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Versions: 00 01                                                         
ECRIT                                                      H. Tschofenig
Internet-Draft                                                   Siemens
Expires: January 19, 2006                                 H. Schulzrinne
                                                             Columbia U.
                                                            M. Shanmugam
                                                           July 18, 2005

        Security Threats and Requirements for Emergency Calling

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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Copyright Notice

   Copyright (C) The Internet Society (2005).


   With the increasing interest to replace parts of the public switched
   telephone network (PSTN) with the IP-based network, the  core
   functionality of PSTN such as emergency services, has to be offered
   when using IP-based technologies.  Since the PSTN and the Internet
   follow different design principles their architecture is quite

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   different.  This fact has to be considered and security threats for
   an IP-based emergency environment have to be re-evaluated.  This
   document investigates the potential threats against the IP based
   emergency architecture.  It focuses on some analysis of threats for
   both the end hosts and the infrastructure that aims to support
   emergency services.

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.   Motivations of Attackers of ECRIT  . . . . . . . . . . . . .   5
   4.   Basic Actors . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.   Security Threats . . . . . . . . . . . . . . . . . . . . . .   9
     5.1  Denial of Service Attacks  . . . . . . . . . . . . . . . .   9
     5.2  Call Identity Spoofing . . . . . . . . . . . . . . . . . .   9
     5.3  Location Spoofing  . . . . . . . . . . . . . . . . . . . .  10
     5.4  Impersonating a PSAP . . . . . . . . . . . . . . . . . . .  10
     5.5  Signaling Message Modification . . . . . . . . . . . . . .  11
     5.6  Modification of the Emergency Call . . . . . . . . . . . .  11
     5.7  Loss of confidentiality  . . . . . . . . . . . . . . . . .  11
     5.8  Replay Attack  . . . . . . . . . . . . . . . . . . . . . .  11
     5.9  Corrupting Configuration Information . . . . . . . . . . .  12
     5.10   Corrupting Database Information  . . . . . . . . . . . .  12
   6.   Security Requirements  . . . . . . . . . . . . . . . . . . .  13
     6.1  Denial of Service Attacks  . . . . . . . . . . . . . . . .  13
     6.2  Call Identity Spoofing . . . . . . . . . . . . . . . . . .  14
     6.3  Location Spoofing  . . . . . . . . . . . . . . . . . . . .  14
     6.4  Impersonating a PSAP . . . . . . . . . . . . . . . . . . .  16
     6.5  Signaling Message Modification . . . . . . . . . . . . . .  16
     6.6  Replay Attack  . . . . . . . . . . . . . . . . . . . . . .  17
     6.7  Loss of confidentiality  . . . . . . . . . . . . . . . . .  17
     6.8  Modification of the Emergency Call . . . . . . . . . . . .  17
     6.9  Corrupting Configuration Information . . . . . . . . . . .  17
     6.10   Corrupting Database Information  . . . . . . . . . . . .  18
     6.11   Location Validation and Verification . . . . . . . . . .  18
   7.   Security Considerations  . . . . . . . . . . . . . . . . . .  20
   8.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  21
   9.   References . . . . . . . . . . . . . . . . . . . . . . . . .  22
     9.1  Normative References . . . . . . . . . . . . . . . . . . .  22
     9.2  Informative References . . . . . . . . . . . . . . . . . .  22
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  22
        Intellectual Property and Copyright Statements . . . . . . .  23

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1.  Introduction

   This document provides an overview of security mechanisms and
   motivations for using them in the VoIP-based emergency services.
   PSTN users can summon help for emergency services such as ambulance,
   fire and police using a well known unique number (e.g., 911 in North
   America, 112 in in Europe).  With the introduction of IP-based
   telephony support for emergency service also has to be provided.  A
   number of protocols and protocol extensions need to interwork in
   order to provide emergency functionality.

   Since the Internet is hostile place, it is important to understand
   the security threats for emergency services.  Otherwise, an adversary
   can use the infrastructure to place fraudulent calls, mount denial of
   service attacks, etc.

   This document focuses on the security threats and security
   requirements for the IP-based emergency service infrastructure only
   without interaction with PSTN infrastructure elements.

   A few discussions within this document are related to emergency
   handling but solutions will not be developed as part of the ECRIT
   working group.  Hence, the are included mainly for completeness and
   to point to the need to investigate additional aspects.  Depending on
   the chosen protocols (for the emergency call itself, for directory
   access related to emergency call routing, for obtaining location
   information from the network, etc.) various solutions might also
   already be available to fulfill these security requirements and to
   address the threats appropriately.

   This document is organized as follows: Section 2 describes basic
   terminology, Section 5 illustrates security threats and Section 6
   lists security requirements.

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Emergency Caller, Public Safety Answering Point (PSAP), Access
   Infrastructure Provider, Application (Voice) Service Provider,
   Emergency Call Taker, etc. is taken from [I-D.schulzrinne-ecrit-

   Additionally, we use the following terms throughout the document:

   Emergency Call Routing Support: This term refers to entities that
      route the emergency call to the appropriate PSAP based on
      information like location information, language, etc.  If SIP is
      used as a protocol for session setup and call routing, for
      example, then this entity would correspond to a SIP proxy.

   Directory: This entity refers to a distributed directory protocol.
      DNS is one example of such as distributed directory but there are
      other protocols that might fulfill the requirements listed in
      [I-D.schulzrinne-ecrit-requirements] for such a protocol.

   Asserted Location Information: The term asserted location information
      refers to the property that the recipient of such an object is
      able to verify that it was generated by a particular party that is
      authorized todo so.

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3.  Motivations of Attackers of ECRIT

   The most obvious motivation for an attacker, in the context of
   emergency, is to hinder user(s) not to avail the emergency service.
   This can be done by variety of means such as impersonating a PSAP,
   directory server, forging or spoofing location, identity etc.,
   However,there are several other potential motivations that cause
   concern.  Attackers might also wish to learn the nature emergency
   information by eavesdropping an emergency call in order to reuse or
   extract the relevant information that might cause potential problems
   to the end hosts such as replay attacks, revealing the identity of
   the user.

   Attackers may want to prevent or delay an emergency caller by
   modifying some information in the message typically modifying the
   loation of the caller, while placing the emergency call.  In some
   cases, this will lead the emergency repsonders to dispatch the
   services to the spoofed ara and might not be available to the other
   callers.  It might also be possible for an attacker to impede the
   users not to reach an appropriate PSAP by corrupting or modifying the
   location of an end host or the information returned from the mapping
   protocol.  Since the  regulatory aspects of the emergency call often
   does not manadate authentication of the caller, it is easy for an
   attacker to forge some data(location information) to an PSAP, thereby
   forcing the emergency responders to dispatch the services, which
   might cause a denial of service to its legitimate users.

   Finally, some attackers may simply want to halt the operation of an
   entire emergency architecture through denial-of-service attacks.

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4.  Basic Actors

   In order to support emergency services covering a large physical area
   various infrastructure elements are necessary: Access Infrastructure
   Providers, Application (Voice) Service Provider, PSAPs as endpoints
   for emergency calls, directory services or other infrastructure
   elements that assist in during the call routing and potentially many
   other entities.

   This section outlines which entities will be considered in the threat
   analysis and shows the high level architecture.

      Information     +-----------------+
          |(1)        |Access           |   +-----------+
          v           |Infrastructure   |   |           |
     +-----------+    |Provider         |   | Directory |
     |           |    | (3)             |   |           |
     | Emergency |<---+-----------------+-->|           |
     | Caller    |    | (2)             |   +-----------+
     |           |<---+-------+         |          ^
     +-----------+    |  +----|---------+------+   |
          ^           |  |   Location   |      |   |
          |           |  |   Information<-+    |   |
          |           +--+--------------+ |(8) |   | (5)
          |              |    +-----------v+   |   |
          |   (4)        |    |Emergency   |   |   |
          +--------------+--->|Call Routing|<--+---+
          |              |    |Support     |   |
          |              |    +------------+   |
          |              |       ^             |
          |              |   (6) |        +----+--+
          |    (7)       |       +------->|       |
          +--------------+--------------->| PSAP  |
                         |                |       |
                         |Application     +----+--+
                         |(Voice)              |
                         |Service              |
                         |Provider             |

                            Figure 1: Framework

   Figure 1 shows the interaction between the entities involved in the
   call.  There are a number of different deployment choices, as it can
   be easily seen from the figure.  The following deployment choices
   need to be highlighted:

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   o  How is location information provided to the end host?  It might
      either be known to the end host itself (due to manual
      configuration or provided via GPS) or available via a third party.
      Even if location information is known to the network it might be
      made available to the end host.  Alternatively, location
      information is used as part of call routing and inserted by

   o  Is the Access Infrastructure Provider also the Application (Voice)
      Service Provider?  In the Internet today these roles are typically
      provided by different entities.  As a consequence, the Application
      (Voice) Service Provider is typically not able to learn the
      physical location of the Emergency Caller.

   Please note that the overlapping squares aim to indicate that certain
   functionality can be collapsed into a single entity.  As an example,
   the Application (Voice) Service Provider might be the same entity as
   the Access Infrastructure Provider and they might also operate the
   PSAP.  There is, however, no requirement that this must be the case.
   Additionally it is worth pointing out that end systems might be its
   own VoSP, e.g., for enterprises or residential users.

   Below, we describe various interactions between the entities shown in
   Figure 1 are described:

   o  (1) Location information might be available to the end host

   o  (2) Location information might, however, also be obtained from the
      Access Infrastructure Provider (e.g., using DHCP or application
      layer signaling protocols).

   o  (3) The Emergency Caller might need to consult a directory to
      determine the PSAP that is appropriate for the physical location
      of the emergency caller (and considering other attributes such as
      a certain language support by the Emergency Call Takers).

   o  (4) The Emergency Caller might get assistance for emergency call
      routing by infrastructure elements (referred as Emergency Call
      Routing Support entities).  In case of SIP these enities are

   o  (5) Individual Emergency Call Routing Support entities might need
      to consult a directory to determine where to route the emergency

   o  (6) The Emergency Call Routing Support entities need to finally
      forward the call, if infrastructure based emergency call routing

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      is used.

   o  (7) The emergency caller might interact directly with the PSAP
      without any Emergency Call Routing Support entities.

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5.  Security Threats

   This section discusses various security threats related to emergency
   call handling.

5.1  Denial of Service Attacks

   A (distributed) denial-of-service attack (DoS attack) on a PSAP, for
   example, might isolate the PSAP from the Internet, for a while, thus
   unable to receive the emergency calls at that time.  Since a
   particular PSAP is responsible for a certain geopraphical boundary,
   attacking a single PSAP means denying the service to the entire area
   (if no other backup PSAP is available).  DoS attacks might appear in
   many different flavors ranging from standard SYN floodings to attacks
   where a  human operator is involved then he has to determine whether
   a call is in fact a true emergency call.  In some cases this might
   lead the case where the emergency staff (police, ambulance, etc.,)
   might need to rush to the indicated emergency scene (potentionally an
   arbitrary location) and will therefore not be available for other
   rescue assignments during that time.

   As such, PSAPs can be seen as a particularly valuable target since
   the consequences of an unreachable PSAP has severe consequences.

   Attacks against the routing infrastructure enables an adversary to
   prevent all nodes attached to this network to sent emergency calls.
   Attacks against entities that assist in the call routing (such as
   attacks against the directory service) might make it difficult or
   impossible for emergency call to reach its intended PSAP.

5.2  Call Identity Spoofing

   If an adversary is able to make emergency calls without the need to
   disclose its identity (such as a SIP URI or NAI) then prank calls
   cannot be traced back.  If the call is proxy-routed, the PSAP will
   not see the IP address of the caller in signaling.  Additionally, it
   might be necessary for the Emergency Call Taker to initiate a voice,
   video or instant messaging exchange towards the Emergency Caller.

   Trying to find an adversary that placed a crank call is difficult if
   somebody uses an open 802.11 access point, even if you can find the
   owner of that access point.  This problem is no different than
   somebody placing an emergency call from a payphone.

   If the adversary is never authenticated (neither to the PSAP nor to
   the Access Infrastructure Provider) then it is possible to trace the
   call back to a make a particular entity accountable.

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   A standard requirement for emergency systems is that emergency calls
   must also be placed in absence of any authentication.  An adversary
   will typically exploit these weaknesses and he will always find
   networks that do not perform network access authentication of the
   user prior to providing network access.  As such, the emergency
   infrastructure cannot neither rely on network access authentication
   nor on authentication of the caller towards the PSAP or the
   Application (Voice) Service Provider.

   It is necessary to point to the fact that authentication in the
   emergency case might require the authorization procedure to be
   skipped.  For example, in an emergency case it is still possible to
   authenticate the user of an emergency call but without considering
   that its credits are exhausted.

5.3  Location Spoofing

   An adversary might want to made-up faked location information in
   order to fool the emergency personnel.  This is made particularly
   easy if the location information is provided by the Emergency Caller
   either via manual configuration or via GPS.  Spoofing is more
   difficult if an entity proving Emergency Call Routing Support inserts
   location information into emergency call signaling.  In this case the
   adversary needs to route the call via some intermediaries.  This is
   possible since these devices are often, by their nature as IP
   devices, addressable from an arbitary physical location.  The usage
   of VPN (or other tunneling mechanisms) and proxies further
   complicates the ability to infer the physical location from the IP
   address seen by the PSAP.

5.4  Impersonating a PSAP

   An adversary might pretend to operate a PSAP.  When either an end
   host or an intermediate device wants to determine the PSAP that is
   responsible for a particular geographical area by sending a query to
   the directory an adversary might return a faked response.  Returning
   an incorrect response message does not require the adversary to be
   somewhere along the path.  It is sufficient for an adversary to be
   located in a broadcast medium and the adversary has to reply as soon
   as a query is observed (if no security protection is utilized).  If
   the response indicates a legitimate but inappropriate (i.e., a PSAP
   that is authoritive for a different geographical area) then the
   emergency call interaction will be able to continue but will suffer
   from delays until the emergency call can be forwarded to the correct
   PSAP, potentially involving human interaction (by the Emergency Call

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5.5  Signaling Message Modification

   An adversary that is located along the signaling path might modify
   the content of emergency calls, such as location information or
   identity information.  This might lead to a denial of service attack
   against the emergency personell, disruption of the emergency call,
   delayed call setup, etc.

   An adversary might want to inject signaling messages to terminate or
   redirect the call to another location.  Dropping or delaying
   signaling messages is also possible for an on-path adversary.

   Depending on the capability of the signaling protocol the range of
   possible attacks might have been documented already.

5.6  Modification of the Emergency Call

   An adversary along the media path might want to modify the data
   traffic part of the emergency call (voice, video or instant message).
   An attacker can change the message on-the-fly and fool the PSAP to
   receive meaningless or bogus messages.  The response messages to
   Emergency Caller might also be subject to change, for example by
   injecting a recorded failure message.

5.7  Loss of confidentiality

   An adversary might eavesdrop an emergency call and use the
   information to future sessions as part of replay attacks.  The
   ability to eavesdrop also allows to learn details about the emergency
   situation which might be of interest for the press or other media
   organizations.  Please note that the location of the adversary is
   important regarding the eavesdropped area.  For example, an adversary
   in a WLAN is typically able to see a small amount of traffic due to
   the coverage area of typical WLAN network.

   Reavealing the true identity of the user as part of the privacy
   override mechanism might conflict with the users privacy settings.

5.8  Replay Attack

   An adversary might want to use eavesdropped information to mount
   attacks in the future.  This might be necessary if information cannot
   be re-created by the adversary (for example, asserted location
   information).  The ability to replay messages or individual objects
   the specific property of these messages and objects is important.
   For example, asserted location information might bind location
   information and a timestamp with a digital signature together that
   makes it difficult to reuse this object beyonds its lifetime.

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   [Editor's Note: It is sometimes hard to tell what are real threats
   and what security threats are addressed already by certain solutions
   outside the scope of the working group.  Addressing all standard
   security threats is a long process if certain mechanisms are required
   in an case that largely or completely mitigate against these

5.9  Corrupting Configuration Information

   An adversary might override all locally configured emergency numbers.
   This might be particular problematic if these emergency numbers are
   dynamically retrieved using some mechanisms.  As such, an Emergency
   Caller would start a call that either leads to a blackhole (as such
   it is a DoS attack), the Emergency Caller connects to a rogue PSAP or
   to an inappropriate PSAP.

5.10  Corrupting Database Information

   An attacker might want to provide emergency callers with wrong
   information about emergency service contact URIs.  An adversary can
   accomplish this goal in various ways, including message modification,
   spoofing or corrupting the database.  Since the database provides a
   pointer(e.g., SIP URI) to an appropriate PSAP, care must be taken
   when retrieving emergency URI.  A succesfull forging will lead the
   user to reach either a false PSAP or to a black hole.

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6.  Security Requirements

   [Editor's Note: A few requirements below are already addressed by a
   number of requirements and solution specific documents today.  In
   order to keep the document short it would be reasonable to focus only
   on the difficult security threats and requiremens for emergency calls
   rather than enumerating everything that could happen to an emergency
   call.  The working group should decide how to proceed with this
   particular issue and what threats and requirements should be
   elaborated in more detail.]

   Compiling security requirements to address the threats listed in the
   previous section might is impacted by several constraints:

      Security mechanisms may lead to a certain performance overhead
      (e.g., several roundtrips).

      A certain security infrastructure is required that might lead to
      deployment problems.  For example, end user certificates,
      certificates for networks, usage of authorization certificate,
      etc. might need to be deployed before any of these mechanisms are

      Many of these aspects are related to regulatory and legal
      requirements that may vary from country to country.  Typically,
      these mechanisms cannot be mandated by an IETF specification.

      Some of the requirements impose solutions that are out-of-scope of
      the ECRIT working group.

   Given the above-listed constraints the requirements that have to be
   addressed by work that is done within ECRIT have to be highlighted.
   Other requirements have to be read as 'if you would like to address
   this threat, then you might want to consider this requirement' rather
   than 'any solution must address fulfill this requirement'.

6.1  Denial of Service Attacks

   It is difficult to address all possible denial of service attacks
   that might lead to disruption of an emergency call since a number of
   IETF protocols are used in order to provide this functionality.
   Hence, care must be taken when protocol extensions are developed that
   the chance for a denial of service attack is not increased.  Even
   without using any security mechanisms (such as authentication and key
   exchange protocols) some degree of security has to be provided.

   It is important to understand that the ability to mount DoS attacks
   must also be considered as part of the architecture work when legal

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   and regulatory requirements are known and need to be fulfilled.

6.2  Call Identity Spoofing

   A standard requirement to prevent identity spoofing is to
   authenticate the Emergency Caller.  Authentication mechanisms that
   require multiple roundtrips and as such might delay the call are
   often not desirable or cannot be mandated.

6.3  Location Spoofing

   An Emergency Caller might in many cases know its own location
   information because it was obtained via civic or geospatial location
   extensions for DHCP, via manual configuration or via GPS.
   Unfortunately, information provided by the end host is untrustworthy
   particularly when it is as important as location information.  Two
   approaches have been discussed in the past that place lead to a few

   o  Location Information is asserted by the Access Infrastructure
      Provider.  As such, the end host might use GPS but uses a protocol
      to allow the network to assert the location information.  This
      approach also has its limitations if the coverage area of the
      wireless network is fairly large.

   o  Location Information is added to the emergency call via an
      Emergency Call Routing Support entity.  Depending on the protocol
      used for call routing and on the properties of this protocol it
      might be necessary to return the asserted location information to
      the end host since intermediate nodes might not be allowed to
      insert objects into the call setup messages (at least not in all
      parts of the messages, such as bodies).  These signaling entities,
      in general, do not know the physiscal location of the user.  Thus,
      they have to rely on somebody else to actually provide the
      location, e.g., the Access Infrastructure Provider.

   As it can be seen from these two options the main difference is based
   on the type of protocol that is used in the message communication.
   This has an impact on the semantic and on the availability of certain
   attributes (such as identities that are used by these protocols) and
   on deployment constraints.  Based on the observation that the Access
   Infrastructure Provider is closest to the end host and is therefore
   the most likely entity that knows something about the physical
   location of the end host it seems to be reasonable to assume that
   some entity that asserts the location information is actually
   available in this particular network.

   In the context of emergency, spoofing can be loosely divided into

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   o  wide-area spoofing - users pretend to be in Germany but actually
      in US.

   o  local-area spoofing - users spoof location information to an
      extend (few miles).

   o  local-area cloning - users pretend to be in place X if they were
      actually there, a while ago.

   The following requirements need to be provided in order for asserted
   location information to accomplish its goals:

   o  Location Information MUST be integrity protected to prevent
      modifications by third parties.

   o  The recipient of the asserted location information object MUST be
      able to determine the party that asserted the location information
      in order to verify the assertion.  As such, authentication of the
      asserting party (the entity that created the assertion) MUST be

   o  The asserted location information MUST include a timestamp to
      limit its validity in order to reduce replay attacks.

   o  The recipient of the asserted location information MUST have a way
      to verify that the asserting party is indeed authorized to create
      such an assertion.  As such, authentication is insufficient if not
      further authorization decision can be associated to the
      authenticated identity.

   o  The recipient of the asserted location information SHOULD have a
      mechanism to determine the Emergency Caller based on the provided

   The last bullet deserves further discussion:  If some information
   about the Emergency Caller identity has to be included, which is only
   for the purpose of traceability then this functionality might not be
   of general use.  This is because an adversary will always find
   networks that do not authenticate the user prior to providing network
   access.  Furthermore, the goal of a number of network access
   authentication protocols is to prevent disclosure of the user
   identity to entities other than to the user's home network.  Note
   that the term 'user idenity' does not require that this identity
   directly points to the 'real' identity of a user.  A court might want
   to require this identity to be resolved and to determine the user
   behind this identity.  Even if the access network would like to
   assertain the user's identity as part of the asserted location
   information it is, in many cases, not even possible for the Access

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   Infrastructure Provider.

   If the authenticated user identity is not available to the Access
   Infrastructure Provider then only a few other identities might be
   useful, such as the IP address or the MAC address.  Other identities,
   such as the Host Identity, might not be available since they are only
   used by very few protocols.  An assertion that indicates the network
   in combination with the IP and/or MAC address (together with a
   timestamp) might provide some limited degree of traceability only if
   the user was authenticated directly to this particular network.
   Providing the IP address allows some obvious attempts to cheat to be
   caught.  Hence, there is the question whether some identity should be
   added at all given the potential limitations and the potential small
   amounts of cut-and-paste attacks.  Using end user based
   authentication in addition to the asserted location information would
   be helpful (e.g., using end user certificates) but will impose a
   serious deployment problem.  Given the fact that emergency calls must
   still be allowed even without end user authentication certainly
   defeats the purpose of these mechanisms.  A partical attempt to
   address some prank calls is to classify emergency calls based on the
   availability of the provided attributes.  If suspicious information
   is being provided that may well be wrong then additional verification
   steps need to be taken.  For example, if a report of a large fire on
   a Manhattan street is received then the PSAP may wait to dispatch
   until it gets a second person to call in.  This approach obviously
   has some limitations as well.

6.4  Impersonating a PSAP

   The Emergency Caller SHOULD be able to determine conclusively that he
   has reached an accredited emergency call center.  This requirement is
   meant to address the threat that a rogue, possibly criminal, entity
   pretends to accept emergency calls and disrupts the emergency

   o  A user agent must be able to authenticate a PSAP.

   Unlike in the PSTN case IP based networks provide a better
   opportunity to spoof a PSAP since physical access to the cable plant
   is required in the PSTN case, while this may not true for the IP

6.5  Signaling Message Modification

   To protect signaling messages against modifications either individual
   attributes SHOULD be protected (such as location objects) or the
   entire signaling message communication SHOULD experience end-to-end
   protection.  This requires integrity and replay protection to be

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   applied.  Authentication of the data sender and the data receiver
   SHOULD be provided to prevent a man-in-the-middle attack.

6.6  Replay Attack

   In order to protect signaling messages (or individual attributes) to
   be replayed in future protocol sessions integrity and replay
   protection mechanisms SHOULD be provided.

6.7  Loss of confidentiality

   In order to prevent leakage of information exchanged during the
   emergency call (both signaling and data traffic) confidentiality
   protection SHOULD be provided.  The mechanisms to accomplish this
   functionality are typically different for the data traffic and for
   the signaling messages and various scenarios, such as hop-by-hop,
   end-to-middle, middle-to-middle and end-to-end security, need to be
   considered.  Particularly the key management aspects for end-to-end
   security mechansisms imposes a deployment burden and hence need to be
   critically analysed in order to determine its applicability in the
   given context.

6.8  Modification of the Emergency Call

   To protect a man-in-the-middle attack i.e disallow the adversary to
   modify or  to inject data traffic into the communication between the
   Emergency Caller and the PSAP, the mechanisms like integrity, replay
   and data origin authentication SHOULD be provided.  Since the
   signaling messages are used to authenticate the end points and to
   distribute the required keying material it is necessary that either
   the key exchange protocol itself and the signaling messages
   experience appropriate security protection.  The term 'appropriate'
   refers to the given context, the used signaling protocol and the key
   exchange protocol.

   Please note that the interactive nature of a voice communication
   already provides a some degree of protection.  However, with the
   introduction of instant messaging the freshness of the Emergency Call
   needs to be provided by other means.

6.9  Corrupting Configuration Information

   Devices SHOULD be assured of the correctness of the local emergency
   numbers that are automatically configured.  If we assume a fixed,
   global emergency service identifier that requires no configuration
   and only configure local "traditional" emergency numbers, users are
   not likely to suddenly dial some random number if a rogue
   configuration server introduces this as an additional emergency

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   number.  The ability to override all locally configured emergency
   number is of more concern.  If the Emergency Caller does not use the
   infrastructure to route the call to the appropriate PSAP then the
   security of the directory service is of importance for security.

6.10  Corrupting Database Information

   If the mapping client i.e., the entity that requests location
   information using a mapping protocol accepts the emergency contact
   information from an unauthenticated mapping server i.e., the entity
   that provides location  information, it is highly possible to receive
   bogus or prank emergency contact URIs.  Particularly the caching
   properties of a distributed directory might be exploited.  To ensure
   the secure retrieval of information, the following properties are

   o  The maping client MUST be able to authenticate the mapping server.

   o  The interaction between the mapping client and the mapping server
      MUST be integrity and replay protected.

   o  The mapping server MUST be provide data origin authentication
      thereby ensuring that the provided data items are indeed from the
      claimed source.

   o  The mapping server MUST provide information to ensure that it is
      authoritive for the provided information.

6.11  Location Validation and Verification

   location validation ensures that an address exists and is mappable to
   a PSAP.  A valid address, however, does not imply that the call
   actually originated from that location.  We refer to location
   verification as the assurance that the call was placed at the
   location claimed, including any error margins provided.

   Verifying a location is generally more difficult than location
   validation as there is currently no generally trusted service that
   can vouch for the location of the caller.  In some cases, AIPs or
   ISPs may be able to indicate the location of the caller with high
   confidence and they may possess cryptographic certificates that are
   trusted by the PSAP.  This may not require a global certification
   authority (CA), as a regional PSAP typically only deals with a modest
   number of larger enterprises and thus could obtain their public keys
   even if self-signed.

   However, even if the AIP or ISP provides a signed location, it is

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   difficult to ensure that such a signed location object is only used
   for calls from that location, as it will have to be copied from a
   location delivery protocol to the call signaling protocol.  For
   example, a third party could obtain such a signed location object and
   use it elsewhere.  Naturally, timestamps will restrict such usage to
   the order of minutes or hours, depending on how often location
   information is updated.  A PSAP may want to be able to answer the
   following questions:

   o  Who originally provided this particular location information?

   o  Did the call originate from that particular geospatial or civic
      address and who says so and how do they know?

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7.  Security Considerations

   This document addresses security threats and security requirements.
   Therefore, security is considered throughout this document.

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8.  IANA Considerations

   This document does not require actions by the IANA.

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9.  References

9.1  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", March 1997.

9.2  Informative References

              Schulzrinne, H. and R. Marshall, "Requirements for
              Emergency Context Resolution with Internet Technologies",
              May 2005.

Authors' Addresses

   Hannes Tschofenig
   Otto-Hahn-Ring 6
   Munich, Bayern  81739

   Email: Hannes.Tschofenig@siemens.com

   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027

   Phone: +1 212 939 7042
   Email: schulzrinne@cs.columbia.edu
   URI:   http://www.cs.columbia.edu/~hgs

   Murugaraj Shanmugam
   Technische Universitat Hamburg-Harburg
   Department of Security in Distributed applications
   Harburger Schlossstrasse 20
   Hamburg-Harburg  21079

   Email: murugaraj.shanmugam@tuhh.de

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