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RFC 3694 - Threat Analysis of the Geopriv Protocol


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Network Working Group                                          M. Danley
Request for Comments: 3694                                   D. Mulligan
Category: Informational Samuelson Law, Technology & Public Policy Clinic
                                                               J. Morris
                                       Center for Democracy & Technology
                                                             J. Peterson
                                                                 NeuStar
                                                           February 2004

                Threat Analysis of the Geopriv Protocol

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document provides some analysis of threats against the Geopriv
   protocol architecture.  It focuses on protocol threats, threats that
   result from the storage of data by entities in the architecture, and
   threats posed by the abuse of information yielded by Geopriv.  Some
   security properties that meet these threats are enumerated as a
   reference for Geopriv requirements.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Habitat of the Geopriv Protocol  . . . . . . . . . . . . . . .  3
   3.  Motivations of Attackers of Geopriv  . . . . . . . . . . . . .  4
   4.  Representative Attacks on Geopriv  . . . . . . . . . . . . . .  5
       4.1.  Protocol Attacks . . . . . . . . . . . . . . . . . . . .  5
             4.1.1.  Eavesdropping and/or Interception  . . . . . . .  5
             4.1.2.  Identity Spoofing  . . . . . . . . . . . . . . .  6
             4.1.3.  Information Gathering  . . . . . . . . . . . . .  7
             4.1.4.  Denial of Service  . . . . . . . . . . . . . . .  8
       4.2.  Host Attacks . . . . . . . . . . . . . . . . . . . . . .  9
             4.2.1.  Data Stored at Servers . . . . . . . . . . . . .  9
             4.2.2.  Data Stored in Devices . . . . . . . . . . . . .  9
             4.2.3.  Data Stored with the Viewer  . . . . . . . . . . 10
             4.2.4.  Information Contained in Rules . . . . . . . . . 10
       4.3.  Usage Attacks  . . . . . . . . . . . . . . . . . . . . . 11
             4.3.1.  Threats Posed by Overcollection  . . . . . . . . 11
   5.  Countermeasures for Usage Violations . . . . . . . . . . . . . 12
       5.1.  Fair Information Practices . . . . . . . . . . . . . . . 12
   6.  Security Properties of the Geopriv Protocol  . . . . . . . . . 13
       6.1.  Rules as Countermeasures . . . . . . . . . . . . . . . . 13
             6.1.1.  Rule Maker Should Define Rules . . . . . . . . . 13
             6.1.2.  Geopriv Should Have Default Rules  . . . . . . . 14
             6.1.3.  Location Recipient Should Not Be Aware of All
                     Rules. . . . . . . . . . . . . . . . . . . . . . 14
             6.1.4.  Certain Rules Should Travel With the LO  . . . . 14
       6.2.  Protection of Identities . . . . . . . . . . . . . . . . 14
             6.2.1.  Short-Lived Identifiers May Protect Target's
                     Identity . . . . . . . . . . . . . . . . . . . . 15
             6.2.2.  Unlinked Pseudonyms May Protect the Location
                     Recipients' Identity . . . . . . . . . . . . . . 15
       6.3.  Security During Transmission of Data . . . . . . . . . . 15
             6.3.1.  Rules May Disallow a Certain Frequency of
                     Requests . . . . . . . . . . . . . . . . . . . . 15
             6.3.2.  Mutual End-Point Authentication  . . . . . . . . 16
             6.3.3.  Data Object Integrity & Confidentiality  . . . . 16
             6.3.4.  Replay Protection  . . . . . . . . . . . . . . . 16
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 16
   10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17
   11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 18

1.  Introduction

   The proliferation of location-based services that integrate tracking
   and navigation capabilities gives rise to significant privacy and
   security concerns.  Such services allow users to identify their own
   location as well as determine the location of others.  In certain
   peer-to-peer exchanges, device identification takes place
   automatically within a defined location perimeter, informing peer
   devices of a given user's identity and availability.  Additionally,
   records of location exchanges can reveal significant information
   about the habits, whereabouts, and associations of individual users.

   The Geopriv requirements allow the Location Object (LO) to support a
   wide variety of uses of Location Information (LI); the Geopriv object
   itself is intended to be technology-neutral, allowing a wide variety
   of devices to provide LI in the form of an LO.  Geopriv also requires
   that many classes of Viewers be capable of requesting LI from a
   Location Server.  The Geopriv requirements account for circumstances
   in which the Target has a contractual relationship with the entities
   that transmit and receive LI and those in which no contract exists.
   Requiring the Geopriv object to support any technology, Target-Viewer
   relationship, or underlying legal framework governing LI, complicates
   the protection of privacy and the security of LI.

   This document analyzes threats to LI in transmission and storage.
   The possibility that the LI will be compromised by these threats
   varies depending on the circumstances.  A server selling location
   information to potential marketers poses a distinctly lower risk than
   an outside individual intercepting a Target's present location to
   commit a physical attack.  It is important that these threats are
   considered as we work towards defining the LO.

   Some of the threats discussed in this document may be outside the
   scope of the Geopriv charter, e.g., threats arising from failure to
   meet contractual obligations.  Nevertheless, a comprehensive
   discussion of threats is necessary to identify desirable security
   properties and counter-measures that will improve the security of the
   LO, and thereby better protect LI.

2.  Habitat of the Geopriv Protocol

   The Geopriv architecture will be deployed in the open Internet - in a
   security environment in which potential attackers can inspect packets
   on the wire, spoof Internet addresses, and launch large-scale
   denial-of-service attacks.  In some architectures, portions of
   Geopriv traffic (especially traffic between the Location Generator
   and an initial Location Server) may occur over managed networks that
   do not interface with the public Internet.

   The protocol itself assumes interaction between a number of logical
   roles, many of which will commonly be implemented in distributed
   network devices (for a full list of Geopriv roles and entities with
   definitions, see [1]).  The endpoints of the common Geopriv
   transactions are the Location Generator (the source of location
   information from the perspective of the network) and the Location
   Recipient.  Both a Location Generator and a Location Recipient may
   have a relationship with a Location Server; the Location Generator
   publishes data to a Location Server (which may provide a grooming/
   filtration function for location information), and the Location
   Recipient requests and/or receives information from the Location
   Server.  This provides two points where Geopriv information could
   require protection across the wire.  Rules can also be passed over
   the network from a Rule Holder to a Location Server; this provides
   another point where the architecture requires security.

   It is important to note that Location Generators and Location
   Recipients may be implemented on low-cost devices for which strong
   cryptographic security is currently prohibitively expensive
   computationally.

3.  Motivations of Attackers of Geopriv

   The most obvious motivation for an attacker of Geopriv is to learn
   the location of a subject who wishes to keep their position private,
   or even for authorized Viewers to ascertain location information with
   a greater degree of precision than the Rule Maker desires.  However,
   there are several other potential motivations that cause concern.
   Attackers might also wish to prevent a Target's location from being
   distributed, or to modify or corrupt location information in order to
   misrepresent the location of the Target, or to redirect the Target's
   location information to a third party that is not authorized to know
   this information.  Attackers may want to identify the associates of a
   Target, or learn the habit or routines of a Target.  Attackers might
   want to learn the identity of all of the parties that are in a
   certain location.  Finally, some attackers may simply want to halt
   the operation of an entire Geopriv system through denial-of-service
   attacks.

   There is also a class of attackers who may be authorized as
   legitimate participants in a Geopriv protocol exchange but who abuse
   location information.  This includes the distribution or accumulation
   of location information outside the parameters of agreements between
   the principals, possibly for commercial purposes or as an act of
   unlawful surveillance.

4.  Representative Attacks on Geopriv

4.1.  Protocol Attacks

4.1.1.  Eavesdropping and/or Interception

   Imagine a location-based computer game, based on traditional hide-
   and-seek, in which a centralized server provides hints as to the
   location of the 'hider' to a set of 'seekers'.  Seekers are given
   access to very coarse location data, whereas a single referee is
   given access to unfiltered and precise location information of the
   hider.  Each seeker has a wireless device (in the Geopriv
   architecture, a Location Recipient) that feeds them coarse
   positioning data from the Location Server.  The hider carries a
   device (a Location Generator employing GPS) that transmits location
   information to the Location Server.

   If one of the seekers wished to cheat by attacking the Geopriv
   protocol, there are a number of ways they could mount such an attack
   in order to learn the precise location of the hider.  They might
   eavesdrop on one of two network connections - either the connection
   between the Location Generator and the Location Server, or the
   connection between the Location Server and the referee's Location
   Recipient (which receives precise information).  They might also
   attempt to impersonate the referee to the Location Server, in order
   to receive unfiltered Location Information.  Alternatively, they
   could impersonate the Location Server to the Location Generator
   carried by the hider, which would also give them access to precise
   location information.  Finally, the cheater could attempt to act as
   the Rule Maker, whereby providing Rules to the Location Server would
   enable the cheater's Location Recipient access to uncoarsened
   location information.

   From these threats, we can derive a need for several security
   properties of the architecture.

   o  Confidentiality is required on both the connection between the
      Location Generator and the Location Server, as well as the
      connection between the Location Server and any given Location
      Recipient.

   o  Location Servers must be capable of authenticating and authorizing
      Location Recipients to prevent impersonation.

   o  Similarly, Location Generators must be capable of authenticating
      and authorizing Location Servers in order to prevent
      impersonation.

   o  Finally, the Location Server must be able to authenticate Rule
      Makers, to make sure that unauthorized parties cannot change
      rules.

4.1.2.  Identity Spoofing

   Consider a case in which the same boss employs two rivals.  One goes
   on a business trip to Cleveland.  Both rivals carry devices that are
   tracked by a Location Generator (such as cell phones which the cell
   carrier can triangulate), and both rivals allow their boss access to
   their (coarse) location information.  The rival that remained home
   wants to hack the Geopriv protocol to make it appear that the
   traveling rival is actually goofing off in South Beach rather than
   attending a dull technology conference in Cleveland.  How would such
   an attack be mounted?

   The attacker might attempt to spoof network traffic from the Location
   Generator to the Location Server (especially if, through some other
   means such as a denial-of-service attack, the Location Generator
   became unable to issue its own reports).  The goal of the attacker
   may be to provide falsified location information appropriate for
   someone in Miami, or perhaps even to replay a genuine location object
   from a previous visit of the rival to Miami.  The attacker might also
   try to spoof traffic from the Location Server to the boss' Location
   Recipient.

   From these threats we can derive a need for several security
   properties of the architecture.

   o  There is a need for the Location Server to authenticate Location
      Generators.

   o  Location Recipients must be capable of authenticating Location
      Servers.

   o  Location information must be protected from replay attacks.

   Identity spoofing may create additional threats when the protocol is
   attacked.  In many circumstances, the identity of the Viewer is the
   basis for controlling whether LI is revealed and, if so, how that LI
   is filtered.  If the identity of that entity is compromised, privacy
   is threatened.  Anyone inside or outside the transaction that is
   capable of impersonating an authorized entity can gain access to
   confidential information, or initiate false transmissions in the
   authorized entity's name.  The ability to spoof the identity of the
   Location Recipient, for example, would create the risk of an
   unauthorized entity accessing both the identity and the location of
   the Target at the moment the LO was sent.

4.1.3.  Information Gathering

   Eavesdropping and interception can also create traffic analysis
   threats as the interceptor collects more data over time.  Traffic
   analysis threats are leveraged by an eavesdropper to determine, from
   the very fact of a network transmission, the relationship between the
   various entities involved.  If an employer sends the location of an
   employee to a customer, an eavesdropper could determine that these
   three entities are somehow interacting with one another.  If
   eavesdropping continues over time, the collection of interactions
   would involve the employer, employees, and all of their customers.
   Such a log of information would reveal that the employer and employee
   frequently were associated with one another, and would reveal which
   clients more frequently dealt with the pair.  Thus, the traffic
   analysis threat creates the risk of eavesdroppers determining the
   Target's associates.

   Traffic analysis might also allow an eavesdropper to ascertain the
   identity or characteristics of targets in a particular location.  By
   observing transmissions between Location Generators in a particular
   location and Location Servers (perhaps by eavesdropping on a wireless
   or wireline LAN scoped to the location in question), and then
   possibly following the data to various Location Recipients, an
   attacker may be able to learn the associates, including the employer,
   of targets in that location, and perhaps to extrapolate further
   identity information.

   If the eavesdropper is able to intercept not only an encrypted LO,
   but the plaintext LI itself, other threats are raised.  Let's return
   to the above example of the employer requesting an employee's
   location information.  In this instance, the interception of one such
   past transaction may reveal the identities and/or locations of all
   three parties involved, in addition to revealing their association.
   In circumstances where there is a log of this data, however, analysis
   could reveal any regular route that the employee may travel in
   visiting customers, a general area that the employee works in, the
   identities and location of the employee's entire customer base, and
   information about how the entities relate.

   Threats based on traffic analysis are difficult to meet with protocol
   security measures, but they are important to note.

   From these threats we can derive a need for several security
   properties of the architecture.

   o  The Rule Maker must be able to define Rules regarding the use of
      their LI.

   o  The connection between the Location Generator and Location Server,
      as well as the connection between the Location Server and Location
      Recipient must remain confidential.

   o  Location Servers must be capable of authenticating Location
      Recipients to prevent impersonation.

   o  Location Servers must be able to authenticate Rule Makers to
      ensure that unauthorized entities cannot change rules.

4.1.4.  Denial of Service

   Parties who wish to deprive entire networks of Geopriv service,
   rather than just targeting particular users, would probably focus
   their efforts on the Location Server.  Since in many scenarios the
   Location Server plays the central role of managing access to location
   information for many devices, it is in such architectures a natural
   single point of failure.

   The Geopriv protocol appears to have some opportunities for
   amplification attacks.  When the Location Generator publishes
   location information, the Location Server acts as an exploder,
   potentially delivering this information to numerous targets.  If the
   Location Generator were to provide very rapid updates of position (as
   many as link speed could accommodate, especially in high-bandwidth
   wireless environments), then were the Location Server to proxy
   information to Seekers at a similar rate, this could become
   problematic when large numbers of Seekers are tracking the same user.

   Also note that most operations associated with the Location Server
   probably require cryptographic authentication.  Cryptographic
   operations entail a computational expense on the part of the Location
   Server.  This could provide an attractive means for attackers to
   flood the Location Server with dummied Geopriv information that is
   spoofed to appear to come from a Location Generator, Location
   Recipient, or the Rule Maker.  Because the Location Server has to
   expend resources to verify credentials presented by these Geopriv
   messages, floods of Geopriv information could have greater impact
   than denial-of-service attacks based on generic packet flooding.

   From these threats we can derive a need for several security
   properties of the architecture.

   o  Location Servers must use stateless authentication challenges and
      similar measures to ensure that authentication attempts will not
      unnecessarily consume system resources.

   o  The Rule Maker must be able to provision policies that limit the
      rate at which Location Information is sent to prevent
      amplification attacks.

4.2.  Host Attacks

4.2.1.  Data Stored at Servers

   LI maintained at a server is subject to many potential risks.  First,
   there may be accidental misuse of LI by the server.  Whether by
   negligence, carelessness, or lack of knowledge, the server may
   accidentally release LI to the wrong Location Recipients, or fail to
   properly filter the LI that is sent out.  Second, the server may
   intentionally misuse LI.  A server may decide to sell a "profile" it
   has compiled of a Target or Location Recipient despite provisions to
   the contrary in the Rule Maker's Rule.  Alternatively, an individual
   working for the server may, for personal gain, misuse access to the
   server to obtain LI.  Third, even with the most secure and trusted
   server, there is the risk that someone outside the system will hack
   into it in order to retrieve LI.  Last, there is always the potential
   that someone would use the legal system to subpoena an individual's
   records from a Server.  Such a process would likely result in the
   revelation of the Target's location information without notice to the
   Target or the Target's consent.

   Data stored at the server may reveal the Target's present location if
   the data is used or intercepted at or near the moment of
   transmission.  If a Target requests a map from their present location
   to a nearby store, and the Location Server sends that information to
   the wrong Location Recipient, the Viewer could know the identity of
   the Target, the Target's current location, and the location where the
   Target might be headed.

   Data stored at the Location Server can also create many of the
   traffic analysis threats discussed in Section 4.1 above.  If access
   is gained not only to the fact of the LO transmission, but also to
   the LI transmitted, anyone with access to that information can put
   together a history of where that Target has been, for how long, and
   with whom.

4.2.2.  Data Stored in Devices

   Because Geopriv is required to work with any given type of technology
   or Device, it is difficult to determine the particular threat
   potential of individual devices.  For example, any device that
   maintains a log of location requests sent, or LOs received, would

   pose a similar threat to the information maintained at a Location
   Server, discussed above.  A court subpoena or warrant for an
   individual's device could additionally reveal a similar log.

   Additionally, depending on the device, there is always the potential
   for data to be compromised in some way.  For a Device with a screen,
   there is always the potential that another individual will have the
   opportunity to view the Device display without the user's knowledge.
   A Device that provides verbal feedback (i.e., to give directions to
   the blind) creates additional potential for LI to be compromised.  If
   the Target/Viewer is sitting in a public place and requests
   directions from the Target's home to another location, anyone who can
   hear the Device output may be able to determine the Target's
   identity, their residence, and possibly the location to which they
   are headed.

   In addition, if the device retained location information and the
   Device were lost or stolen, someone other than the Rule Maker could
   potentially access information regarding who LI was sent to and when,
   as well as potentially the location of the Target during each
   transaction.  Such information could enable an entity to determine
   significant private information based on who the owner of the Device
   has associated with in the past, as well as each location where the
   Target has been and for how long.

4.2.3.  Data Stored with the Viewer

   The threats posed here are similar to those discussed above in
   relation to Location Servers and Devices.  The main purpose of
   separating out threats posed by data stored at the Viewer is to show
   that, depending on the complexity of the transaction and the other
   entities involved, data storage at various points in the transaction
   can bring rise to the same types of privacy risks.

4.2.4.  Information Contained in Rules

   In many instances, the Rules a Rule Maker creates will reveal
   information either about the Rule Maker or the Target.  A rule that
   degrades all information sent out by approximately 25 miles might
   tell an interceptor how to determine the Target's true location.  A
   Rule that states, "Tell my boss what room I'm in when I'm in the
   building, but when I'm outside the building between 9 a.m. and 5 p.m.
   tell him I'm in the building," would reveal a lot more information
   than most employees would desire.  Any boss who was the Location
   Recipient who received LI that specified "in the building" would then
   realize that the employee was elsewhere.

   In addition, if an entity had access to a log of data at the Location
   Server or at a Device, knowledge of the content of Rules would enable
   a sort of "decoding" of the location information of the device to
   something more accurate.  Thus, my boss could not only tell where I
   am at this minute, but could tell how many times over the last year I
   had been outside the building between 9 a.m. and 5 p.m.

   The Rules themselves may also reveal information about the Target.  A
   rule such as the one above would clearly reveal the employment
   relationship between the two individuals, as well as the fact that
   the employee was hiding something from the employer.

   In combination with other information, the location information may
   enable the identification of the Target.

4.3.  Usage Attacks

4.3.1.  Threats Posed by Overcollection

   Weak or absent default privacy rules would also compromise LI.
   Without default Rules for LOs, it is likely that a large number of
   Devices would reveal LI by default.  Privacy rules should control the
   collection, use, disclosure, and retention of Location Information.
   These rules must comply with fair information practices - these
   practices are further discussed in Section 5.1.

   While technically savvy Device users may create privacy rules to
   protect their LI, many individuals will lack the skill or motivation
   to do so.  Thus, left to their own devices many individuals would
   likely be left without privacy rules for their LI.  This in turn
   would leave these users' LI entirely vulnerable to various attacks.
   Default rules are necessary to address this problem.

   Without default rules, for example, a device might signal out to
   anyone nearby at regular intervals, respond to anyone nearby who
   queried it, or send signals out to unknown entities.

   The lack of a default rule of "Do not re-distribute," would allow the
   Location Server to pass the Target's location information on to
   others.  Lack of a default rule limiting the retention of LI could
   increase the risk posed by inappropriate use and access to stored
   data.

   While defining default privacy rules is beyond the scope of this
   document, default rules are necessary to limit the privacy risks
   posed by the use of services and devices using LI.

5.  Countermeasures for Usage Violations

5.1.  Fair Information Practices

   Principles of fair information practices require entities that handle
   personal information to meet certain obligations with respect to its
   collection, use, maintenance and security, and give individuals whose
   personal information is collected certain due process-like rights in
   the handling of their information.  Fair information practices are
   designed to prevent specific threats posed by the collection of
   personal information about individuals.  For this reason, fair
   information practices are "countermeasures" that should be reflected
   in technical systems that handle personal information and the Rules
   that govern their use.  A brief discussion of fair information
   practices may be beneficial in formulating requirements for the LO.

   There are seven main principles of fair information practices:

   1. Openness: The existence of a record-keeping system for personal
      information must be known, along with a description of the main
      purpose and uses of the data.  Thus, any entity that collects LI
      should inform individuals that this information is being collected
      and inform them about what the LI is being used for.  Openness is
      designed to prevent the creation of secret systems.

   2. Individual Participation: Individuals should have a right to view
      all information collected about them, and to be able to correct or
      remove data that is not timely, accurate, relevant, or complete.
      The practice of individual participation acknowledges that
      sometimes information that is collected may be inaccurate or
      inappropriate.

   3. Collection Limitation: Data should be collected by lawful and fair
      means and should be collected, where appropriate, with the
      knowledge or consent of the subject.  Data collection should be
      minimized to that which is necessary to support the transaction.
      Placing limits on collection helps protect individuals from the
      dangers of overcollection - both in terms of collecting too much
      information, or of collecting information for too long of a time
      period.

   4. Data Quality: Personal data should be relevant to the purposes for
      which it is collected and used; personal information should be
      accurate, complete, and timely.  The requirement of data quality
      is designed to prevent particular kinds of harms that can flow
      from the use (appropriate or inappropriate) of personal
      information.

   5. Finality: There should be limits to the use and disclosure of
      personal data: data should be used only for purposes specified at
      the time of collection; data should not be otherwise used or
      disclosed without the consent of the data subject or other legal
      authority.  A consumer who provides LI to a business in order to
      receive directions, for example, does not provide that information
      for any other purpose.  The business should then only use that LI
      to provide directions, and not for other purposes.

   6. Security: Personal Data should be protected by reasonable security
      safeguards against such risks as loss, unauthorized access,
      destruction, use, modification, or disclosure.  While some
      security measures may take place outside of the LO (i.e., limiting
      employee access to Location Servers), other measures may be done
      through the LO or LO applications.

   7. Accountability: Record keepers should be accountable for complying
      with fair information practices.  It will typically be easier for
      an individual to enforce these practices if they are explicitly
      written - either in the Rules written by the Rule Maker, or in
      contracts between the individual and a trusted entity.

6.  Security Properties of the Geopriv Protocol

   The countermeasures suggested below reflect the threats discussed in
   this document.  There is thus some overlap between the proposed
   security properties listed below, and the requirements in [1].

6.1.  Rules as Countermeasures

   The sections below are designed to illustrate that in many instances
   threats to LI can be limited through clear, unavoidable rules
   determined by Rule Makers.

6.1.1.  Rule Maker Should Define Rules

   The Rule Maker for a given Device will generally be either the user
   of, or owner of, the Device.  In certain circumstances, the Rule
   Maker may be both of these entities.  Depending on the device, the
   Rule Maker may or may not be the individual most closely aligned with
   the Target.  For instance, a child carrying a cell phone may be the
   Target, but the parent of that child would likely be the Rule Maker
   for the Device.  Giving the Rule Maker control is a potential
   opportunity to buttress the consent component of the collection
   limitation and finality principles discussed above.

6.1.2.  Geopriv Should Have Default Rules

   Because some Rule Makers may not be informed about the role Rules
   play in the disclosure of their LI, Geopriv should include default
   Rules.  The Rule Maker is, of course, always free to change his or
   her Rules to provide more or less protection.  To protect privacy and
   physical safety, default Rules should, at a minimum, limit disclosure
   and retention of LI.

   Default Rules are also necessary for so-called "dumb" Location
   Generators (LG).  If a LG is unable to determine the Rules set by the
   Rule Maker before publishing the LO on to a Location Server, it is
   important that some default Rules protect that LO in transit, and
   ensure that the LO is eventually only sent to authorized Location
   Recipients.  These default LG Rules would help prevent many of the
   threats discussed in this document.  The Rule Maker should be able to
   determine the content of these default Rules at any time.

6.1.3.  Location Recipient Should Not Be Aware of All Rules

   A Viewer should not be aware of the full Rules defined by the Rule
   Maker.  The Viewer will only need to be aware of those Rules it must
   obey (i.e., those regarding its use and retention of the LI).  Other
   Rules, such as those specifying the accuracy or filtering of the LI,
   or rules that do not cover the given interaction should not be
   revealed to the Viewer.  This countermeasure is consistent with the
   minimization component of the collection limitation principle and
   ensures that the Rule Maker reveals only what he intends to reveal.

6.1.4.  Certain Rules Should Travel With the LO

   Security of LI at the device level is a bit complicated, as the Rule
   Maker has no real control over what is done with the LI once it
   arrives at the Location Recipient.  If certain Rules travel with the
   LO, the Rule Maker can encourage Viewer compliance with its Rules.
   Potentially, a Rule could travel with the LO indicating when it was
   time to purge the data, preventing the compilation of a "log" of the
   Target's LI on any Device involved in the transmission of the LO.
   Allowing Rules to travel with the LO has the potential to limit the
   opportunity for traffic analysis attacks.

6.2.  Protection of Identities

   Identities are an extremely important component of the LO.  While, in
   many instances, some form of identification of the Target, Rule
   Maker, and Viewer will be necessary for authentication, there are
   various methods to separate these authentication "credentials" from
   the true identity of these devices.  These countermeasures are

   particularly useful in that compromise of a log of LI, no matter
   where the source, is less threatening to privacy when the Target's
   identity is stripped.

6.2.1.  Short-Lived Identifiers May Protect Target's Identity

   Short-Lived identifiers would allow the using protocol to hide the
   true identity of the Rule Maker and the Target from Location Servers
   or Location Recipients.  These identifiers would still allow
   authentication, ensuring that only appropriate Location Recipients
   received the LO.  At the same time, however, making these identifiers
   short-lived helps prevent any association of a true identity of a
   Target with particular habits and associates.

6.2.2.  Unlinked Pseudonyms May Protect the Location Recipients'
        Identity

   Unlinked pseudonyms would protect the identity of the Location
   Recipients in much the same manner as short-lived identifiers would
   protect the Target's identity.  When using both, any record that a
   Location Server had of a transaction would have two "credentials"
   associated with an LI transmission: one linked to the Target and one
   linked to the Location Recipient.  These credentials would allow the
   Location Server to authenticate the transmission without ever
   acquiring knowledge of the true identities of the individuals
   associated with each side of the transaction.

6.3.  Security During Transmission of Data

   The attacks described in this document motivate the following
   security properties for the connections between the Location
   Generator and Location Server, the Location Server and Rule Maker,
   and the Location Server and Location Recipient:

6.3.1.  Rules May Disallow a Certain Frequency of Requests

   The Rule Maker might be able to set a Rule that disallows a certain
   number of requests made within a specific period of time.  This type
   of arrangement would allow the Rule Maker to somewhat prevent
   attackers from detecting patterns in randomly coarsened data.  To an
   "untrusted" Location Recipient, for example, to whom the Rule Maker
   only wants to reveal LI that is coarsened to the level of a city,
   only one request might be honored every 2 hours.  This would prevent
   Location Recipients from sending repeated requests to gain more
   accurate presence information.

   Similarly, thresholds on notifications of location information can
   help to combat amplification attacks.

6.3.2.  Mutual End-Point Authentication

   Authentication is crucial to the security of LI during transmission.
   The Location Server must be capable of authenticating Location
   Recipients to prevent impersonation.  Location Generators must be
   capable of authenticating Location Servers to ensure that raw
   location information is not sent to improper entities.  Additionally,
   Location Servers must be able to authenticate Rule Makers to ensure
   that unauthorized entities cannot change Rules.

6.3.3.  Data Object Integrity & Confidentiality

   The LO must maintain integrity at all points of communication between
   Location Servers and Location Recipients.  Confidentiality is
   required on both the connection between the Location Generator and
   the Location Server, as well as on the connection between the
   Location Server and any given Location Recipient.  Confidentiality of
   Rules sent over the network to the Location Server is of comparable
   importance.

6.3.4.  Replay Protection

   Replay protection prevents an attacker from capturing a particular
   piece of location information and replaying it at a later time in
   order to convince Viewers of an erroneous location for the target.
   Both Location Recipients and Location Servers, depending on their
   capabilities, may need replay protection.

7.  Security Considerations

   This informational document characterizes potential security threats
   targeting the Geopriv architecture.

8.  IANA Considerations

   This document introduces no additional considerations for IANA.

9.  Informative References

   [1]  Cuellar, J., Morris, J., Mulligan, D., Peterson, J. and J. Polk,
        "Geopriv Requirements", RFC 3693, January 2004.

10.  Authors' Addresses

   Michelle Engelhardt Danley
   Samuelson Law, Technology & Public Policy Clinic
   Boalt Hall School of Law
   University of California
   Berkeley, CA  94720
   USA

   EMail: mre213@nyu.edu
   URI:   http://www.law.berkeley.edu/cenpro/samuelson/

   Deirdre Mulligan
   Samuelson Law, Technology & Public Policy Clinic
   Boalt Hall School of Law
   University of California
   Berkeley, CA  94720
   USA

   EMail: dmulligan@law.berkeley.edu
   URI:   http://www.law.berkeley.edu/cenpro/samuelson/

   John B. Morris, Jr.
   Center for Democracy & Technology
   1634 I Street NW
   Suite 1100
   Washington, DC  20006
   USA

   EMail: jmorris@cdt.org
   URI:   http://www.cdt.org

   Jon Peterson
   NeuStar, Inc.
   1800 Sutter St
   Suite 570
   Concord, CA  94520
   USA

   Phone: +1 925/363-8720
   EMail: jon.peterson@neustar.biz
   URI:   http://www.neustar.biz/

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