Trustworthy Location
draft-ietf-ecrit-trustworthy-location-10
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (ecrit WG) | |
|---|---|---|---|
| Authors | Hannes Tschofenig , Henning Schulzrinne , Dr. Bernard D. Aboba | ||
| Last updated | 2014-05-31 | ||
| Replaces | draft-tschofenig-ecrit-trustworthy-location | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
| Reviews |
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| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Roger Marshall | ||
| Shepherd write-up | Show Last changed 2014-02-03 | ||
| IESG | IESG state | IESG Evaluation::AD Followup | |
| Consensus boilerplate | Unknown | ||
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| Responsible AD | Alissa Cooper | ||
| Send notices to | ecrit-chairs@tools.ietf.org, draft-ietf-ecrit-trustworthy-location@tools.ietf.org | ||
| IANA | IANA review state | Version Changed - Review Needed |
draft-ietf-ecrit-trustworthy-location-10
ECRIT Working Group H. Tschofenig
INTERNET-DRAFT ARM Ltd.
Category: Informational H. Schulzrinne
Expires: December 1, 2014 Columbia University
B. Aboba (ed.)
Microsoft Corporation
31 May 2014
Trustworthy Location
draft-ietf-ecrit-trustworthy-location-10.txt
Abstract
The trustworthiness of location information is critically important
for some location-based applications, such as emergency calling or
roadside assistance.
This document focuses on the security issues arising from conveyance
of location within an emergency call, and describes mechanisms
availble to convey location in a manner that is inherently secure and
reliable. It also provides guidelines for assessing the
trustworthiness of location information.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 1, 2014.
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Copyright Notice
Copyright (c) 2014 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
carefully, as they describe your rights and restrictions with respect
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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Literature review . . . . . . . . . . . . . . . . . . . . 5
2. Threat Model . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Location Spoofing . . . . . . . . . . . . . . . . . . . . 8
2.2. Identity Spoofing . . . . . . . . . . . . . . . . . . . . 9
3. Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Signed Location by Value . . . . . . . . . . . . . . . . . 10
3.2. Location by Reference . . . . . . . . . . . . . . . . . . 14
3.3. Proxy Adding Location . . . . . . . . . . . . . . . . . . 17
4. Location Trust Assessment . . . . . . . . . . . . . . . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 20
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1. Informative references . . . . . . . . . . . . . . . . . . 22
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
Several public and commercial services depend upon location
information in their operations. This includes emergency services
(such as fire, ambulance and police) as well as commercial services
such as food delivery and roadside assistance.
For circuit-switched calls from landlines, as well as for Voice over
IP (VoIP) services only supporting emergency service calls from
stationary devices, location provided to the Public Safety Answering
Point (PSAP) is determined from a lookup using the calling telephone
number. As a result, for landlines or stationary VoIP, spoofing of
caller identification can result in the PSAP incorrectly determining
the caller's location. Problems relating to calling party number and
Caller ID assurance have been analyzed by the "Secure Telephone
Identity Revisited" [STIR] Working Group as described in "Secure
Telephone Identity Problem Statement and Requirements" [I-D.ietf-
stir-problem-statement]. In addition to the work underway in STIR,
other mechanisms exist for validating caller identification. For
example, as noted in [EENA], one mechanism for validating caller
identification information (as well as the existence of an emergency)
is for the PSAP to call the user back, as described in [RFC7090].
Given the existing work on caller identification, this document
focuses on the additional threats that are introduced by the support
of IP-based emergency services in nomadic and mobile devices, in
which location may be conveyed to the PSAP within the emergency call.
Ideally, a call taker at a PSAP should be able to assess, in real-
time, the level of trust that can be placed on the information
provided within a call. This includes automated location conveyed
along with the call and location information communicated by the
caller, as well as identity information relating to the caller or the
device initiating the call. Where real-time assessment is not
possible, it is important to be able to determine the source of the
call after the fact, so as to be able to enforce accountability.
This document defines terminology (including the meaning of
"trustworthy location") in Section 1.1, reviews existing work in
Section 1.2, describes the threat model in Section 2, outlines
potential solutions in Section 3, covers trust assessment in Section
4 and discusses security considerations in Section 5.
1.1. 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 [RFC2119].
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The definitions of "Internet Access Provider (IAP)", "Internet
Service Provider (ISP)" and "Voice Service Provider (VSP)" are taken
from "Requirements for Emergency Context Resolution with Internet
Technologies" [RFC5012].
The definition of a "hoax call" is taken from "False Emergency Calls"
[EENA].
The definition of "Target" and "Device" is taken from "An
Architecture for Location and Location Privacy in Internet
Applications" [RFC6280].
The term "location determination method" refers to the mechanism used
to determine the location of a Target. This may be something
employed by a location information server (LIS), or by the Target
itself. It specifically does not refer to the location configuration
protocol (LCP) used to deliver location information either to the
Target or the Recipient. This term is re-used from "GEOPRIV PIDF-LO
Usage Clarification, Considerations, and Recommendations" [RFC5491].
The term "source" is used to refer to the LIS, node, or device from
which a Recipient (Target or Third-Party) obtains location
information.
Additionally, the terms Location-by-Value (LbyV), Location-by-
Reference (LbyR), Location Configuration Protocol, Location
Dereference Protocol, and Location Uniform Resource Identifier (URI)
are re-used from "Requirements for a Location-by-Reference Mechanism"
[RFC5808].
"Trustworthy Location" is defined as location information that can be
attributed to a trusted source, has been protected against
modification in transmit, and has been assessed as trustworthy.
"Location Trust Assessment" refers to the process by which the
reliability of location information can be assessed. This topic is
discussed in Section 4.
The following additional terms apply to location spoofing:
"Place Shifting" is where the attacker constructs a Presence
Information Data Format Location Object (PIDF-LO) for a location
other than where they are currently located. In some cases, place
shifting can be limited in range (e.g., within the coverage area of a
particular cell tower).
"Time Shifting" is where the attacker uses or re-uses location
information that was valid in the past, but is no longer valid
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because the attacker has moved.
"Location Theft" is where the attacker captures a Target's location
information and presents it as their own. Location theft can occur
in a single instance, or may be continuous (e.g., where the attacker
has gained control over the victim's device). Location theft may
also be combined with time shifting to present someone else's
location information after the original Target has moved.
"Identity Spoofing" is where the attacker forges or obscures their
identity so as to prevent themselves from being identified as the
source of the attack. One class of identity spoofing attack involves
the forging of call origin identification.
1.2. Literature Review
There is existing work on the problem of hoax calls, as well as
analyses of aspects of the security of emergency services, threats to
geographic location privacy, threats relating to spoofing of caller
identification and modification of location information in transit.
This section reviews the literature.
1.2.1. Hoax Calls
Hoax calls have been a problem for emergency services dating back to
the time of street corner call boxes. The European Emergency Number
Association (EENA) has noted [EENA]: "False emergency calls divert
emergency services away from people who may be in life-threatening
situations and who need urgent help. This can mean the difference
between life and death for someone in trouble." As a result,
considerable attention has been focused on the problem.
EENA [EENA] has attempted to define terminology and describe best
current practices for dealing with false emergency calls. Reducing
the number of hoax calls represents a challenge, since emergency
services authorities in most countries are required to answer every
call (whenever possible). Where the caller cannot be identified, the
ability to prosecute is limited.
A particularly dangerous form of hoax call is "swatting" - a hoax
emergency call that draws a response from law enforcement prepared
for a violent confrontation (e.g. a fake hostage situation that
results in dispatching of a "Special Weapons And Tactics" (SWAT)
team). In 2008 the Federal Bureau of Investigation (FBI) issued a
warning [Swatting] about an increase in the frequency and
sophistication of these attacks.
As noted in [EENA], many documented cases of "swatting" involve not
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only the faking of an emergency, but also falsification or
obfuscation of identity. In general, the ability to identify the
caller also appears to influence the incidence of hoax calls. Where
a Voice Service Provider enables setting of the outbound caller
identification without checking it against the authenticated
identity, forging caller identification is trivial. Similarly where
an attacker can gain entry to a Private Branch Exchange (PBX), they
can then subsequently use that access to launch a denial of service
attack against the PSAP, or to make fraudulent emergency calls.
Where emergency calls have been allowed from handsets lacking a SIM
card, or where ownership of the SIM card cannot be determined, the
frequency of hoax calls has often been unacceptably high
[TASMANIA][UK][SA].
However, to date there have been few documented cases of hoax calls
that have arisen from conveyance of untrustworthy location
information within an emergency call, which is the focus of this
document.
1.2.2. Existing IETF Work
The Internet architecture for emergency calling is described in
"Framework for Emergency Calling Using Internet Multimedia" [RFC6443]
and "Best Current Practice for Communications Services in Support of
Emergency Calling" [RFC6881]. The conveyance of location information
within the Session Initiation Protocol (SIP) is described in
"Location Conveyance for the Session Initiation Protocol" [RFC6442],
which in the Security Considerations (Section 7) includes discussion
of privacy, authentication and integrity concerns relating to
conveyed location. Note that while [RFC6442] does not prohibit the
conveyance of location within non-emergency calls, in practice,
location conveyance requires additional infrastructure as described
in [RFC6443]. As a result, privacy issues inherent in conveyance of
location within non-emergency calls are not considered within
[RFC6442].
"Secure Telephone Identity Threat Model" [I-D.ietf-stir-threats]
analyzes threats relating to impersonation and obscuring of calling
party numbers, reviewing the capabilities available to attackers, and
the scenarios in which attacks are launched.
"An Architecture for Location and Location Privacy in Internet
Applications" [RFC6280] describes an architecture for privacy-
preserving location-based services in the Internet, focusing on
authorization, security and privacy requirements for the data formats
and protocols used by these services. Within the Security
Considerations (Section 5), mechanisms for ensuring the security of
the location distribution chain are discussed; these include
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mechanisms for hop-by-hop confidentiality and integrity protection as
well as end-to-end assurance. As noted in Section 6.3:
"there are three critical steps in the placement of an emergency
call, each involving location information:
1. Determine the location of the caller.
2. Determine the proper Public Safety Answering Point (PSAP) for the
caller's location.
3. Send a SIP INVITE message, including the caller's location, to the
PSAP."
"Geopriv Requirements" [RFC3693] focuses on the authorization,
security and privacy requirements of location-dependent services,
including emergency services. Within the Security Considerations
(Section 8), this includes discussion of emergency services
authentication (Section 8.3), and issues relating to identity and
anonymity (Section 8.4).
"Threat Analysis of the Geopriv Protocol" [RFC3694] describes threats
against geographic location privacy, including protocol threats,
threats resulting from the storage of geographic location data, and
threats posed by the abuse of information.
"Security Threats and Requirements for Emergency Call Marking and
Mapping" [RFC5069] reviews security threats associated with the
marking of signalling messages and the process of mapping locations
to Universal Resource Identifiers (URIs) that point to PSAPs. RFC
5069 describes attacks on the emergency services system, such as
attempting to deny system services to all users in a given area, to
gain fraudulent use of services and to divert emergency calls to non-
emergency sites. In addition, it describes attacks against
individuals, including attempts to prevent an individual from
receiving aid, or to gain information about an emergency, as well as
attacks on emergency services infrastructure elements, such as
mapping discovery and mapping servers.
2. Threat Model
To provide a structured analysis we distinguish between three
adversary models:
External adversary model: The end host, e.g., an emergency caller
whose location is going to be communicated, is honest and the
adversary may be located between the end host and the location
server or between the end host and the PSAP. None of the
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emergency service infrastructure elements act maliciously.
Malicious infrastructure adversary model: The emergency call routing
elements, such as the Location Information Server (LIS), the
Location-to-Service Translation (LoST) infrastructure, used for
mapping locations to PSAP address, or call routing elements, may
act maliciously.
Malicious end host adversary model: The end host itself acts
maliciously, whether the owner is aware of this or whether it is
acting under the control of a third party.
Since previous work describes attacks against infrastructure elements
(e.g. location servers, call route servers, mapping servers) or the
emergency services IP network, as well as threats from attackers
attempting to snoop location in transit, this document focuses on the
threats arising from end hosts providing false location information
within emergency calls (the malicious end host adversary model).
Since the focus is on malicious hosts, we do not cover threats that
may arise from attacks on infrastructure that hosts depend on to
obtain location. For example, end hosts may obtain location from
civilian GPS, which is vulnerable to spoofing [GPSCounter] or from
third party Location Service Providers (LSPs) which may be vulnerable
to attack or may not provide location accuracy suitable for emergency
purposes.
Also, we do not cover threats arising from inadequate location
infrastructure. For example, a stale wiremap or an inaccurate access
point location database could be utilized by the Location Information
Server (LIS) or the end host in its location determination, thereby
leading to an inaccurate determination of location. Similarly, a
Voice Service Provider (VSP) (and indirectly a LIS) could utilize the
wrong identity (such as an IP address) for location lookup, thereby
providing the end host with misleading location information.
2.1. Location Spoofing
Where location is attached to the emergency call by an end host, the
end host can fabricate a PIDF-LO and convey it within an emergency
call. The following represent examples of location spoofing:
Place shifting: Trudy, the adversary, pretends to be at an
arbitrary location.
Time shifting: Trudy pretends to be at a location she was a
while ago.
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Location theft: Trudy observes or obtains Alice's location and
replays it as her own.
2.2. Identity Spoofing
While this document does not focus on the problems created by
determination of location based on spoofed caller identification, the
ability to ascertain identity is important, since the threat of
punishment reduces hoax calls. As an example, calls from pay phones
are subject to greater scrutiny by the call taker.
With calls originating on an IP network, at least two forms of
identity are relevant, with the distinction created by the split
between the IAP and the VSP:
(a) network access identity such as might be determined via
authentication (e.g., using the Extensible Authentication Protocol
(EAP) [RFC3748]);
(b) caller identity, such as might be determined from authentication
of the emergency caller at the VoIP application layer.
If the adversary did not authenticate itself to the VSP, then
accountability may depend on verification of the network access
identity. However, this also may not have been authenticated, such
as in the case where an open IEEE 802.11 Access Point is used to
initiate a hoax emergency call. Although endpoint information such
as the IP or MAC address may have been logged, tying this back to the
device owner may be challenging.
Unlike the existing telephone system, VoIP emergency calls can
provide an identity that need not necessarily be coupled to a
business relationship with the IAP, ISP or VSP. However, due to the
time-critical nature of emergency calls, multi-layer authentication
is undesirable, so that in most cases, only the device placing the
call will be able to be identified. Furthermore, deploying
additional credentials for emergency service purposes (such as
certificates) increases costs, introduces a significant
administrative overhead and is only useful if widely deployed.
3. Solutions
This section presents two mechanisms which can be used to enable
location to be authenticated: signed location by value (Section 3.1),
which provides for authentication and integrity protection of the
PIDF-LO, and location-by-reference (Section 3.2), which enables
location to be obtained by the PSAP via direct contact with the
location server. In addition, a mechanism is presented which
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protects against location forgery by the end host: proxy added
location (Section 3.3). Since at the time of this writing there is
no completed specification for signed location by value, only an
expired straw-man proposal, it should be understood that only the
location-by-reference and proxy added location mechanisms are
suitable for deployment.
In order to provide authentication and integrity protection for the
Session Initiation Protocol (SIP) messages conveying location,
several security approaches are available. It is possible to ensure
that modification of the identity and location in transit can be
detected by the location recipient (e.g., the PSAP), using
cryptographic mechanisms, as described in "Enhancements for
Authenticated Identity Management in the Session Initiation Protocol"
[RFC4474]. However, compatibility with Session Border Controllers
(SBCs) that modify integrity-protected headers has proven to be an
issue in practice. As a result, SIP over Transport Layer Security
(TLS) is currently a more deployable mechanism to provide per-message
authentication and integrity protection hop-by-hop.
3.1. Signed Location by Value
With location signing, a location server signs the location
information before it is sent to the Target. The signed location
information is then sent to the location recipient, who verifies it.
Figure 1 shows the communication model with the target requesting
signed location in step (a), the location server returns it in step
(b) and it is then conveyed to the location recipient in step (c) who
verifies it. For SIP, the procedures described in "Location
Conveyance for the Session Initiation Protocol" [RFC6442] are
applicable for location conveyance.
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+-----------+ +-----------+
| | | Location |
| LIS | | Recipient |
| | | |
+-+-------+-+ +----+------+
^ | --^
| | --
Geopriv |Req. | --
Location |Signed |Signed -- Protocol Conveying
Configuration |Loc. |Loc. -- Location (e.g. SIP)
Protocol |(a) |(b) -- (c)
| v --
+-+-------+-+ --
| Target / | --
| End Host +
| |
+-----------+
Figure 1: Location Signing
A straw-man proposal for location signing is provided in "Digital
Signature Methods for Location Dependability" [I-D.thomson-geopriv-
location-dependability]. Note that since this document is no longer
under development, location signing cannot be considered deployable
at the time of this writing.
In order to limit replay attacks, this document proposes the addition
of a "validity" element to the PIDF-LO, including a "from" sub-
element containing the time that location information was validated
by the signer, as well as an "until" sub-element containing the last
time that the signature can be considered valid.
One of the consequences of including an "until" element is that even
a stationary target would need to periodically obtain a fresh PIDF-
LO, or incur the additional delay of querying during an emergency
call.
Although privacy-preserving procedures may be disabled for emergency
calls, by design, PIDF-LO objects limit the information available for
real-time attribution. As noted in [RFC5985] Section 6.6:
The LIS MUST NOT include any means of identifying the Device in
the PIDF-LO unless it is able to verify that the identifier is
correct and inclusion of identity is expressly permitted by a Rule
Maker. Therefore, PIDF parameters that contain identity are
either omitted or contain unlinked pseudonyms [RFC3693]. A
unique, unlinked presentity URI SHOULD be generated by the LIS for
the mandatory presence "entity" attribute of the PIDF document.
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Optional parameters such as the "contact" and "deviceID" elements
[RFC4479] are not used.
Also, the device referred to in the PIDF-LO may not necessarily be
the same entity conveying the PIDF-LO to the PSAP. As noted in
[RFC6442] Section 1:
In no way does this document assume that the SIP user agent client
that sends a request containing a location object is necessarily
the Target. The location of a Target conveyed within SIP
typically corresponds to that of a device controlled by the
Target, for example, a mobile phone, but such devices can be
separated from their owners, and moreover, in some cases, the user
agent may not know its own location.
Without the ability to tie the target identity to the identity
asserted in the SIP message, it is possible for an attacker to cut
and paste a PIDF-LO obtained by a different device or user into a SIP
INVITE and send this to the PSAP. This cut and paste attack could
succeed even when a PIDF-LO is signed, or [RFC4474] is implemented.
To address location-spoofing attacks, [I-D.thomson-geopriv-location-
dependability] proposes addition of an "identity" element which could
include a SIP URI (enabling comparison against the identity asserted
in the SIP headers) or an X.509v3 certificate. If the target was
authenticated by the LIS, an "authenticated" attribute is added.
However, inclusion of an "identity" attribute could enable location
tracking, so that a "hash" element is also proposed which could
contain a hash of the content of the "identity" element instead. In
practice, such a hash would not be much better for real-time
validation than a pseudonym.
Location signing cannot deter attacks in which valid location
information is provided. For example, an attacker in control of
compromised hosts could launch a denial-of-service attack on the PSAP
by initiating a large number of emergency calls, each containing
valid signed location information. Since the work required to verify
the location signature is considerable, this could overwhelm the PSAP
infrastructure.
However, while DDOS attacks are unlikely to be deterred by location
signing, accurate location information would limit the subset of
compromised hosts that could be used for an attack, as only hosts
within the PSAP serving area would be useful in placing emergency
calls.
Location signing is also difficult when the host obtains location via
mechanisms such as GPS, unless trusted computing approaches, with
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tamper-proof GPS modules, can be applied. Otherwise, an end host can
pretend to have a GPS device, and the recipient will need to rely on
its ability to assess the level of trust that should be placed in the
end host location claim.
[NENA-i2] Section 3.7 includes operational recommendations relating
to location signing:
Location determination is out of scope for NENA, but we can offer
guidance on what should be considered when designing mechanisms to
report location:
1. The location object should be digitally signed.
2. The certificate for the signer (LIS operator) should be
rooted in VESA. For this purpose, VPC and ERDB operators
should issue certs to LIS operators.
3. The signature should include a timestamp.
4. Where possible, the Location Object should be refreshed
periodically, with the signature (and thus the timestamp)
being refreshed as a consequence.
5. Anti-spoofing mechanisms should be applied to the Location
Reporting method.
[Note: The term Valid Emergency Services Authority (VESA) refers
to the root certificate authority. VPC stands for VoIP
Positioning Center and ERDB stands for the Emergency Service Zone
Routing Database.]
As noted above, signing of location objects implies the development
of a trust hierarchy that would enable a certificate chain provided
by the LIS operator to be verified by the PSAP. Rooting the trust
hierarchy in VESA can be accomplished either by having the VESA
directly sign the LIS certificates, or by the creation of
intermediate Certificate Authorities (CAs) certified by the VESA,
which will then issue certificates to the LIS. In terms of the
workload imposed on the VESA, the latter approach is highly
preferable. However, this raises the question of who would operate
the intermediate CAs and what the expectations would be.
In particular, the question arises as to the requirements for LIS
certificate issuance, and how they would compare to requirements for
issuance of other certificates such as an SSL/TLS web certificate.
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3.2. Location by Reference
Location-by-reference was developed so that end hosts can avoid
having to periodically query the location server for up-to-date
location information in a mobile environment. Additionally, if
operators do not want to disclose location information to the end
host without charging them, location-by-reference provides a
reasonable alternative. Also, since location-by-reference enables
the PSAP to directly contact the location server, it avoids potential
attacks by intermediaries. As noted in "A Location Dereference
Protocol Using HTTP-Enabled Location Delivery (HELD)" [RFC6753], a
location reference can be obtained via HTTP-Enabled Location Delivery
(HELD) [RFC5985].
Figure 2 shows the communication model with the target requesting a
location reference in step (a), the location server returns the
reference in step (b), and it is then conveyed to the location
recipient in step (c). The location recipient needs to resolve the
reference with a request in step (d). Finally, location information
is returned to the Location Recipient afterwards. For location
conveyance in SIP, the procedures described in [RFC6442] are
applicable.
+-----------+ Geopriv +-----------+
| | Location | Location |
| LIS +<------------->+ Recipient |
| | Dereferencing | |
+-+-------+-+ Protocol (d) +----+------+
^ | --^
| | --
Geopriv |Req. | --
Location |LbyR |LbyR -- Protocol Conveying
Configuration |(a) |(b) -- Location (e.g. SIP)
Protocol | | -- (c)
| V --
+-+-------+-+ --
| Target / | --
| End Host +
| |
+-----------+
Figure 2: Location by Reference
Where location by reference is provided, the recipient needs to
deference the LbyR in order to obtain location. The details for the
dereferencing operations vary with the type of reference, such as a
HTTP, HTTPS, SIP, SIPS URI or a SIP presence URI.
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For location-by-reference, the location server needs to maintain one
or several URIs for each target, timing out these URIs after a
certain amount of time. References need to expire to prevent the
recipient of such a Uniform Resource Locator (URL) from being able to
permanently track a host and to offer garbage collection
functionality for the location server.
Off-path adversaries must be prevented from obtaining the target's
location. The reference contains a randomized component that
prevents third parties from guessing it. When the location recipient
fetches up-to-date location information from the location server, it
can also be assured that the location information is fresh and not
replayed. However, this does not address location theft.
With respect to the security of the de-reference operation, [RFC6753]
Section 6 states:
TLS MUST be used for dereferencing location URIs unless
confidentiality and integrity are provided by some other
mechanism, as discussed in Section 3. Location Recipients MUST
authenticate the host identity using the domain name included in
the location URI, using the procedure described in Section 3.1 of
[RFC2818]. Local policy determines what a Location Recipient does
if authentication fails or cannot be attempted.
The authorization by possession model (Section 4.1) further relies
on TLS when transmitting the location URI to protect the secrecy
of the URI. Possession of such a URI implies the same privacy
considerations as possession of the PIDF-LO document that the URI
references.
Location URIs MUST only be disclosed to authorized Location
Recipients. The GEOPRIV architecture [RFC6280] designates the
Rule Maker to authorize disclosure of the URI.
Protection of the location URI is necessary, since the policy
attached to such a location URI permits anyone who has the URI to
view the associated location information. This aspect of security
is covered in more detail in the specification of location
conveyance protocols, such as [RFC6442].
For authorizing access to location-by-reference, two authorization
models were developed: "Authorization by Possession" and
"Authorization via Access Control Lists". With respect to
"Authorization by Possession" [RFC6753] Section 4.1 notes:
In this model, possession -- or knowledge -- of the location URI
is used to control access to location information. A location URI
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might be constructed such that it is hard to guess (see C8 of
[RFC5808]), and the set of entities that it is disclosed to can be
limited. The only authentication this would require by the LS is
evidence of possession of the URI. The LS could immediately
authorize any request that indicates this URI.
Authorization by possession does not require direct interaction
with Rule Maker; it is assumed that the Rule Maker is able to
exert control over the distribution of the location URI.
Therefore, the LIS can operate with limited policy input from a
Rule Maker.
Limited disclosure is an important aspect of this authorization
model. The location URI is a secret; therefore, ensuring that
adversaries are not able to acquire this information is paramount.
Encryption, such as might be offered by TLS [RFC5246] or S/MIME
[RFC5751], protects the information from eavesdroppers.
Using possession as a basis for authorization means that, once
granted, authorization cannot be easily revoked. Cancellation of
a location URI ensures that legitimate users are also affected;
application of additional policy is theoretically possible but
could be technically infeasible. Expiration of location URIs
limits the usable time for a location URI, requiring that an
attacker continue to learn new location URIs to retain access to
current location information.
In situations where "Authorization by Possession" is not suitable
(such as where location hiding [RFC6444] is required), the
"Authorization via Access Control Lists" model may be preferred.
Without the introduction of hierarchy, it would be necessary for the
PSAP to obtain client certificates or Digest credentials for all the
LISes in its coverage area, to enable it to successfully dereference
LbyRs. In situations with more than a few LISes per PSAP, this would
present operational challenges.
A certificate hierarchy providing PSAPs with client certificates
chaining to the VESA could be used to enable the LIS to authenticate
and authorize PSAPs for dereferencing. Note that unlike PIDF-LO
signing (which mitigates against modification of PIDF-LOs), this
merely provides the PSAP with access to a (potentially unsigned)
PIDF-LO, albeit over a protected TLS channel.
Another approach would be for the local LIS to upload location
information to a location aggregation point who would in turn manage
the relationships with the PSAP. This would shift the management
burden from the PSAPs to the location aggregation points.
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3.3. Proxy Adding Location
Instead of relying upon the end host to provide location, is possible
for a proxy that has the ability to determine the location of the end
point (e.g., based on the end host IP or MAC address) to retrieve and
add or override location information.
The use of proxy-added location is primarily applicable in scenarios
where the end host does not provide location. As noted in [RFC6442]
Section 4.1:
A SIP intermediary SHOULD NOT add location to a SIP request that
already contains location. This will quite often lead to
confusion within LRs. However, if a SIP intermediary adds
location, even if location was not previously present in a SIP
request, that SIP intermediary is fully responsible for addressing
the concerns of any 424 (Bad Location Information) SIP response it
receives about this location addition and MUST NOT pass on
(upstream) the 424 response. A SIP intermediary that adds a
locationValue MUST position the new locationValue as the last
locationValue within the Geolocation header field of the SIP
request.
A SIP intermediary MAY add a Geolocation header field if one is
not present -- for example, when a user agent does not support the
Geolocation mechanism but their outbound proxy does and knows the
Target's location, or any of a number of other use cases (see
Section 3).
As noted in [RFC6442] Section 3.3:
This document takes a "you break it, you bought it" approach to
dealing with second locations placed into a SIP request by an
intermediary entity. That entity becomes completely responsible
for all location within that SIP request (more on this in Section
4).
While it is possible for the proxy to override location included by
the end host, [RFC6442] Section 3.4 notes the operational
limitations:
Overriding location information provided by the user requires a
deployment where an intermediary necessarily knows better than an
end user -- after all, it could be that Alice has an on-board GPS,
and the SIP intermediary only knows her nearest cell tower. Which
is more accurate location information? Currently, there is no way
to tell which entity is more accurate or which is wrong, for that
matter. This document will not specify how to indicate which
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location is more accurate than another.
The disadvantage of this approach is the need to deploy application
layer entities, such as SIP proxies, at IAPs or associated with IAPs.
This requires a standardized VoIP profile to be deployed at every end
device and at every IAP. This might impose interoperability
challenges.
Additionally, the IAP needs to take responsibility for emergency
calls, even for customers they have no direct or indirect
relationship with. To provide identity information about the
emergency caller from the VSP it would be necessary to let the IAP
and the VSP to interact for authentication (see, for example,
"Diameter Session Initiation Protocol (SIP) Application" [RFC4740]).
This interaction along the Authentication, Authorization and
Accounting infrastructure is often based on business relationships
between the involved entities. An arbitrary IAP and VSP are unlikely
to have a business relationship. In case the interaction between the
IAP and the VSP fails due to the lack of a business relationship then
typically a fall-back would be provided where no emergency caller
identity information is made available to the PSAP and the emergency
call still has to be completed.
4. Location Trust Assessment
The ability to assess the level of trustworthiness of conveyed
location information is important, since this makes it possible to
understand how much value should be placed on location information,
as part of the decision making process. As an example, if automated
location information is understood to be highly suspect or is absent,
a call taker can put more effort into verifying the authenticity of
the call and to obtaining location information from the caller.
Location trust assessment has value regardless of whether the
location itself is authenticated (e.g. signed location) or is
obtained directly from the location server (e.g. location-by-
reference) over security transport, since these mechanisms do not
provide assurance of the validity or provenance of location data.
To prevent location-theft attacks, the "entity" element of the PIDF-
LO is of limited value if an unlinked pseudonym is provided in this
field. However, if the LIS authenticates the target, then the
linkage between the pseudonym and the target identity can be
recovered after the fact.
As noted in [I.D.thomson-geopriv-location-dependability], if the
location object was signed, the location recipient has additional
information on which to base their trust assessment, such as the
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validity of the signature, the identity of the target, the identity
of the LIS, whether the LIS authenticated the target, and the
identifier included in the "entity" field.
Caller accountability is also an important aspect of trust
assessment. Can the individual purchasing the device or activating
service be identified or did the call originate from a non-service
initialized (NSI) device whose owner cannot be determined? Prior to
the call, was the caller authenticated at the network or application
layer? In the event of a hoax call, can audit logs be made available
to an investigator, or can information relating to the owner of an
unlinked pseudonym be provided, enabling investigators to unravel the
chain of events that lead to the attack?
In practice, the source of the location data is important for
location trust assessment. For example, location provided by a
Location Information Server (LIS) whose administrator has an
established history of meeting emergency location accuracy
requirements (e.g. Phase II) may be considered more reliable than
location information provided by a third party Location Service
Provider (LSP) that disclaims use of location information for
emergency purposes.
However, even where an LSP does not attempt to meet the accuracy
requirements for emergency location, it still may be able to provide
information useful in assessing about how reliable location
information is likely to be. For example, was location determined
based on the nearest cell tower or 802.11 Access Point (AP), or was a
triangulation method used? If based on cell tower or AP location
data, was the information obtained from an authoritative source (e.g.
the tower or AP owner) and when was the last time that the location
of the tower or access point was verified?
For real-time validation, information in the signaling and media
packets can be cross checked against location information. For
example, it may be possible to determine the city, state, country or
continent associated with the IP address included within SIP Via: or
Contact: headers, or the media source address, and compare this
against the location information reported by the caller or conveyed
in the PIDF-LO. However, in some situations only entities close to
the caller may be able to verify the correctness of location
information.
Real-time validation of the timestamp contained within PIDF-LO
objects (reflecting the time at which the location was determined) is
also challenging. To address time-shifting attacks, the "timestamp"
element of the PIDF-LO, defined in [RFC3863], can be examined and
compared against timestamps included within the enclosing SIP
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message, to determine whether the location data is sufficiently
fresh. However, the timestamp only represents an assertion by the
LIS, which may or may not be trustworthy. For example, the recipient
of the signed PIDF-LO may not know whether the LIS supports time
synchronization, or whether it is possible to reset the LIS clock
manually without detection. Even if the timestamp was valid at the
time location was determined, a time period may elapse between when
the PIDF-LO was provided and when it is conveyed to the recipient.
Periodically refreshing location information to renew the timestamp
even though the location information itself is unchanged puts
additional load on LISes. As a result, recipients need to validate
the timestamp in order to determine whether it is credible.
While this document focuses on the discussion of real-time
determination of suspicious emergency calls, the use of audit logs
may help in enforcing accountability among emergency callers. For
example, in the event of a hoax call, information relating to the
owner of the unlinked pseudonym could be provided to investigators,
enabling them to unravel the chain of events that lead to the attack.
However, while auditability is an important deterrent, it is likely
to be of most benefit in situations where attacks on the emergency
services system are likely to be relatively infrequent, since the
resources required to pursue an investigation are likely to be
considerable. However, although real-time validation based on PIDF-
LO elements is challenging, where LIS audit logs are available (such
as where a law enforcement agency can present a subpoena), linking of
a pseudonym to the device obtaining location can be accomplished in a
post-mortem.
Where attacks are frequent and continuous, automated mechanisms are
required. For example, it might be valuable to develop mechanisms to
exchange audit trails information in a standardized format between
ISPs and PSAPs / VSPs and PSAPs or heuristics to distinguish
potentially fraudulent emergency calls from real emergencies. While
a Completely Automated Public Turing test to tell Computers and
Humans Apart (CAPTCHA) may be applied to suspicious calls to lower
the risk from bot-nets, this is quite controversial for emergency
services, due to the risk of delaying or rejecting valid calls.
5. Security Considerations
IP-based emergency services face a number of security threats that do
not exist within the legacy system. Mechanically placing a large
number of emergency calls that appear to come from different
locations is difficult in a legacy environment. Also, in the current
system, it would be very difficult for an attacker from country 'Foo'
to attack the emergency services infrastructure located in country
'Bar'.
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However, within an IP-based emergency services a number of these
attacks become much easier to mount. Emergency services have three
finite resources subject to denial of service attacks: the network
and server infrastructure, call takers and dispatchers, and the first
responders, such as fire fighters and police officers. Protecting
the network infrastructure is similar to protecting other high-value
service providers, except that location information may be used to
filter call setup requests, to weed out requests that are out of
area. Even for large cities PSAPs may only have a handful of call
takers on duty. So even if call takers can, by questioning the
caller, eliminate many hoax calls, PSAPs can be overwhelmed even by a
small-scale attack. Finally, first responder resources are scarce,
particularly during mass-casualty events.
Attackers may want to modify, prevent or delay emergency calls. In
some cases, this will lead the PSAP to dispatch emergency personnel
to an emergency that does not exist and, hence, the personnel might
not be available to other callers. It might also be possible for an
attacker to impede the users from reaching an appropriate PSAP by
modifying the location of an end host or the information returned
from the mapping protocol. In some countries, regulators may not
require the authenticated identity of the emergency caller (e.g.
emergency calls placed from PSTN pay phones or SIM-less cell phones).
Furthermore, if identities can easily be crafted (as it is the case
with many VoIP offerings today), then the value of emergency caller
authentication itself might be limited. As a result, attackers can
forge emergency call information with a lower risk of being held
accountable.
The above-mentioned attacks are mostly targeting individual emergency
callers or a very small fraction of them. If attacks are, however,
launched against the mapping architecture (see "Location-URL Mapping
Architecture and Framework" [RFC5582] or against the emergency
services IP network (including PSAPs), a larger region and a large
number of potential emergency callers are affected. The call takers
themselves are a particularly scarce resource and if human
interaction by these call takers is required then this can very
quickly have severe consequences.
Although it is important to ensure that location information cannot
be faked there will be many GPS-enabled devices that will find it
difficult to utilize any of the solutions described in Section 3. It
is also unlikely that users will be willing to upload their location
information for "verification" to a nearby location server located in
the access network.
Nevertheless, it should be understood that mounting several of the
attacks described in this document is non-trivial. Location theft
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requires the attacker to be in proximity to the location being
spoofed, or to either collude with another endhost or gain control of
an endhost so as to obtain its location. Time shifting attacks
require that the attacker visit the location and submit it before the
location information is considered stale, while travelling rapidly
away from that location to avoid apprehension. Obtaining a PIDF-LO
from a spoofed IP address requires that the attacker be on the path
between the HELD requester and the LIS.
6. IANA Considerations
This document does not require actions by IANA.
7. References
7.1. Informative References
[I-D.ietf-stir-problem-statement]
Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement", Internet draft (work in
progress), draft-ietf-stir-problem-statement-05.txt, May 2014.
[I-D.ietf-stir-threats]
Peterson, J., "Secure Telephone Identity Threat Model",
Internet draft (work in progress), draft-ietf-stir-
threats-02.txt, February 2014.
[EENA] EENA, "False Emergency Calls", EENA Operations Document,
Version 1.1, May 2011, http://www.eena.org/ressource/static/
files/2012_05_04-3.1.2.fc_v1.1.pdf
[GPSCounter]
Warner, J. S. and R. G. Johnston, "GPS Spoofing
Countermeasures", Los Alamos research paper LAUR-03-6163,
December 2003.
[NENA-i2] "08-001 NENA Interim VoIP Architecture for Enhanced 9-1-1
Services (i2)", December 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP over TLS", RFC 2818, May 2000.
[RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and J.
Polk, "Geopriv Requirements", RFC 3693, February 2004.
Tschofenig, et. al Informational [Page 22]
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[RFC3694] Danley, M., Mulligan, D., Morris, J. and J. Peterson, "Threat
Analysis of the Geopriv Protocol", RFC 3694, February 2004.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
[RFC3863] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W. and
J. Peterson, "Presence Information Data Format (PIDF)", RFC
3863, August 2004.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
RFC 4474, August 2006.
[RFC4479] Rosenberg, J., "A Data Model for Presence", RFC 4479, July
2006.
[RFC4740] Garcia-Martin, M., Belinchon, M., Pallares-Lopez, M., Canales-
Valenzuela, C., and K. Tammi, "Diameter Session Initiation
Protocol (SIP) Application", RFC 4740, November 2006.
[RFC5012] Schulzrinne, H. and R. Marshall, "Requirements for Emergency
Context Resolution with Internet Technologies", RFC 5012,
January 2008.
[RFC5069] Taylor, T., Tschofenig, H., Schulzrinne, H. and M. Shanmugam,
"Security Threats and Requirements for Emergency Call Marking
and Mapping", RFC 5069, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Level Security
(TLS) Protocol Version 1.2", RFC 5246, August 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.
[RFC5582] Schulzrinne, H., "Location-to-URL Mapping Architecture and
Framework", RFC 5582, September 2009.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.2 Message Specification", RFC
5751, January 2010.
[RFC5808] Marshall, R., "Requirements for a Location-by-Reference
Mechanism", RFC 5808, May 2010.
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[RFC5985] Barnes, M., "HTTP Enabled Location Delivery (HELD)", RFC 5985,
September 2010.
[RFC6280] Barnes, R., et. al, "An Architecture for Location and Location
Privacy in Internet Applications", RFC 6280, July 2011.
[RFC6442] Polk, J., Rosen, B. and J. Peterson, "Location Conveyance for
the Session Initiation Protocol", RFC 6442, December 2011.
[RFC6443] Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
"Framework for Emergency Calling Using Internet Multimedia",
RFC 6443, December 2011.
[RFC6444] Schulzrinne, H., Liess, L., Tschofenig, H., Stark, B., and A.
Kuett, "Location Hiding: Problem Statement and Requirements",
RFC 6444, January 2012.
[RFC6753] Winterbottom, J., Tschofenig. H., Schulzrinne, H. and M.
Thomson, "A Location Dereference Protocol Using HTTP-Enabled
Location Delivery (HELD)", RFC 6753, October 2012.
[RFC6881] Rosen, B. and J. Polk, "Best Current Practice for
Communications Services in Support of Emergency Calling", BCP
181, RFC 6881, March 2013.
[RFC7090] Schulzrinne, H., Tschofenig, H., Holmberg, C. and M. Patel,
"Public Safety Answering Point (PSAP) Callback", RFC 7090,
April 2014.
[SA] "Saudi Arabia - Illegal sale of SIMs blamed for surge in hoax
calls", Arab News, May 4, 2010,
http://www.menafn.com/qn_news_story_s.asp?StoryId=1093319384
[STIR] IETF, "Secure Telephone Identity Revisited (stir) Working
Group", http://datatracker.ietf.org/wg/stir/charter/, October
2013.
[Swatting]
"Don't Make the Call: The New Phenomenon of 'Swatting',
Federal Bureau of Investigation, February 4, 2008,
http://www.fbi.gov/news/stories/2008/february/swatting020408
[TASMANIA]
"Emergency services seek SIM-less calls block", ABC News
Online, August 18, 2006,
http://www.abc.net.au/elections/tas/2006/news/stories/
1717956.htm?elections/tas/2006/
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[UK] "Rapper makes thousands of prank 999 emergency calls to UK
police", Digital Journal, June 24, 2010,
http://www.digitaljournal.com/article/293796?tp=1
Acknowledgments
We would like to thank the members of the IETF ECRIT working group,
including Marc Linsner and Brian Rosen, for their input at IETF 85
that helped get this documented pointed in the right direction. We
would also like to thank members of the IETF GEOPRIV WG, including
Andrew Newton, Murugaraj Shanmugam, Martin Thomson, Richard Barnes
and Matt Lepinski for their feedback to previous versions of this
document. Thanks also to Pete Resnick, Adrian Farrel, Alissa Cooper,
Bert Wijnen and Meral Shirazipour who provided review comments in
IETF last call.
Authors' Addresses
Hannes Tschofenig
ARM Ltd.
110 Fulbourn Rd
Cambridge CB1 9NJ
Great Britain
Email: Hannes.tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building, New York, NY 10027
US
Phone: +1 212 939 7004
Email: hgs@cs.columbia.edu
URI: http://www.cs.columbia.edu
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: bernard_aboba@hotmail.com
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