate to FALSE if a <location-condition>
element is unsupported. A <location-condition> is considered TRUE if
any of the <location> elements understood by the rule evaluator is
TRUE.
4.1. Geodetic Location Condition Profile
The geodetic location profile is identified by the token 'geodetic-
condition'. Rule Makers use this profile by placing a Geography
Markup Language [GML] <Circle> element within the <location> element
(as described in Section 5.2.3 of [RFC5491]).
The <location> element containing the information for the geodetic
location profile evaluates to TRUE if the current location of the
Target is completely within the described location (see Section
6.1.15.3 of [OGC-06-103r4]). Note that the Target's actual location
might be represented by any of the location shapes described in
[RFC5491]. If the geodetic location of the Target is unknown, then
the <location> element containing the information for the geodetic
location profile evaluates to FALSE.
Implementations MUST support the World Geodetic System 1984 (WGS 84)
[NIMA.TR8350.2-3e] coordinate reference system using the formal
identifier from the European Petroleum Survey Group (EPSG) Geodetic
Parameter Dataset (as formalized by the Open Geospatial Consortium
(OGC)):
2D: WGS 84 (latitude, longitude), as identified by the URN
"urn:ogc:def:crs:EPSG::4326". This is a two-dimensional CRS.
A Coordinate Reference System (CRS) MUST be specified using the above
URN notation only; implementations do not need to support user-
defined CRSs.
Implementations MUST specify the CRS using the "srsName" attribute on
the outermost geometry element. The CRS MUST NOT be changed for any
sub-elements. The "srsDimension" attribute MUST be omitted, since
the number of dimensions in these CRSs is known.
4.2. Civic Location Condition Profile
The civic location profile is identified by the token 'civic-
condition'. Rule Makers use this profile by placing a <civicAddress>
element, defined in [RFC5139], within the <location> element.
All child elements of a <location> element that carry <civicAddress>
elements MUST evaluate to TRUE (i.e., logical AND) in order for the
<location> element to evaluate to TRUE. For each child element, the
value of that element is compared to the value of the same element in
the Target's civic location. The child element evaluates to TRUE if
the two values are identical based on an octet-by-octet comparison.
A <location> element containing a <civic-condition> profile evaluates
to FALSE if a civic address is not present for the Target. For
example, this could occur if location information has been removed by
other rules or other transmitters of location information or if only
the geodetic location is known. In general, it is RECOMMENDED
behavior for an LS not to apply a translation from geodetic location
to civic location (i.e., geocode the location).
5. Actions
This document does not define location-specific actions.
6. Transformations
This document defines several elements that allow Rule Makers to
specify transformations that
o reduce the accuracy of the returned location information, and
o set the basic authorization policies carried inside the PIDF-LO.
6.1. Set Retransmission-Allowed
This element specifies a change to or the creation of a value for the
<retransmission-allowed> element in the PIDF-LO. The data type of
the <set-retransmission-allowed> element is a boolean.
If the value of the <set-retransmission-allowed> element is set to
TRUE, then the <retransmission-allowed> element in the PIDF-LO MUST
be set to TRUE. If the value of the <set-retransmission-allowed>
element is set to FALSE, then the <retransmission-allowed> element in
the PIDF-LO MUST be set to FALSE.
If the <set-retransmission-allowed> element is absent, then the value
of the <retransmission-allowed> element in the PIDF-LO MUST be kept
unchanged, or if the PIDF-LO is created for the first time, then the
value MUST be set to FALSE.
6.2. Set Retention-Expiry
This transformation asks the LS to change or set the value of the
<retention-expiry> element in the PIDF-LO. The data type of the
<set-retention-expiry> element is a non-negative integer.
The value provided with the <set-retention-expiry> element indicates
seconds, and these seconds are added to the time that the LS provides
location. A value of zero requests that the information is not
retained.
If the <set-retention-expiry> element is absent, then the value of
the <retention-expiry> element in the PIDF-LO is kept unchanged, or
if the PIDF-LO is created for the first time, then the value MUST be
set to the current date.
6.3. Set Note-Well
This transformation asks the LS to change or set the value of the
<note-well> element in the PIDF-LO. The data type of the <set-note-
well> element is a string.
The value provided with the <set-note-well> element contains a
privacy statement as a human-readable text string, and an 'xml:lang'
attribute denotes the language of the human-readable text.
If the <set-note-well> element is absent, then the value of the
<note-well> element in the PIDF-LO is kept unchanged, or if the
PIDF-LO is created for the first time, then no content is provided
for the <note-well> element.
6.4. Keep Ruleset Reference
This transformation specifies whether the <external-ruleset> element
in the PIDF-LO carries the extended authorization rules defined in
[RFC4745]. The data type of the <keep-rule-reference> element is
boolean.
If the value of the <keep-rule-reference> element is set to TRUE,
then the <external-ruleset> element in the PIDF-LO is kept unchanged
when included. If the value of the <keep-rule-reference> element is
set to FALSE, then the <external-ruleset> element in the PIDF-LO MUST
NOT contain a reference to an external rule set. The reference to
the ruleset is removed, and no rules are carried as MIME bodies (in
case of Content-ID (cid:) URIs [RFC2392]).
If the <keep-rule-reference> element is absent, then the value of the
<external-ruleset> element in the PIDF-LO is kept unchanged when
available, or if the PIDF-LO is created for the first time, then the
<external-ruleset> element MUST NOT be included.
6.5. Provide Location
The <provide-location> element contains child elements of a specific
location profile that controls the granularity of returned location
information. This form of location granularity reduction is also
called 'obfuscation' and is defined in [DUCKHAM05] as
the means of deliberately degrading the quality of information
about an individual's location in order to protect that
individual's location privacy.
Location obscuring presents a number of technical challenges. The
algorithms provided in this document are provided as examples only.
A discussion of the technical constraints on location obscuring is
included in Section 13.5.
The functionality of location granularity reduction depends on the
type of location provided as input. This document defines two
profiles for reduction, namely:
o civic-transformation: If the <provide-location> element has a
<provide-civic> child element, then civic location information is
disclosed as described in Section 6.5.1, subject to availability.
o geodetic-transformation: If the <provide-location> element has a
<provide-geo> child element, then geodetic location information is
disclosed as described in Section 6.5.2, subject to availability.
The <provide-location> element MUST contain the 'profile' attribute
if it contains child elements, and the child elements MUST be
appropriate for the profile.
If the <provide-location> element has no child elements, then civic
as well as geodetic location information is disclosed without
reducing its granularity, subject to availability. In this case, the
profile attribute MUST NOT be included.
6.5.1. Civic Location Profile
This profile uses the token 'civic-transformation'. This profile
allows civic location transformations to be specified by means of the
<provide-civic> element that restricts the level of civic location
information the LS is permitted to disclose. The symbols of these
levels are: 'country', 'region', 'city', 'building', and 'full'.
Each level is given by a set of civic location data items such as
<country> and <A1>, ..., <POM>, as defined in [RFC5139]. Each level
includes all elements included by the lower levels.
The 'country' level includes only the <country> element; the 'region'
level adds the <A1> element; the 'city' level adds the <A2> and <A3>
elements; the 'building' level and the 'full' level add further civic
location data as shown below.
full
{<country>, <A1>, <A2>, <A3>, <A4>, <A5>, <A6>, <PRD>, <POD>,
<STS>, <HNO>, <HNS>, <LMK>, <LOC>, <PC>, <NAM>, <FLR>,
<BLD>,<UNIT>,<ROOM>,<PLC>, <PCN>, <POBOX>, <ADDCODE>, <SEAT>
<RD>, <RDSEC>, <RDBR>, <RDSUBBR>, <PRM>, <POM>}
|
|
building
{<country>, <A1>, <A2>, <A3>, <A4>, <A5>, <A6>, <PRD>
<POD>, <STS>, <HNO>, <HNS>, <LMK>, <PC>,
<RD>, <RDSEC>, <RDBR>, <RDSUBBR> <PRM>, <POM>}
|
|
city
{<country>, <A1>, <A2>, <A3>}
|
|
region
{<country>, <A1>}
|
|
country
{<country>}
|
|
none
{}
The default value is "none".
The schema of the <provide-civic> element is defined in Section 8.
6.5.2. Geodetic Location Profile
This profile uses the token 'geodetic-transformation' and refers only
to the Coordinate Reference System (CRS) WGS 84
(urn:ogc:def:crs:EPSG::4326, 2D). This profile allows geodetic
location transformations to be specified by means of the <provide-
geo> element that may restrict the returned geodetic location
information based on the value provided in the 'radius' attribute.
The value of the 'radius' attribute expresses the radius in meters.
The schema of the <provide-geo> element is defined in Section 8.
The algorithm proceeds in six steps. The first two steps are
independent of the measured position to be obscured and should be run
only once or very infrequently for each region and desired
uncertainty. The steps are:
1. Choose a geodesic projection with Cartesian coordinates and a
surface you want to cover. Limit the worst-case distortion of
the map as noted below.
2. Given a desired uncertainty radius "d", choose a grid of so-
called "landmarks" at a distance of at least d units apart from
each other.
3. Given a measured location M=(m,n) on the surface, calculate its 4
closest landmarks on the grid, with coordinates: SW = (l,b),
SE=(r,b), NW=(l,t), NE=(r,t). Thus, l<=m<r and b<=n<t. See
notes below.
4. Let x=(m-l)/(r-l) and y=(n-b)/(t-b).
x and y are thus the scaled local coordinates of the point M in
the small grid square that contains it, where x and y range
between 0 and 1.
5. Let p = 0.2887 (=sqrt(3)/6) and q = 0.7113 (=1-p). Determine
which of the following eight cases holds:
C1. x < p and y < p
C2. p <= x < q and y < x and y < 1-x
C3. q <= x and y < p
C4. p <= y < q and x <= y and y < 1-x
C5. p <= y < q and y < x and 1-x <= y
C6. x < p and q <= y
C7. p <= x < q and x <= y and 1-x <= y
C8. q <= x and q <= y
6. Depending on the case, let C (=Center) be
C1: SW
C2: SW or SE
C3: SE
C4: SW or NW
C5: SE or NE
C6: NW
C7: NW or NE
C8: NE
Return the circle with center C and radius d.
Notes:
Regarding Step 1:
The scale of a map is the ratio of a distance (a straight line) on
the map to the corresponding air distance on the ground. For maps
covering larger areas, a map projection from a sphere (or
ellipsoid) to the plane will introduce distortion, and the scale
of the map is not constant. Also, note that the real distance on
the ground is taken along great circles, which may not correspond
to straight lines on the map, depending on the projection used.
Let us measure the (length) distortion of the map as the quotient
between the maximal and the minimal scales on the map. The
distortion MUST be below 1.5. (The minimum distortion is 1.0: if
the region of the map is small, then the scale may be taken as a
constant over the whole map).
Regarding Step 3:
SW is mnemonic for southwest, b for bottom, l for left (SW=(l,b)),
etc., but the directions of the geodesic projection may be
arbitrary, and thus SW may not be southwest of M, but it will be
left and below M *on the map*.
7. Examples
This section provides a few examples for authorization rules using
the extensions defined in this document.
7.1. Rule Example with Civic Location Condition
This example illustrates a single rule that employs the civic
location condition. It matches if the current location of the Target
equals the content of the child elements of the <location> element.
Requests match only if the Target is at a civic location with country
set to 'Germany', state (A1) set to 'Bavaria', city (A3) set to
'Munich', city division (A4) set to 'Perlach', street name (A6) set
to 'Otto-Hahn-Ring', and house number (HNO) set to '6'.
No actions and transformation child elements are provided in this
rule example. The actions and transformation could include presence-
specific information when the Geolocation Policy framework is applied
to the Presence Policy framework (see [RFC5025]).
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy"
xmlns:gp="urn:ietf:params:xml:ns:geolocation-policy">
<rule id="AA56i09">
<conditions>
<gp:location-condition>
<gp:location
profile="civic-condition"
xml:lang="en"
label="Siemens Neuperlach site 'Legoland'"
xmlns="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr">
<country>DE</country>
<A1>Bavaria</A1>
<A3>Munich</A3>
<A4>Perlach</A4>
<A6>Otto-Hahn-Ring</A6>
<HNO>6</HNO>
</gp:location>
</gp:location-condition>
</conditions>
<actions/>
<transformations/>
</rule>
</ruleset>
7.2. Rule Example with Geodetic Location Condition
This example illustrates a rule that employs the geodetic location
condition. The rule matches if the current location of the Target is
inside the area specified by the polygon. The polygon uses the EPSG
4326 coordinate reference system. No altitude is included in this
example.
<?xml version="1.0" encoding="UTF-8"?>
<ruleset
xmlns="urn:ietf:params:xml:ns:common-policy"
xmlns:gp="urn:ietf:params:xml:ns:geolocation-policy"
xmlns:gml="http://www.opengis.net/gml"
xmlns:gs="http://www.opengis.net/pidflo/1.0">
<rule id="BB56A19">
<conditions>
<gp:location-condition>
<gp:location
xml:lang="en"
label="Sydney Opera House"
profile="geodetic-condition">
<gs:Circle srsName="urn:ogc:def:crs:EPSG::4326">
<gml:pos>-33.8570029378 151.2150070761</gml:pos>
<gs:radius uom="urn:ogc:def:uom:EPSG::9001">1500
</gs:radius>
</gs:Circle>
</gp:location>
</gp:location-condition>
</conditions>
<transformations/>
</rule>
</ruleset>
7.3. Rule Example with Civic and Geodetic Location Condition
This example illustrates a rule that employs a mixed civic and
geodetic location condition. Depending on the available type of
location information, namely civic or geodetic location information,
one of the location elements may match.
<?xml version="1.0" encoding="UTF-8"?>
<ruleset
xmlns="urn:ietf:params:xml:ns:common-policy"
xmlns:gp="urn:ietf:params:xml:ns:geolocation-policy"
xmlns:gml="http://www.opengis.net/gml"
xmlns:gs="http://www.opengis.net/pidflo/1.0">
<rule id="AA56i09">
<conditions>
<gp:location-condition>
<gp:location profile="civic-condition"
xmlns="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr">
<country>DE</country>
<A1>Bavaria</A1>
<A3>Munich</A3>
<A4>Perlach</A4>
<A6>Otto-Hahn-Ring</A6>
<HNO>6</HNO>
</gp:location>
<gp:location profile="geodetic-condition">
<gs:Circle srsName="urn:ogc:def:crs:EPSG::4326">
<gml:pos>-34.410649 150.87651</gml:pos>
<gs:radius uom="urn:ogc:def:uom:EPSG::9001">1500
</gs:radius>
</gs:Circle>
</gp:location>
</gp:location-condition>
</conditions>
<actions/>
<transformations/>
</rule>
</ruleset>
7.4. Rule Example with Location-Based Transformations
This example shows the transformations specified in this document.
The <provide-civic> element indicates that the available civic
location information is reduced to building level granularity. If
geodetic location information is requested, then a granularity
reduction is provided as well.
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy"
xmlns:gp="urn:ietf:params:xml:ns:geolocation-policy"
xmlns:lp="urn:ietf:params:xml:ns:basic-location-profiles">
<rule id="AA56i09">
<conditions/>
<actions/>
<transformations>
<gp:set-retransmission-allowed>false
</gp:set-retransmission-allowed>
<gp:set-retention-expiry>86400</gp:set-retention-expiry>
<gp:set-note-well xml:lang="en">My privacy policy goes here.
</gp:set-note-well>
<gp:keep-rule-reference>false
</gp:keep-rule-reference>
<gp:provide-location
profile="civic-transformation">
<lp:provide-civic>building</lp:provide-civic>
</gp:provide-location>
<gp:provide-location
profile="geodetic-transformation">
<lp:provide-geo radius="500"/>
</gp:provide-location>
</transformations>
</rule>
</ruleset>
The following rule describes the shorthand notation for making the
current location of the Target available to Location Recipients
without granularity reduction.
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy"
xmlns:gp="urn:ietf:params:xml:ns:geolocation-policy">
<rule id="AA56ia9">
<conditions/>
<actions/>
<transformations>
<gp:provide-location/>
</transformations>
</rule>
</ruleset>
7.5. Location Obfuscation Example
Suppose you want to obscure positions in the continental USA.
Step 1:
First, you choose a geodesic projection. If you are measuring
location as latitude and longitude, a natural choice is to take a
rectangular projection. One latitudinal degree corresponds to
approximately 110.6 kilometers, while a good approximation of a
longitudinal degree at latitude phi is (pi/180)*M*cos(phi), where
pi is approximately 3.1415, and M is the Earth's average
meridional radius, approximately 6,367.5 km. For instance, one
longitudinal degree at 30 degrees (say, New Orleans) is 96.39 km,
while the formula given offers an estimation of 96.24, which is
good enough for our purposes.
We will set up a grid not only for the continental USA, but for
the whole earth between latitudes 25 and 50 degrees, and thus will
cover also the Mediterranean, South Europe, Japan, and the north
of China. As will be seen below, the grid distortion (for not too
large grids in this region) is approx cos(25)/cos(50), which is
1.4099.
As origin of our grid, we choose the point at latitude 25 degrees
and longitude 0 (Greenwich). The latitude 25 degrees is chosen to
be just south of Florida and thus south of the continental USA.
(On the Southern Hemisphere, the origin should be north of the
region to be covered; if the region crosses the Equator, the
origin should be on the Equator. In this way, it is guaranteed
that the latitudinal degree has the largest distance at the
latitude of the origin).
At 25 degrees, one degree in east-west direction corresponds to
approximately (pi/180)*M*cos(25) = 100.72 km.
The same procedure, basically, produces grids for
* 45 degrees south to 45 degrees north: Tropics and subtropics,
Africa, Australia
* 25 to 50 degrees (both north or south): Continental United
States, Mediterranean, most of China; most of Chile and
Argentina, New Zealand
* 35 to 55 degrees (both north or south): Southern and Central
Europe
* 45 to 60 degrees (both north or south): Central and Northern
Europe, Canada
* 55 to 65 degrees (both north or south): most of Scandinavia
* 60 to 70 degrees (both north or south): Alaska
Since we do not want to change the grid system often (this would
leak more information about obscured locations when they are
repeatedly visited), the algorithm should prefer to use the grids
discussed above, with origin at the Greenwich meridian and at
latitudes o=0, o=25, o=35, o=45, 0=55, and o=60 degrees (north) or
at latitudes o=-25, o=-35, o=-45, 0=-55, and o=-60 degrees (the
minus to indicate "south").
Our choice for the continental USA is o=25.
For locations close to the poles, a different projection should be
used (not discussed here).
Step 2:
To construct the grid, we start with our chosen origin and place
grid points at regular intervals along each of the axes (north-
south and east-west) with a distance d between each.
We will now construct a grid for a desired uncertainty of d =
100km. At our origin, 100 km correspond roughly to d1 = 100/
100.72 = 0.993 degrees in an east-west direction and to d2 = 100/
110.6 = 0.904 degrees in a north-south direction.
The (i,j)-point in the grid (i and j are integers) has longitude
d1*i and latitude 25+d2*j, measured in degrees. More generally,
if the grid has origin at coordinates (0,o), measured in degrees,
the (i,j)-point in the grid has coordinates (longitude = d1*i,
latitude = o+d2*j). The grid has almost no distortion at the
latitude of the origin, but it does as we go further away from it.
The distance between two points in the grid at 25 degrees latitude
is indeed approximately 100 km, but just above the Canadian
border, on the 50th degree, it is 0.993*(pi/180)*M*cos(50) =
70.92km. Thus, the grid distortion is 100/70.92 = 1.41, which is
acceptable (<1.5). (In the north-south direction, the grid has
roughly no distortion; the vertical distance between two
neighboring grid points is approximately 100 km).
Step 3:
Now suppose you measure a position at M, with longitude -105 (the
minus sign is used to denote 105 degrees *west*; without minus,
the point is in China, 105 degrees east) and latitude 40 degrees
(just north of Denver, CO). The point M is 105 degrees west and
15 degrees north of our origin (which has longitude 0 and latitude
25).
Let "floor" be the function that returns the largest integer
smaller or equal to a floating point number. To calculate SW, the
closest point of the grid on the southwest of M=(m,n), we
calculate
i= floor(m/d1) = floor(-105/0.993) = -106
j= floor(n-o/d2) = floor(15/0.904) = 16
Those are the indexes of SW on the grid. The coordinates of SW
are then: (d1*i, 25+d2*j) = (-105.242, 39.467).
Thus:
l=d1*floor(m/d1) = -105.243
r=l+d1 = -105.243+0.993 = -104.250
b=o+d2*floor(n-o/d2) = 39.467
t=b+d2 = 39.467+0.904 = 40.371
These are the formulas for l, r, b, and t in the general case of
Cartesian projections based on latitude and longitude.
Step 4:
Calculate x and y, the local coordinates of the point M in the
small grid square that contains it. This is easy:
x=(m-l)/(r-l) = [-105 -(-105.243)]/0.993 = 0.245
y=(n-b)/(t-b) = [40 - 39.467]/0.904 = 0.590
Step 5:
First, compare x with p (0.2887) and 1-p (0.7113). x is smaller
than p. Therefore, only cases 1, 4, or 6 could hold.
Also, compare y with p (0.2887) and 1-p (0.7113). y is between
them: p <= y < q. Thus, we must be in case 4. To check, compare
y (0.59) with x (0.245) and 1-x. y is larger than x and smaller
than 1-x. We are in case C4 (p <= y < q and x <= y and y < 1-x).
Step 6:
Now we choose either SW or NW as the center of the circle.
The obscured location is the circle with radius 100 km and center
in SW (coordinates: -105.243, 39.467) or NW (coordinates:
-105.243, 40.371).
8. XML Schema for Basic Location Profiles
This section defines the location profiles used as child elements of
the transformation element.
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema
targetNamespace="urn:ietf:params:xml:ns:basic-location-profiles"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<!-- profile="civic-transformation" -->
<xs:element name="provide-civic" default="none">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="full"/>
<xs:enumeration value="building"/>
<xs:enumeration value="city"/>
<xs:enumeration value="region"/>
<xs:enumeration value="country"/>
<xs:enumeration value="none"/>
</xs:restriction>
</xs:simpleType>
</xs:element>
<!-- profile="geodetic-transformation" -->
<xs:element name="provide-geo">
<xs:complexType>
<xs:attribute name="radius" type="xs:integer"/>
</xs:complexType>
</xs:element>
</xs:schema>
9. XML Schema for Geolocation Policy
This section presents the XML schema that defines the Geolocation
Policy schema described in this document. The Geolocation Policy
schema extends the Common Policy schema (see [RFC4745]).
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema
targetNamespace="urn:ietf:params:xml:ns:geolocation-policy"
xmlns:gp="urn:ietf:params:xml:ns:geolocation-policy"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<!-- Import Common Policy-->
<xs:import namespace="urn:ietf:params:xml:ns:common-policy"/>
<!-- This import brings in the XML language attribute xml:lang-->
<xs:import namespace="http://www.w3.org/XML/1998/namespace"
schemaLocation="http://www.w3.org/2001/xml.xsd"/>
<!-- Geopriv Conditions -->
<xs:element name="location-condition"
type="gp:locationconditionType"/>
<xs:complexType name="locationconditionType">
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:choice minOccurs="1" maxOccurs="unbounded">
<xs:element name="location" type="gp:locationType"
minOccurs="1" maxOccurs="unbounded"/>
<xs:any namespace="##other" processContents="lax"
minOccurs="0" maxOccurs="unbounded"/>
</xs:choice>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
<xs:complexType name="locationType">
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:choice minOccurs="1" maxOccurs="unbounded">
<xs:any namespace="##other" processContents="lax"
minOccurs="0" maxOccurs="unbounded"/>
</xs:choice>
<xs:attribute name="profile" type="xs:string"/>
<xs:attribute name="label" type="xs:string"/>
<xs:attribute ref="xml:lang" />
</xs:restriction>
</xs:complexContent>
</xs:complexType>
<!-- Geopriv transformations -->
<xs:element name="set-retransmission-allowed"
type="xs:boolean" default="false"/>
<xs:element name="set-retention-expiry"
type="xs:integer" default="0"/>
<xs:element name="set-note-well"
type="gp:notewellType"/>
<xs:element name="keep-rule-reference"
type="xs:boolean" default="false"/>
<xs:element name="provide-location"
type="gp:providelocationType"/>
<xs:complexType name="notewellType">
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attribute ref="xml:lang" />
</xs:extension>
</xs:simpleContent>
</xs:complexType>
<xs:complexType name="providelocationType">
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:choice minOccurs="0" maxOccurs="unbounded">
<xs:any namespace="##other" processContents="lax"
minOccurs="0" maxOccurs="unbounded"/>
</xs:choice>
<xs:attribute name="profile" type="xs:string" />
</xs:restriction>
</xs:complexContent>
</xs:complexType>
</xs:schema>
10. XCAP Usage
This section defines the details necessary for clients to manipulate
geolocation privacy documents from a server using XCAP. If used as
part of a presence system, it uses the same Application Unique ID
(AUID) as those rules. See [RFC5025] for a description of the XCAP
usage in context with presence authorization rules.
10.1. Application Unique ID
XCAP requires application usages to define a unique Application
Unique ID (AUID) in either the IETF tree or a vendor tree. This
specification defines the "geolocation-policy" AUID within the IETF
tree, via the IANA registration in Section 11.
10.2. XML Schema
XCAP requires application usages to define a schema for their
documents. The schema for geolocation authorization documents is
described in Section 9.
10.3. Default Namespace
XCAP requires application usages to define the default namespace for
their documents. The default namespace is
urn:ietf:params:xml:ns:geolocation-policy.
10.4. MIME Media Type
XCAP requires application usages to define the MIME media type for
documents they carry. Geolocation privacy authorization documents
inherit the MIME type of Common Policy documents, application/
auth-policy+xml.
10.5. Validation Constraints
This specification does not define additional constraints.
10.6. Data Semantics
This document discusses the semantics of a geolocation privacy
authorization.
10.7. Naming Conventions
When a Location Server receives a request to access location
information of some user foo, it will look for all documents within
http://[xcaproot]/geolocation-policy/users/foo and use all documents
found beneath that point to guide authorization policy.
10.8. Resource Interdependencies
This application usage does not define additional resource
interdependencies.
10.9. Authorization Policies
This application usage does not modify the default XCAP authorization
policy, which is that only a user can read, write, or modify his/her
own documents. A server can allow privileged users to modify
documents that they do not own, but the establishment and indication
of such policies is outside the scope of this document.
11. IANA Considerations
There are several IANA considerations associated with this
specification.
11.1. Geolocation Policy XML Schema Registration
This section registers an XML schema in the IETF XML Registry as per
the guidelines in [RFC3688].
URI: urn:ietf:params:xml:schema:geolocation-policy
Registrant Contact: IETF Geopriv Working Group (geopriv@ietf.org),
Hannes Tschofenig (hannes.tschofenig@nsn.com).
XML: The XML schema to be registered is contained in Section 9. Its
first line is
<?xml version="1.0" encoding="UTF-8"?>
and its last line is
</xs:schema>
11.2. Geolocation Policy Namespace Registration
This section registers a new XML namespace in the IETF XML Registry
as per the guidelines in [RFC3688].
URI: urn:ietf:params:xml:ns:geolocation-policy
Registrant Contact: IETF Geopriv Working Group (geopriv@ietf.org),
Hannes Tschofenig (hannes.tschofenig@nsn.com).
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>Geolocation Policy Namespace</title>
</head>
<body>
<h1>Namespace for Geolocation Authorization Policies</h1>
<h2>urn:ietf:params:xml:schema:geolocation-policy</h2>
<p>See <a href="http://www.rfc-editor.org/rfc/rfc6772.txt">
RFC 6772</a>.</p>
</body>
</html>
END
11.3. Geolocation Policy Location Profile Registry
This document creates a registry of location profile names for the
Geolocation Policy framework. Profile names are XML tokens. This
registry will operate in accordance with RFC 5226 [RFC5226],
Specification Required.
This document defines the following profile names:
geodetic-condition: Defined in Section 4.1.
civic-condition: Defined in Section 4.2.
geodetic-transformation: Defined in Section 6.5.2.
civic-transformation: Defined in Section 6.5.1.
11.4. Basic Location Profile XML Schema Registration
This section registers an XML schema in the IETF XML Registry as per
the guidelines in [RFC3688].
URI: urn:ietf:params:xml:schema:basic-location-profiles
Registrant Contact: IETF Geopriv Working Group (geopriv@ietf.org),
Hannes Tschofenig (hannes.tschofenig@nsn.com).
XML: The XML schema to be registered is contained in Section 8. Its
first line is
<?xml version="1.0" encoding="UTF-8"?>
and its last line is
</xs:schema>
11.5. Basic Location Profile Namespace Registration
This section registers a new XML namespace in the IETF XML Registry
as per the guidelines in [RFC3688].
URI: urn:ietf:params:xml:ns:basic-location-profiles
Registrant Contact: IETF Geopriv Working Group (geopriv@ietf.org),
Hannes Tschofenig (hannes.tschofenig@nsn.com).
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>Basic Location Profile Namespace</title>
</head>
<body>
<h1>Namespace for Basic Location Profile</h1>
<h2>urn:ietf:params:xml:schema:basic-location-profiles</h2>
<p>See <a href="http://www.rfc-editor.org/rfc/rfc6772.txt">
RFC 6772</a>.</p>
</body>
</html>
END
11.6. XCAP Application Usage ID
This section registers an XCAP Application Unique ID (AUID) in the
"XML-XCAP Application Unique IDs" registry according to the IANA
procedures defined in [RFC4825].
Name of the AUID: geolocation-policy
Description: Geolocation privacy rules are documents that describe
the permissions that a Target has granted to Location Recipients that
access information about his/her geographic location.
12. Internationalization Considerations
The policies described in this document are mostly meant for machine-
to-machine communications; as such, many of its elements are tokens
not meant for direct human consumption. If these tokens are
presented to the end user, some localization may need to occur. The
policies are, however, supposed to be created with the help of
humans, and some of the elements and attributes are subject to
internationalization considerations. The content of the <label>
element is meant to be provided by a human (the Rule Maker) and also
displayed to a human. Furthermore, the location condition element
(<location-condition>, using the civic location profile, see
Section 4.2) and the <set-note-well> element (see Section 6.3) may
contain non-US-ASCII letters.
The geolocation policies utilize XML, and all XML processors are
required to understand UTF-8 and UTF-16 encodings. Therefore, all
entities processing these policies MUST understand UTF-8- and UTF-16-
encoded XML. Additionally, geolocation policy-aware entities MUST
NOT encode XML with encodings other than UTF-8 or UTF-16.
13. Security Considerations
13.1. Introduction
This document aims to allow users to prevent unauthorized access to
location information and to restrict access to information dependent
on the location of the Target, using location-based conditions. This
is accomplished using authorization policies. This work builds on a
series of other documents: security requirements are described in
[RFC6280] and a discussion of generic security threats is available
with [RFC3694]. Aspects of combining permissions in cases of
multiple occurrence are addressed in [RFC4745].
In addition to the authorization policies, mechanisms for obfuscating
location information are described. A theoretical treatment of
location obfuscation is provided in [DUCKHAM05] and in [IFIP07].
[DUCKHAM05] provides the foundation, and [IFIP07] illustrates three
different types of location obfuscation by enlarging the radius, by
shifting the center, and by reducing the radius. The algorithm in
Section 6.5.2 for geodetic location information obfuscation uses
these techniques.
The requirements for protecting privacy-sensitive location
information vary. The two obfuscation algorithms in this document
provide a basis for protecting against unauthorized disclosure of
location information, but they have limitations. Application and
user requirements vary widely; therefore, an extension mechanism is
support for defining and using different algorithms.
13.2. Obfuscation
Whenever location information is returned to a Location Recipient, it
contains the location of the Target. This is also true when location
is obfuscated, i.e., the Location Server does not lie about the
Target's location but instead hides it within a larger location
shape. Even without the Target's movement, there is a danger that
information will be revealed over time. While the Target's location
is not revealed within a particular region of the grid, the size of
that returned region matters as well as the precise location of the
Target within that region. Returning location shapes that are
randomly computed will over time reveal more and more information
about the Target.
Consider Figure 1, which shows three ellipses, a dotted area in the
middle, and the Target's true location marked as 'x'. The ellipses
illustrate the location shapes as received by a potential Location
Recipient over time for requests of a Target's location information.
Collecting information about the returned location information over
time allows the Location Recipient to narrow the potential location
of the Target down to the dotted area in the center of the graph.
For this purpose, the algorithm described in Section 6.5.2 uses a
grid that ensures the same location information is reported while the
Target remains in the same geographical area.
,-----.
,----,-'. `-.
,-' / `-. \
,' / _...._ `. \
/ ,-'......`._\ :
; /|...........\: |
| / :.....x......+ ;
: | \...........;| /
\ | \........./ | /
`. \ `-.....,' ,''
'-.\ `-----'|
``.-----' ,'
`._ _,'
`'''
Figure 1: Obfuscation: A Static Target
An obscuring method that returns different results for consecutive
requests can be exploited by recipients wishing to use this property.
Rate limiting the generation of new obscured locations or providing
the same obscured location to recipients for the same location might
limit the information that can be obtained. Note, however, that
providing a new obscured location based on a change in location
provides some information to recipients when they observe a change in
location.
When the Target is moving, then the location transformations reveal
information when switching from one privacy region to another one.
For example, when a transformation indicates that civic location is
provided at a 'building' level of granularity, floor levels, room
numbers, and other details normally internal to a building would be
hidden. However, when the Target moves from one building to the next
one, then the movement would still be recognizable as the disclosed
location information would be reflected by the new civic location
information indicating the new building. With additional knowledge
about building entrances and floor plans, it would be possible to
learn additional information.
13.3. Algorithm Limitations
The algorithm presented in Section 6.5.2 has some issues where
information is leaked: when moving, when switching from one privacy
region to another one, and also when the user regularly visits the
same location.
The first issue arises if the algorithm provides different location
information (privacy region) only when the previous one becomes
inapplicable. The algorithm discloses new information the moment
that the Target is on the border of the old privacy region.
Another issue arises if the algorithm produces the different values
for the same location that is repeatedly visited. Suppose a user
goes home every night. If the reported obfuscated locations are all
randomly chosen, an analysis can reveal the home location with high
precision.
In addition to these concerns, the combination of an obscured
location with public geographic information (highways, lakes,
mountains, cities, etc.) may yield much more precise location
information than is desired. But even without it, just observing
movements, once or multiple times, any obscuring algorithm can leak
information about velocities or positions. Suppose a user wants to
disclose location information with a radius of r. The privacy
region, a circle with that radius, has an area of A = pi * r^2. An
adversary, observing the movements, will deduce that the target is
visiting, was visiting, or regularly visits, a region of size A1,
smaller than A. The ratio A1/A should be, even in the worst case,
larger than a fixed known number, in order that the user can predict
the worst-case information leakage. The choices of Section 6.5.2 are
such that this maximum leakage can be established: by any statistical
procedures, without using external information (highways, etc., as
discussed above), the quotient A1/A is larger than 0.13 (= 1/(5*1.5)
). Thus, for instance, when choosing a provided location of size
1000 km^2, he will be leaking, in worst case, the location within a
region of size 130 km^2.
13.4. Usability
There is the risk that end users are specifying their location-based
policies in such a way that very small changes in location yields a
significantly different level of information disclosure. For
example, a user might want to set authorization policies differently
when they are in a specific geographical area (e.g., at home, in the
office). Location might be the only factor in the policy that
triggers a very different action and transformation to be executed.
The accuracy of location information is not always sufficient to
unequivocally determine whether a location is within a specific
boundary [GEOPRIV-UNCERTAINTY]. In some situations, uncertainty in
location information could produce unexpected results for end users.
Providing adequate user feedback about potential errors arising from
these limitation can help prevent unintentional information leakage.
Users might create policies that are nonsensical. To avoid such
cases, the software used to create the authorization policies should
perform consistency checks, and when authorization policies are
uploaded to the policy servers, then further checks can be performed.
When XCAP is used to upload authorization policies, then built-in
features of XCAP can be utilized to convey error messages back to the
user about an error condition. Section 8.2.5 of [RFC4825] indicates
that some degree of application-specific checking is provided when
authorization policies are added, modified, or deleted. The XCAP
protocol may return a 409 response with a response that may contain a
detailed conflict report containing the <constraint-failure> element.
A human-readable description of the problem can be indicated in the
'phrase' attribute of that element.
13.5. Limitations of Obscuring Locations
Location-obscuring attempts to remove information about the location
of a Target. The effectiveness of location obscuring is determined
by how much uncertainty a Location Recipient has about the location
of the Target. A location-obscuring algorithm is effective if the
Location Recipient cannot recover a location with better uncertainty
than the obscuring algorithm was instructed to add.
Effective location obscuring is difficult. The amount of information
that can be recovered by a determined and resourceful Location
Recipient can be considerably more than is immediately apparent. A
concise summary of the challenges is included in [DUCKHAM10].
A Location Recipient in possession of external information about the
Target or geographical area that is reported can make assumptions or
guesses aided by that information to recover more accurate location
information. This is true even when a single location is reported,
but it is especially true when multiple locations are reported for
the same Target over time.
Furthermore, a Location Recipient that attempts to recover past
locations for a Target can use later-reported locations to further
refine any recovered location. A location-obscuring algorithm
typically does not have any information about the future location of
the Target.
The degree to which location information can be effectively degraded
by an obscuring algorithm depends on the information that is used by
the obscuring algorithm. If the information available to the
obscuring algorithm is both more extensive and more effectively
employed than the information available to the Location Recipient,
then location obscuring might be effective.
Obscured locations can still serve a purpose where a Location
Recipient is willing to respect privacy. A privacy-respecting
Location Recipient can choose to interpret the existence of
uncertainty as a request from a Rule Maker to not recover location.
Location obscuring is unlikely to be effective against a more
determined or resourceful adversary. Withholding location
information entirely is perhaps the most effective method of ensuring
that it is not recovered.
As a final caution, we note that omitted data also conveys some
information. Selective withholding of information reveals that there
is something worth hiding. That information might be used to reveal
something of the information that is being withheld. For example, if
location is only obscured around a user's home and office, then the
lack of location for that user and the current time will likely mean
that the user is at home at night and in the office during the day,
defeating the purpose of the controls.
14. References
14.1. Normative References
[GML] OpenGIS, "OpenGIS Geography Markup Language (GML)
Implementation Specification, Version 3.1.1, OGC 03-
105r1", July 2004,
<http://portal.opengeospatial.org/files/
?artifact_id=4700>.
[NIMA.TR8350.2-3e]
"Department of Defense (DoD) World Geodetic System 1984
(WGS 84), Third Edition", NIMA TR8350.2, January 2000.
[OGC-06-103r4]
OpenGIS, "OpenGIS Implementation Specification for
Geographic information - Simple feature access - Part 1:
Common architecture", May 2011,
<http://www.opengeospatial.org/standards/sfa?>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[RFC4745] Schulzrinne, H., Tschofenig, H., Morris, J., Cuellar, J.,
Polk, J., and J. Rosenberg, "Common Policy: A Document
Format for Expressing Privacy Preferences", RFC 4745,
February 2007.
[RFC5139] Thomson, M. and J. Winterbottom, "Revised Civic Location
Format for Presence Information Data Format Location
Object (PIDF-LO)", RFC 5139, February 2008.
[RFC5491] Winterbottom, J., Thomson, M., and H. Tschofenig, "GEOPRIV
Presence Information Data Format Location Object (PIDF-LO)
Usage Clarification, Considerations, and Recommendations",
RFC 5491, March 2009.
14.2. Informative References
[DUCKHAM05]
Duckham, M. and L. Kulik, "A Formal Model of Obfuscation
and Negotiation for Location Privacy", In Proc. of the 3rd
International Conference PERVASIVE 2005, Munich, Germany,
May 2005.
[DUCKHAM10]
Duckham, M., "Moving Forward: Location Privacy and
Location Awareness", In Proc. 3rd ACM SIGSPATIAL Workshop
on Security and Privacy in GIS and LBS (SPRINGL), ACM,
November 2010.
[GEO-SHAPE]
Thomson, M., "Geodetic Shapes for the Representation of
Uncertainty in PIDF-LO", Work in Progress, December 2006.
[GEOPRIV-UNCERTAINTY]
Thomson, M. and J. Winterbottom, "Representation of
Uncertainty and Confidence in PIDF-LO", Work in Progress,
March 2012.
[IFIP07] Ardagna, C., Cremonini, M., Damiani, E., De Capitani di
Vimercati, S., and P. Samarati, "Location Privacy
Protection through Obfuscation-Based Techniques",
Proceedings of the 21st Annual IFIP WG 11.3 Working
Conference on Data and Applications Security, Redondo
Beach, CA, USA, July 2007.
[RFC2392] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, August 1998.
[RFC2778] Day, M., Rosenberg, J., and H. Sugano, "A Model for
Presence and Instant Messaging", RFC 2778, February 2000.
[RFC3694] Danley, M., Mulligan, D., Morris, J., and J. Peterson,
"Threat Analysis of the Geopriv Protocol", RFC 3694,
February 2004.
[RFC4079] Peterson, J., "A Presence Architecture for the
Distribution of GEOPRIV Location Objects", RFC 4079,
July 2005.
[RFC4119] Peterson, J., "A Presence-based GEOPRIV Location Object
Format", RFC 4119, December 2005.
[RFC4825] Rosenberg, J., "The Extensible Markup Language (XML)
Configuration Access Protocol (XCAP)", RFC 4825, May 2007.
[RFC5025] Rosenberg, J., "Presence Authorization Rules", RFC 5025,
December 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6280] Barnes, R., Lepinski, M., Cooper, A., Morris, J.,
Tschofenig, H., and H. Schulzrinne, "An Architecture for
Location and Location Privacy in Internet Applications",
BCP 160, RFC 6280, July 2011.
Appendix A. Acknowledgments
This document is informed by the discussions within the IETF GEOPRIV
working group, including discussions at the GEOPRIV interim meeting
in Washington, D.C., in 2003.
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>, and Jon
Peterson <jon.peterson@neustar.biz> for their help in improving the
quality of this document.
We would like to thank Christian Guenther for his help with an
earlier version of this document. Furthermore, we would like to
thank Johnny Vrancken for his document reviews in September 2006,
December 2006 and January 2007. James Winterbottom provided a
detailed review in November 2006. Richard Barnes gave a detailed
review in February 2008.
This document uses text from "Geodetic Shapes for the Representation
of Uncertainty in PIDF-LO" [GEO-SHAPE], authored by Martin Thomson.
We would like to thank Matt Lepinski and Richard Barnes for their
comments regarding the geodetic location transformation procedure.
Richard provided us with a detailed text proposal.
Robert Sparks, and Warren Kumari deserve thanks for their input on
the location obfuscation discussion. Robert implemented various
versions of the algorithm in the graphical language "Processing" and
thereby helped us tremendously to understand problems with the
previously illustrated algorithm.
We would like to thank Dan Romascanu, Yoshiko Chong, and Jari
Urpalainen for their last call comments.
Finally, we would like to thank the following individuals for their
feedback as part of the IESG, GenArt, and SecDir review: Jari Arkko,
Lisa Dusseault, Eric Gray, Sam Hartman, Russ Housley, Cullen
Jennings, Chris Newman, Jon Peterson, Tim Polk, Carl Reed, and Brian
Rosen.
Although John Morris is currently employed by the U.S. Government, he
participated in the development of this document in his personal
capacity, and the views expressed in the document may not reflect
those of his employer.
Appendix B. Pseudocode
This section provides an informal description for the algorithm
described in 6.5.2 and 7.5 as pseudocode. In addition to the
algorithm, it randomly chooses among equi-distant landmarks based on
the previous location.
Constants
P = sqrt(3)/6 // approx 0.2887
q = 1 - p // approx 0.7113
Parameters
prob: real // prob is a parameter in the range
// 0.5 <= prob <=1
// recommended is a value for prob between 0.7 and 0.9
// the default of prob is 0.8
Inputs
M = (m,n) : real * real
// M is a pair of reals: m and n
// m is the longitude and n the latitude,
// respectively, of the measured location
// The values are given as real numbers, in the
// range: -180 < m <= 180; -90 < n < 90
// minus values for longitude m correspond to "West"
// minus values for latitude n correspond to "South"
radius : integer // the 'radius' or uncertainty,
// measured in meters
prev-M = (prev-m1, prev-n1): real * real
// the *previously* provided location, if available
// prev-m1 is the longitude and
// prev-n1 the latitude, respectively
o : real
// this is the reference latitude for the geodesic projection
// The value of 'o' is chosen according to the table below.
// The area you want to project MUST be included in
// between a minimal latitude and a maximal latitude
// given by the two first columns of the table.
// (Otherwise the transformation is not available).
// +------+------+--------------------------+-------+
// | min | max | | |
// | lat | lat | Examples | o |
// +------+------+--------------------------+-------+
// | | | Tropics and subtropics | |
// | -45 | 45 | Africa | 0 |
// | | | Australia | |
// +------+------+--------------------------+-------+
// | | | Continental US | |
// | 25 | 50 | Mediterranean | 25 |
// | | | most of China | |
// +------+------+--------------------------+-------+
// | | | | |
// | 35 | 55 | Southern and Central | 35 |
// | | | Europe | |
// +------+------+--------------------------+-------+
// | | | | |
// | 45 | 60 | Central and Northern | 45 |
// | | | Europe | |
// +------+------+--------------------------+-------+
// | | | | |
// | 55 | 65 | most of Scandinavia | 55 |
// | | | | |
// +------+------+--------------------------+-------+
// | | | | |
// | 60 | 70 | | 60 |
// | | | | |
// +------+------+--------------------------+-------+
// | | | most of | |
// | -50 | -25 | Chile and Argentina | -50 |
// | | | New Zealand | |
// +------+------+--------------------------+-------+
// | | | | |
// | -35 | -55 | | -35 |
// | | | | |
// +------+------+--------------------------+-------+
// | | | | |
// | -45 | -60 | | -45 |
// | | | | |
// +------+------+--------------------------+-------+
// | | | | |
// | -55 | -65 | | -55 |
// | | | | |
// +------+------+--------------------------+-------+
// | | | | |
// | -60 | -70 | | -60 |
// | | | | |
// +------+------+--------------------------+-------+
Outputs
M1 = (m1,n1) : real * real // longitude and latitude,
// respectively, of the provided location
Local Variables
d, d1, d2, l, r, b, t, x, y: real
SW, SE, NW, NE: real * real
// pairs of real numbers, interpreted as coordinates
// longitude and latitude, respectively
temp : Integer[1..8]
Function
choose(Ma, Mb: real * real): real * real;
// This function chooses either Ma or Mb
// depending on the parameter 'prob'
// and on prev-M1, the previous value of M1:
// If prev-M1 == Ma choose Ma with probability 'prob'
// If prev-M1 == Mb choose Mb with probability 'prob'
// Else choose Ma or Mb with probability 1/2
Begin
rand:= Random[0,1];
// a real random number between 0 and 1
If prev-M1 == Ma Then
If rand < prob Then choose := Ma;
Else choose := Mb; EndIf
Elseif prev-M1 == Mb Then
If rand < prob Then choose := Mb;
Else choose := Ma; EndIf
Else
If rand < 0.5 Then choose := Ma;
Else choose := Mb; EndIf
End // Function choose
Main // main procedure
Begin
d := radius/1000; // uncertainity, measured in km
d1:= (d * 180) / (pi*M*cos(o));
d2:= d / 110.6;
l := d1*floor(m/d1)
// "floor" returns the largest integer
// smaller or equal to a floating point number
r := l+d1;
b := o+d2*floor(n-o/d2);
t := b+d2;
x := (m-l)/(r-l);
y := (n-b)/(t-b);
SW := (l,b);
SE := (r,b);
NW := (l,t);
NE := (r,t);
If x < p and y < p Then M1 := SW;
Elseif x < p and q <= y Then M1 := NW;
Elseif q <= x and y < p Then M1 := SE;
Elseif q <= x and q <= y Then M1 := NE;
Elseif p <= x and x < q and y < x and y < 1-x
Then M1 := choose(SW,SE);
Elseif p <= y and y < q and x <= y and y < 1-x
Then M1 := choose(SW,NW);
Elseif p <= y and y < q and y < x and 1-x <= y
Then M1 := choose(SE,NE);
Elseif p <= x and x < q and x <= y and 1-x <= y
Then M1 := choose(NW,NE);
Endif
End // Main
Authors' Addresses
Henning Schulzrinne (editor)
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
Hannes Tschofenig (editor)
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
EMail: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Jorge R. Cuellar
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
EMail: Jorge.Cuellar@siemens.com
James Polk
Cisco
2200 East President George Bush Turnpike
Richardson, Texas 75082
USA
Phone: +1 817-271-3552
EMail: jmpolk@cisco.com
John B. Morris, Jr.
EMail: ietf@jmorris.org
Martin Thomson
Microsoft
3210 Porter Drive
Palo Alto, CA 94304
USA
Phone: +1 650-353-1925
EMail: martin.thomson@gmail.com