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RFC 7340

Informational
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Secure Telephone Identity Problem Statement and Requirements

 


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Internet Engineering Task Force (IETF)                       J. Peterson
Request for Comments: 7340                                 NeuStar, Inc.
Category: Informational                                   H. Schulzrinne
ISSN: 2070-1721                                      Columbia University
                                                           H. Tschofenig
                                                          September 2014


      Secure Telephone Identity Problem Statement and Requirements

Abstract

   Over the past decade, Voice over IP (VoIP) systems based on SIP have
   replaced many traditional telephony deployments.  Interworking VoIP
   systems with the traditional telephone network has reduced the
   overall level of calling party number and Caller ID assurances by
   granting attackers new and inexpensive tools to impersonate or
   obscure calling party numbers when orchestrating bulk commercial
   calling schemes, hacking voicemail boxes, or even circumventing
   multi-factor authentication systems trusted by banks.  Despite
   previous attempts to provide a secure assurance of the origin of SIP
   communications, we still lack effective standards for identifying the
   calling party in a VoIP session.  This document examines the reasons
   why providing identity for telephone numbers on the Internet has
   proven so difficult and shows how changes in the last decade may
   provide us with new strategies for attaching a secure identity to SIP
   sessions.  It also gives high-level requirements for a solution in
   this space.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7340.

Page 2 
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
   2. Problem Statement ...............................................4
   3. Terminology .....................................................6
   4. Use Cases .......................................................6
      4.1. VoIP-to-VoIP Call ..........................................7
      4.2. VoIP-PSTN-VoIP Call ........................................7
      4.3. PSTN-to-VoIP Call ..........................................8
      4.4. VoIP-to-PSTN Call ..........................................9
      4.5. PSTN-VoIP-PSTN Call .......................................10
      4.6. PSTN-to-PSTN Call .........................................11
   5. Limitations of Current Solutions ...............................11
      5.1. P-Asserted-Identity .......................................12
      5.2. SIP Identity ..............................................14
      5.3. VIPR ......................................................17
   6. Environmental Changes ..........................................19
      6.1. Shift to Mobile Communication .............................19
      6.2. Failure of Public ENUM ....................................19
      6.3. Public Key Infrastructure Developments ....................20
      6.4. Prevalence of B2BUA Deployments ...........................20
      6.5. Stickiness of Deployed Infrastructure .....................20
      6.6. Concerns about Pervasive Monitoring .......................21
      6.7. Relationship with Number Assignment and Management ........21
   7. Basic Requirements .............................................22
   8. Acknowledgments ................................................23
   9. Security Considerations ........................................23
   10. Informative References ........................................23

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1.  Introduction

   In many communication architectures that allow users to communicate
   with other users, the need arises for identifying the originating
   party that initiates a call or a messaging interaction.  The desire
   to identify communication parties in end-to-end communication derives
   from the need to implement authorization policies (to grant or reject
   call attempts) but has also been utilized for charging.  While there
   are a number of ways to enable identification, this functionality has
   been provided by the Session Initiation Protocol (SIP) [RFC3261] by
   using two main types of approaches, namely, P-Asserted-Identity (PAI)
   [RFC3325] and SIP Identity [RFC4474], which are described in more
   detail in Section 5.  The goal of these mechanisms is to validate
   that the originator of a call is authorized to claim an originating
   identifier.  Protocols like the Extensible Messaging and Presence
   Protocol (XMPP) use mechanisms that are conceptually similar to those
   offered by SIP.

   Although solutions have been standardized, it turns out that the
   current deployment situation is unsatisfactory, and even worse, there
   is little indication that it will improve in the future.  In
   [SECURE-ORIGIN], we illustrate what challenges arise.  In particular,
   interworking with different communication architectures (e.g., SIP,
   Public Switched Telephone Network (PSTN), XMPP, Real-Time
   Communications on the Web (RTCWeb)) or other forms of mediation
   breaks the end-to-end semantic of the communication interaction and
   destroys any identification capabilities.  (In this document, we use
   the term "PSTN" colloquially rather than in a legal or policy sense,
   as a common shorthand for the circuit-switched analog and time-
   division multiplexing (TDM) digital telephone system, often using
   Signaling System #7 (SS7) to control call setup and teardown.)
   Furthermore, the use of different identifiers (e.g., E.164 numbers
   vs. SIP URIs) creates challenges for determining who is able to claim
   "ownership" for a specific identifier; although domain-based
   identifiers (sip:user@example.com) might use certificate or DNS-
   related approaches to determine who is able to claim "ownership" of
   the URI, telephone numbers do not yet have any similar mechanism
   defined.

   After the publication of the PAI and SIP Identity specifications
   ([RFC3325] and [RFC4474], respectively), further attempts have been
   made to tackle the topic but, unfortunately, with little success, due
   to the complexity of deploying solutions and the long list of (often
   conflicting) requirements.  A number of years have passed since the
   last attempts were made to improve the situation, and we therefore
   believe it is time to give it another try.  With this document, we
   would like to start to develop a common understanding of the problem

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   statement as well as basic requirements to develop a vision on how to
   advance the state of the art and to initiate technical work to enable
   secure call origin identification.

2.  Problem Statement

   In the classical Public Switched Telephone Network, there were a
   limited number of carriers, all of whom trusted each other to provide
   accurate caller origination information in an environment without any
   cryptographic validation.  In some cases, national telecommunication
   regulation codified these obligations.  This model worked as long as
   the number of entities was relatively small, easily identified (e.g.,
   in the manner carriers are certified in the United States), and
   subject to effective legal sanctions in case of misbehavior.
   However, for some time, these assumptions have no longer held true.
   For example, entities that are not traditional telecommunication
   carriers, possibly located outside the country whose country code
   they are using, can act as voice service providers.  While there was
   a clear distinction between customers and service providers in the
   past, VoIP service providers can now easily act as customers or
   either originating or transit providers.  Moreover, the problem is
   not limited to voice communications, as growth in text messaging has
   made it another vector for bulk unsolicited commercial messaging
   relying on impersonation of a source telephone number or, sometimes,
   an SMS short code.  For telephony, Caller ID spoofing has become
   common, with a small subset of entities either ignoring abuse of
   their services or willingly serving to enable fraud and other illegal
   behavior.

   For example, recently, enterprises and public safety organizations
   have been subjected to telephony denial-of-service attacks [TDOS].
   In this case, an individual claiming to represent a collections
   company for payday loans starts the extortion scheme with a phone
   call to an organization.  Failing to get payment from an individual
   or organization, the criminal organization launches a barrage of
   phone calls with spoofed numbers, preventing the targeted
   organization from receiving legitimate phone calls.  Other boiler-
   room organizations use number spoofing to place illegal "robocalls"
   (automated telemarketing; see, for example, the US Federal
   Communications Commission webpage on this topic [ROBOCALL-FCC]).
   Robocalls are a problem that has been recognized already by various
   regulators; for example, the US Federal Trade Commission (FTC)
   recently organized a robocall competition to solicit ideas for
   creating solutions that will block illegal robocalls
   [ROBOCALL-CHALLENGE].  Criminals may also use number spoofing to
   impersonate banks or bank customers to gain access to information or
   financial accounts.

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   In general, number spoofing is used in two ways: impersonation and
   anonymization.  For impersonation, the attacker pretends to be a
   specific individual.  Impersonation can be used for pretexting, where
   the attacker obtains information about the individual impersonated
   and, for example, activates credit cards, or for harassment, e.g.,
   causing utility services to be disconnected, take-out food to be
   delivered, or police to respond to a non-existing hostage situation
   ("swatting"; see [SWATTING]).  Some voicemail systems can be set up
   so that they grant access to stored messages without a password,
   relying solely on the caller identity.  As an example, in the News
   International phone-hacking scandal [NEWS-HACK], employees of the
   newspaper were accused of engaging in phone hacking by utilizing
   Caller ID spoofing to get access to voicemail.  For numbers where the
   caller has suppressed textual caller identification, number spoofing
   can be used to retrieve this information, stored in the so-called
   Calling Name (CNAM) database.  For anonymization, the caller does not
   necessarily care whether the number is in service or who it is
   assigned to and may switch rapidly and possibly randomly between
   numbers.  Anonymization facilitates automated illegal telemarketing
   or telephony denial-of-service attacks, as described above, as it
   makes it difficult to identify perpetrators and craft policies to
   block them.  It also makes tracing such calls much more labor-
   intensive, as each call has to be identified in each transit carrier
   hop-by-hop, based on destination number and time of call.

   It is insufficient to simply outlaw all spoofing of originating
   telephone numbers because the entities spoofing numbers are already
   committing other crimes and are thus unlikely to be deterred by legal
   sanctions.  Secure origin identification should prevent impersonation
   and, to a lesser extent, anonymization.  However, if numbers are easy
   and cheap to obtain, and if the organizations assigning identifiers
   cannot or will not establish the true corporate or individual
   identity of the entity requesting such identifiers, robocallers will
   still be able to switch between many different identities.

   The problem space is further complicated by a number of use cases
   where entities in the telephone network legitimately send calls on
   behalf of others, including "Find-Me/Follow-Me" services.
   Ultimately, any SIP entity can receive an INVITE request and forward
   it to any other entity, and the recipient of a forwarded message has
   little means to ascertain which recipient a call should legitimately
   target (see [SIP-SECURITY]).  Also, in some cases, third parties may

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   need to temporarily use the identity of another individual or
   organization with full consent of the "owner" of the identifier.  For
   example:

   Doctors' offices:  Physicians calling their patients using their cell
      phones would like to replace their mobile phone number with the
      number of their office to avoid being called back by patients on
      their personal phone.

   Call centers:  Call centers operate on behalf of companies, and the
      called party expects to see the Caller ID of the company, not the
      call center.

3.  Terminology

   The following terms are defined in this document:

   In-band Identity Conveyance:  In-band conveyance is the presence of
      call origin identification information conveyed within the control
      plane protocol(s) setting up a call.  Any in-band solution must
      accommodate in-band intermediaries such as Back-to-Back User
      Agents (B2BUAs).

   Out-of-Band Identity Verification:  Out-of-band verification
      determines whether the telephone number used by the calling party
      actually exists, whether the calling entity is entitled to use the
      number, and whether a call has recently been made from this phone
      number.  This approach is needed because the in-band technique
      does not work in all cases, as when certain intermediaries are
      involved or due to interworking with circuit-switched networks.

   Authority Delegation Infrastructure:  The delegation authority
      infrastructure determines how the authority over telephone numbers
      is used when numbers are ported and delegated.  It also describes
      how the existing numbering infrastructure is reused to maintain
      the lifecycle of number assignments.

   Canonical Telephone Number:  In order for either in-band conveyance
      or out-of-band verification to work, entities must be able to
      canonicalize telephone numbers to arrive at a common syntactical
      form.

4.  Use Cases

   In order to explain the requirements and other design assumptions, we
   will explain some of the scenarios that need to be supported by any
   solution.  To reduce clutter, the figures do not show call-routing

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   elements such as SIP proxies of voice or text service providers.  We
   generally assume that the PSTN component of any call path cannot be
   altered.

4.1.  VoIP-to-VoIP Call

   For the VoIP-to-VoIP communication case, a group of service providers
   that offer interconnected VoIP service exchange calls using SIP end-
   to-end but may also deliver some calls via circuit-switched
   facilities, as described in separate use cases below.  These service
   providers use telephone numbers as source and destination
   identifiers, either as the user component of a SIP URI (e.g.,
   sip:12125551234@example.com) or as a tel URI [RFC3966].

   As illustrated in Figure 1, if Alice calls Bob, the call will use SIP
   end-to-end.  (The call may or may not traverse the Internet.)

               +------------+
               |  IP-based  |
               |  SIP Phone |<--+
               |  of Bob    |   |
               |+19175551234|   |
               +------------+   |
                                |
      +------------+            |
      |  IP-based  |            |
      |  SIP Phone |       ------------
      |  of Alice  |      /     |      \
      |+12121234567|    //      |       \\
      +------------+   //      ,'        \\\
          |          ///      /             -----
          |       ////      ,'                  \\\\
          |      /        ,'                        \
          |     |       ,'                           |
          +---->|......:       IP-based              |
                |              Network               |
                 \                                  /
                  \\\\                         ////
                      -------------------------

                        Figure 1: VoIP-to-VoIP Call

4.2.  VoIP-PSTN-VoIP Call

   Frequently, two VoIP-based service providers are not directly
   connected by VoIP and use Time Division Multiplexer (TDM) circuits to
   exchange calls, leading to the IP-PSTN-IP use case.  In this use
   case, Dan's Voice Service Provider (VSP) is not a member of the

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   interconnect federation Alice's and Bob's VSP belongs to.  As far as
   Alice is concerned, Dan is not accessible via IP, and the PSTN is
   used as an interconnection network.  Figure 2 shows the resulting
   exchange.

                                          --------
                                      ////        \\\\
                               +--- >|      PSTN      |
                               |     |                |
                               |      \\\\        ////
                               |          --------
                               |             |
                               |             |
                               |             |
     +------------+         +--+----+        |
     |  IP-based  |         | PSTN  |        |
     |  SIP Phone |       --+ VoIP  +-       v
     |  of Alice  |      /  |  GW   | \  +---+---+
     |+12121234567|    //    `'''''''  \\| PSTN  |
     +------------+   //       |        \+ VoIP  +
         |          ///        |         |  GW   |\
         |       ////          |          `'''''''\\      +------------+
         |      /              |             |     \      |  IP-based  |
         |     |               |             |      |     |   Phone    |
         +---->|---------------+             +------|---->|  of Dan    |
               |                                    |     |+12039994321|
                \             IP-based             /      +------------+
                 \\\\         Network         ////
                     -------------------------

                         Figure 2: IP-PSTN-IP Call

   Note: A B2BUA/Session Border Controller (SBC) exhibits behavior that
   looks similar to this scenario since the original call content would,
   in the worst case, be re-created on the call origination side.

4.3.  PSTN-to-VoIP Call

   Consider Figure 3, where Carl is using a PSTN phone and initiates a
   call to Alice.  Alice is using a VoIP-based phone.  The call from
   Carl traverses the PSTN and enters the Internet via a PSTN/VoIP
   gateway.  This gateway attaches some identity information to the
   call, for example, based on the caller identification information it
   had received through the PSTN, if available.

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                  --------
              ////        \\\\
          +->|      PSTN      |--+
          |  |                |  |
          |   \\\\        ////   |
          |       --------       |
          |                      |
          |                      v
          |                 +----+-------+
      +---+------+          |PSTN / VoIP |              +-----+
      |PSTN Phone|          |Gateway     |              |SIP  |
      |of Carl   |          +----+-------+              |UA   |
      +----------+               |                      |Alice|
                               INVITE                   +-----+
                                 |                         ^
                                 V                         |
                          +---------------+              INVITE
                          |VoIP           |                |
                          |Interconnection|   INVITE   +-------+
                          |Provider(s)    |----------->+       |
                          +---------------+            |Alice's|
                                                       |VSP    |
                                                       |       |
                                                       +-------+

                        Figure 3: PSTN-to-VoIP Call

4.4.  VoIP-to-PSTN Call

   Consider Figure 4, where Alice calls Carl.  Carl uses a PSTN phone,
   and Alice uses an IP-based phone.  When Alice initiates the call, the
   E.164 number is translated to a SIP URI and subsequently to an IP
   address.  The call of Alice traverses her VoIP provider, where the
   call origin identification information is added.  It then hits the
   PSTN/VoIP gateway.  It is desirable that the gateway verify that
   Alice can claim the E.164 number she is using before it populates the
   corresponding calling party number field in telephone network
   signaling.  Carl's phone must be able to verify that it is receiving
   a legitimate call from the calling party number it will render to
   Carl.

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        +-------+                                        +-----+  -C
        |PSTN   |                                        |SIP  |  |a
        |Phone  |<----------------+                      |UA   |  |l
        |of Carl|                 |                      |Alice|  |l
        +-------+                 |                      +-----+  |i
                   ---------------------------              |     |n
               ////                           \\\\          |     |g
              |               PSTN                |       INVITE  |
              |                                   |         |     |P
               \\\\                           ////          |     |a
                   ---------------------------              |     |r
                                  ^                         |     |t
                                  |                         v     |y
                             +------------+             +--------+|
                             |PSTN / VoIP |<--INVITE----|VoIP    ||D
                             |Gateway     |             |Service ||o
                             +------------+             |Provider||m
                                                        |of Alice||a
                                                        +--------+|i
                                                                  -n

                        Figure 4: VoIP-to-PSTN Call

4.5.  PSTN-VoIP-PSTN Call

   Consider Figure 5, where Carl calls Alice.  Both users have PSTN
   phones, but interconnection between the two circuit-switched parts of
   the PSTN is accomplished via an IP network.  Consequently, Carl's
   operator uses a PSTN-to-VoIP gateway to route the call via an IP
   network to a gateway to break out into the PSTN again.

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                                                     +----------+
                                                     |PSTN Phone|
               --------                              |of Alice  |
           ////        \\\\                          +----------+
       +->|      PSTN      |------+                       ^
       |  |                |      |                       |
       |   \\\\        ////       |                       |
       |       --------           |                    --------
       |                          v                ////        \\\\
       |                       ,-------+          |      PSTN      |
       |                       |PSTN   |          |                |
   +---+------+              __|VoIP GW|_          \\\\        ////
   |PSTN Phone|             /  '`''''''' \             --------
   |of Carl   |           //      |       \\              ^
   +----------+          //       |        \\\            |
                       ///        -. INVITE   -----       |
                    ////            `-.           \\\\    |
                   /                   `..            \   |
                  |    IP-based           `._       ,--+----+
                  |    Network               `.....>|VoIP   |
                  |                                 |PSTN GW|
                   \                                '`'''''''
                    \\\\                         ////
                        -------------------------

                       Figure 5: PSTN-VoIP-PSTN Call

4.6.  PSTN-to-PSTN Call

   For the "legacy" case of a PSTN-to-PSTN call, otherwise beyond
   improvement, we may be able to use out-of-band IP connectivity at
   both the originating and terminating carrier to validate the call
   information.

5.  Limitations of Current Solutions

   From the inception of SIP, the From header field value has held an
   arbitrary user-supplied identity, much like the From header field
   value of an SMTP email message.  During work on [RFC3261], efforts
   began to provide a secure origin for SIP requests as an extension to
   SIP.  The so-called "short term" solution, the P-Asserted-Identity
   header described in [RFC3325], is deployed fairly widely, even though
   it is limited to closed trusted networks where end-user devices
   cannot alter or inspect SIP messages and offers no cryptographic
   validation.  As P-Asserted-Identity is used increasingly across
   multiple networks, it cannot offer any protection against identity
   spoofing by intermediaries or entities that allow untrusted entities

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   to set the P-Asserted-Identity information.  An overview of
   addressing spam in SIP and an explanation of how it differs from
   similar problems with email appeared in [RFC5039].

   Subsequent efforts to prevent calling-origin identity spoofing in SIP
   include the SIP Identity effort (the "long-term" identity solution)
   [RFC4474] and Verification Involving PSTN Reachability (VIPR)
   [VIPR-OVERVIEW].  SIP Identity attaches a new header field to SIP
   requests containing a signature over the From header field value
   combined with other message components to prevent replay attacks.
   SIP Identity is meant to prevent both (a) SIP UAs from originating
   calls with spoofed From headers and (b) intermediaries, such as SIP
   proxies, from launching man-in-the-middle attacks by altering calls
   as they pass through the intermediaries.  The VIPR architecture
   attacked a broader range of problems relating to spam, routing, and
   identity with a new infrastructure for managing rendezvous and
   security, which operated alongside of SIP deployments.

   As we will describe in more detail below, both SIP Identity and VIPR
   suffer from serious limitations that have prevented their deployment
   on a significant scale, but they may still offer ideas and protocol
   building blocks for a solution.

5.1.  P-Asserted-Identity

   The P-Asserted-Identity header field of SIP [RFC3325] provides a way
   for trusted network entities to share with one another an
   authoritative identifier for the originator of a call.  The value of
   P-Asserted-Identity cannot be populated by a user, though if a user
   wants to suggest an identity to the trusted network, a separate
   header (P-Preferred-Identity) enables them to do so.  The features of
   the P-Asserted-Identity header evolved as part of a broader effort to
   reach parity with traditional telephone network signaling mechanisms
   for selectively sharing and restricting presentation of the calling
   party number at the user level while still allowing core network
   elements to know the identity of the user for abuse prevention and
   accounting.

   In order for P-Asserted-Identity to have these properties, it
   requires the existence of a trust domain as described in [RFC3324].
   Any entity in the trust domain may add a P-Asserted-Identity header
   to a SIP message, and any entity in the trust domain may forward a
   message with a P-Asserted-Identity header to any other entity in the
   trust domain.  If a trusted entity forwards a SIP request to an
   untrusted entity, however, the P-Asserted-Identity header must first
   be removed; most end-user devices are outside trust domains.  Sending
   a P-Asserted-Identity request to an untrusted entity could leak
   potentially private information, such as the network-asserted calling

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   party number in a case where a caller has requested presentation
   restriction.  This concept of a trust domain is modeled on the
   trusted network of devices that operate the traditional telephone
   network.

   P-Asserted-Identity has been very successful in telephone replacement
   deployments of SIP.  It is an extremely simple in-band mechanism,
   requiring no cryptographic operations.  Since it is so reminiscent of
   legacy mechanisms in the traditional telephone network and interworks
   so seamlessly with those protocols, it has naturally been favored by
   providers comfortable with these operating principles.

   In practice, a trust domain exhibits many of the same merits and
   flaws as the traditional telephone network when it comes to securing
   a calling party number.  Any trusted entity may provide P-Asserted-
   Identity, and a recipient of a SIP message has no direct assurance of
   who generated the P-Asserted-Identity header field value: all trust
   is transitive.  Trust domains are dictated by business arrangements
   more than by security standards; thus, the level of assurance of
   P-Asserted-Identity is only as good as the least trustworthy member
   of a trust domain.  Since the contents of P-Asserted-Identity are not
   intended for consumption by end users, end users must trust that
   their service provider participates in an appropriate trust domain,
   as there will be no direct evidence of the trust domain in the SIP
   signaling that end-user devices receive.  Since the mechanism is so
   closely modeled on the traditional telephone network, it is unlikely
   to provide a higher level of security than that.

   Since [RFC3325] was written, the whole notion of "P-" headers
   intended for use in private SIP domains has also been deprecated (see
   [RFC5727]) largely because of overwhelming evidence that these
   headers were being used outside of private contexts and leaking into
   the public Internet.  It is unclear how many deployments that make
   use of P-Asserted-Identity in fact conform to the Spec(T)
   requirements of [RFC3324].

   P-Asserted-Identity also complicates the question of which URI should
   be presented to a user when a call is received.  Per [RFC3261], SIP
   user agents would render the contents of the From header field to a
   user when receiving an INVITE request, but what if the P-Asserted-
   Identity contains a more trustworthy URI, and presentation is not
   restricted?  Subsequent proposals have suggested additional header
   fields to carry different forms of identity related to the caller,
   including billing identities.  As the calling identities in a SIP
   request proliferate, the question of how to select one to render to
   the end user becomes more difficult to answer.

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5.2.  SIP Identity

   The SIP Identity mechanism [RFC4474] provides two header fields for
   securing identity information in SIP requests: the Identity and
   Identity-Info header fields.  Architecturally, the SIP Identity
   mechanism assumes a classic "SIP trapezoid" deployment in which an
   authentication service, acting on behalf of the originator of a SIP
   request, attaches identity information to the request that provides
   partial integrity protection; a verification service acting on behalf
   of the recipient validates the integrity of the request when it is
   received.

   The Identity header field value contains a signature over a hash of
   selected elements of a SIP request, including several header field
   values (most significantly, the From header field value) and the
   entirety of the body of the request.  The set of header field values
   was chosen specifically to prevent cut-and-paste attacks; it requires
   the verification service to retain some state to guard against
   replays.  The signature over the body of a request has different
   properties for different SIP methods, but all prevent tampering by
   man-in-the-middle attacks.  For a SIP MESSAGE request, for example,
   the signature over the body covers the actual message conveyed by the
   request: it is pointless to guarantee the source of a request if a
   man in the middle can change the content of the message, as in that
   case the message content is created by an attacker.  Similar threats
   exist against the SIP NOTIFY method.  For a SIP INVITE request, a
   signature over the Session Description Protocol (SDP) body is
   intended to prevent a man in the middle from changing properties of
   the media stream, including the IP address and port to which media
   should be sent, as this provides a means for the man in the middle to
   direct session media to a resource that the originator did not
   specify and thus impersonate an intended listener.

   The Identity-Info header field value contains a URI designating the
   location of the certificate corresponding to the private key that
   signed the hash in the Identity header.  That certificate could be
   passed by-value along with the SIP request, in which case a cid URI
   appears in Identity-Info, or by-reference, for example, when the
   Identity-Info header field value has the URL of a service that
   delivers the certificate.  [RFC4474] imposes further constraints
   governing the subject of that certificate, namely, that it must cover
   the domain name indicated in the domain component of the URI in the
   From header field value of the request.

Top      ToC       Page 15 
   The SIP Identity mechanism, however, has two fundamental limitations
   that have precluded its deployment: first, it provides identity only
   for domain names rather than other identifiers, and second, it does
   not tolerate intermediaries that alter the bodies, or certain header
   fields, of SIP requests.

   As deployed, SIP predominantly mimics the structures of the telephone
   network and thus uses telephone numbers as identifiers.  Telephone
   numbers in the From header field value of a SIP request may appear as
   the user part of a SIP URI or, alternatively, in an independent tel
   URI.  The certificate designated by the Identity-Info header field as
   specified, however, corresponds only to the domain portion of a SIP
   URI in the From header field.  As such, [RFC4474] does not have any
   provision to identify the assignee of a telephone number.  While it
   could be the case that the domain name portion of a SIP URI signifies
   a carrier (like "att.com") to whom numbers are assigned, the SIP
   Identity mechanism provides no assurance that a particular number has
   been assigned to any specific carrier.  For a tel URI, moreover, it
   is unclear in [RFC4474] what entity should hold a corresponding
   certificate.  A caller may not want to reveal the identity of its
   service provider to the callee and may thus prefer tel URIs in the
   From header field.

   This lack of authority gives rise to a whole class of SIP Identity
   problems when dealing with telephone numbers, as is explored in
   [CONCERNS].  That document shows how the Identity header of a SIP
   request targeting a telephone number (embedded in a SIP URI) could be
   dropped by an intermediate domain, which then modifies and re-signs
   the request, all without alerting the verification service: the
   verification service has no way of knowing which original domain
   signed the request.  Provided that the local authentication service
   is complicit, an originator can claim virtually any telephone number,
   impersonating any chosen Caller ID from the perspective of the
   verifier.  Both of these attacks are rooted in the inability of the
   verification service to ascertain a specific certificate that is
   authoritative for a telephone number.

   Moreover, as deployed, SIP is highly mediated and is mediated in ways
   that [RFC3261] did not anticipate.  As request routing commonly
   depends on policies dissimilar to [RFC3263], requests transit
   multiple intermediate domains to reach a destination; some forms of
   intermediaries in those domains may effectively reinitiate the
   session.

   One of the main reasons that SIP deployments mimic the PSTN
   architecture is because the requirement for interconnection with the
   PSTN remains paramount: a call may originate in SIP and terminate on
   the PSTN, or vice versa.  Worse still, a PSTN-to-PSTN call may

Top      ToC       Page 16 
   transit a SIP network in the middle, or vice versa.  This necessarily
   reduces SIP's feature set to the least common denominator of the
   telephone network and mandates support for telephone numbers as a
   primary calling identifier.

   Interworking with non-SIP networks makes end-to-end identity
   problematic.  When a PSTN gateway sends a call to a SIP network, it
   creates the INVITE request anew, regardless of whether a previous leg
   of the call originated in a SIP network that later delivered the call
   to the PSTN.  As these gateways are not necessarily operated by
   entities that have any relationship to the number assignee, it is
   unclear how they could provide an identity signature that a verifier
   should trust.  Moreover, how could the gateway know that the calling
   party number it receives from the PSTN is actually authentic?  And
   when a gateway receives a call via SIP and terminates a call to the
   PSTN, how can that gateway verify that a telephone number in the From
   header field value is authentic before it presents that number as the
   calling party number in the PSTN?

   Similarly, some SIP networks deploy intermediaries that act as back-
   to-back user agents (B2BUAs), typically in order to provide policy or
   interworking functions at network boundaries (hence, the nickname
   "Session Border Controller").  These functions range from topology
   hiding, to alterations necessary to interoperate successfully with
   particular SIP implementations, to simple network address translation
   from private address space.  To implement these functions, these
   entities modify SIP INVITE requests in transit, potentially changing
   the From, Contact, and Call-ID header field values, as well as
   aspects of the SDP, including especially the IP addresses and ports
   associated with media.  Consequently, a SIP request exiting a B2BUA
   does not necessarily bear much resemblance to the original request
   received by the B2BUA, just as an SS7 request exiting a PSTN gateway
   may transform all aspects of the SIP request in the VoIP leg of the
   call.  An Identity signature provided for the original INVITE has no
   bearing on the post-B2BUA INVITE, and, were the B2BUA to preserve the
   original Identity header, any verification service would detect a
   violation of the integrity protection.

   The SIP community has long been aware of these problems with
   [RFC4474] in practical deployments.  Some have therefore proposed
   weakening the security constraints of [RFC4474] so that at least some
   deployments of B2BUAs will be compatible with integrity protection of
   SIP requests.  However, such solutions do not address the key
   problems identified above: the lack of any clear authority for
   telephone numbers and the fact that some INVITE requests are
   generated by intermediaries rather than endpoints.  Removing the

Top      ToC       Page 17 
   signature over the SDP from the Identity header will not, for
   example, make it any clearer how a PSTN gateway should assert
   identity in an INVITE request.

5.3.  VIPR

   Verification Involving PSTN Reachability (VIPR) directly attacks the
   twin problems of identifying number assignees on the Internet and
   coping with intermediaries that may modify signaling.  To address the
   first problem, VIPR relies on the PSTN itself: it discovers which
   endpoints on the Internet are reachable via a particular PSTN number
   by calling the number on the PSTN to determine whom a call to that
   number will reach.  As VIPR-enabled Internet endpoints associated
   with PSTN numbers are discovered, VIPR provides a rendezvous service
   that allows the endpoints of a call to form an out-of-band connection
   over the Internet; this connection allows the endpoints to exchange
   information that secures future communications and permits direct,
   unmediated SIP connections.

   VIPR provides these services within a fairly narrow scope of
   applicability.  Its seminal use case is the enterprise IP Private
   Branch Exchange (IPBX), a device that has both PSTN connectivity and
   Internet connectivity, which serves a set of local users with
   telephone numbers; after a PSTN call has connected successfully and
   then ended, the PBX searches a distributed hash table to see if any
   VIPR-compatible devices have advertised themselves as a route for the
   unfamiliar number on the Internet.  If advertisements exist, the
   originating PBX then initiates a verification process to determine
   whether the entity claiming to be the assignee of the unfamiliar
   number in fact received the successful call: this involves verifying
   details such as the start and stop times of the call.  If the
   destination verifies successfully, the originating PBX provisions a
   local database with a route for that telephone number to the URI
   provided by the proven destination.  Moreover, the destination gives
   a token to the originator that can be inserted in future call setup
   messages to authenticate the source of future communications.

   Through this mechanism, the VIPR system provides a suite of
   properties, ones that go well beyond merely securing the origins of
   communications.  It also provides a routing system that dynamically
   discovers mappings between telephone numbers and URIs, effectively
   building an ad hoc ENUM database in every VIPR implementation.  The
   tokens exchanged over the out-of-band connection established by VIPR
   also provide an authorization mechanism for accepting calls over the
   Internet, which significantly reduces the potential for spam.
   Because the token can act as a cookie due to the presence of this

Top      ToC       Page 18 
   out-of-band connectivity, the VIPR token is less susceptible to cut-
   and-paste attacks and thus needs to cover far less of a SIP request
   with its signature.

   Due to its narrow scope of applicability and the details of its
   implementation, VIPR has some significant limitations.  The most
   salient for the purposes of this document is that it only has bearing
   on repeated communications between entities: it has no solution to
   the classic "robocall" problem, where the target typically receives a
   call from a number that has never called before.  All of VIPR's
   strengths in establishing identity and spam prevention kick in only
   after an initial PSTN call has been completed and subsequent attempts
   at communication begin.  Every VIPR-compliant entity, moreover,
   maintains its own stateful database of previous contacts and
   authorizations, which lends itself more to aggregators like IP PBXs
   that may front for thousands of users than to individual phones.
   That database must be refreshed by periodic PSTN calls to determine
   that control over the number has not shifted to some other entity;
   figuring out when data has grown stale is one of the challenges of
   the architecture.  As VIPR requires compliant implementations to
   operate both a PSTN interface and an IP interface, it has little
   apparent applicability to ordinary desktop PCs or similar devices
   with no ability to place direct PSTN calls.

   The distributed hash table (DHT) also creates a new attack surface
   for impersonation.  Attackers who want to pose as the owners of
   telephone numbers can advertise themselves as routes to a number in
   the hash table.  VIPR has no inherent restriction on the number of
   entities that may advertise themselves as routes for a number; thus,
   an originator may find multiple advertisements for a number on the
   DHT even when an attack is not in progress.  Attackers may learn from
   these validation attempts which VIPR entities recently placed calls
   to the target number, even if they cannot impersonate the target
   since they lack the PSTN call detail information.  It may be that
   this information is all the attacker hopes to glean.  The fact that
   advertisements and verifications are public results from the public
   nature of the DHT that VIPR creates.  The public DHT prevents any
   centralized control or attempts to impede communications, but those
   come at the cost of apparently unavoidable privacy losses.

   Because of these limitations, VIPR, much like SIP Identity, has had
   little impact in the marketplace.  Ultimately, VIPR's utility as an
   identity mechanism is limited by its reliance on the PSTN, especially
   its need for an initial PSTN call to complete before any of VIPR's
   benefits can be realized, and by the drawbacks of the highly public
   exchanges required to create the out-of-band connection between VIPR
   entities.  As such, there is no obvious solution to providing secure
   origin services for SIP on the Internet today.

Top      ToC       Page 19 
6.  Environmental Changes

6.1.  Shift to Mobile Communication

   In the years since [RFC4474] was conceived, there have been a number
   of fundamental shifts in the communications marketplace.  The most
   transformative has been the precipitous rise of mobile smartphones,
   which are now arguably the dominant communications device in the
   developed world.  Smart phones have both a PSTN and an IP interface,
   as well as SMS and Multimedia Messaging Service (MMS) capabilities.
   This suite of tools suggests that some of the techniques proposed by
   VIPR could be adapted to the smartphone environment.  The installed
   base of smartphones is, moreover, highly upgradable and permits rapid
   adoption of out-of-band rendezvous services for smartphones that
   bypass the PSTN.  Mobile messaging services that use telephone
   numbers as identities allow smartphone users to send text messages to
   one another over the Internet rather than over the PSTN.  Like VIPR,
   such services create an out-of-band connection over the Internet
   between smartphones; unlike VIPR, the rendezvous service is provided
   by a trusted centralized database rather than by a DHT, and it is the
   centralized database that effectively verifies and asserts the
   telephone number of the sender of a message.  While such messaging
   services are specific to the users of the specific service, it seems
   clear that similar databases could be provided by neutral third
   parties in a position to coordinate between endpoints.

6.2.  Failure of Public ENUM

   At the time [RFC4474] was written, the hopes for establishing a
   certificate authority for telephone numbers on the Internet largely
   rested on public ENUM deployment.  The e164.arpa DNS tree established
   for ENUM could have grown to include certificates for telephone
   numbers or at least for number ranges.  It is now clear, however,
   that public ENUM as originally envisioned has little prospect for
   adoption.  That said, some national authorities for telephone numbers
   are migrating their provisioning services to the Internet and issuing
   credentials that express authority for telephone numbers to secure
   those services.  These new authorities for numbers could provide to
   the public Internet the necessary signatory authority for securing
   calling party numbers.  While these systems are far from universal,
   the authors of this document believe that a solution devised for the
   North American Numbering Plan could have applicability to other
   country codes.

Top      ToC       Page 20 
6.3.  Public Key Infrastructure Developments

   There have been a number of recent high-profile compromises of web
   certificate authorities.  The presence of numerous (in some cases,
   hundreds) trusted certificate authorities in modern web browsers has
   become a significant security liability.  As [RFC4474] relied on web
   certificate authorities, this too provides new lessons for any work
   on revising [RFC4474], namely, that innovations like DNS-Based
   Authentication of Named Entities (DANE) [RFC6698], which designate a
   specific certificate preferred by the owner of a DNS name, could
   greatly improve the security of a SIP Identity mechanism and,
   moreover, that when considering new certificate authorities for
   telephone numbers, we should be wary of excessive pluralism.  While a
   chain of delegation with a progressively narrowing scope of authority
   (e.g., from a regulatory entity, to a carrier, to a reseller, to an
   end user) is needed to reflect operational practices, there is no
   need to have multiple roots or peer entities that both claim
   authority for the same telephone number or number range.

6.4.  Prevalence of B2BUA Deployments

   Given the prevalence of established B2BUA deployments, we may have a
   further opportunity to review the elements signed using the SIP
   Identity mechanism [RFC4474] and to decide on the value of
   alternative signature mechanisms.  Separating the elements necessary
   for (a) securing the From header field value and preventing replays
   from (b) the elements necessary to prevent men-in-the-middle from
   tampering with messages may also yield a strategy for identity that
   will be practicable in some highly mediated networks.  Solutions in
   this space must, however, remain mindful of the requirements for
   securing cryptographic material necessary to support Datagram
   Transport Layer Security for Secure RTP (DTLS-SRTP) or future
   security mechanisms.

6.5.  Stickiness of Deployed Infrastructure

   One thing that has not changed, and is not likely to change in the
   future, is the transitive nature of trust in the PSTN.  When a call
   from the PSTN arrives at a SIP gateway with a calling party number,
   the gateway will have little chance of determining whether the
   originator of the call was authorized to claim that calling party
   number.  Due to roaming and countless other factors, calls on the
   PSTN may emerge from administrative domains that were not assigned
   the originating number.  This use case will remain the most difficult
   to tackle for an identity system and may prove beyond repair.  It
   does, however, seem that with the changes in the solution space, and

Top      ToC       Page 21 
   a better understanding of the limits of [RFC4474] and VIPR, we are
   today in a position to reexamine the problem space and find solutions
   that can have a significant impact on the secure origins problem.

6.6.  Concerns about Pervasive Monitoring

   While spoofing the origins of communication is a source of numerous
   security concerns, solutions for identifying communications must also
   be mindful of the security risks of pervasive monitoring (see
   [RFC7258]).  Identifying information, once it is attached to
   communications, can potentially be inspected by parties other than
   the intended recipient and collected for any number of reasons.  As
   stated above, the purpose of this work is not to eliminate anonymity;
   furthermore, to be viable and in the public interest, solutions
   should not facilitate the unauthorized collection of calling data.

6.7.  Relationship with Number Assignment and Management

   Currently, telephone numbers are typically managed in a loose
   delegation hierarchy.  For example, a national regulatory agency may
   task a private, neutral entity with administering numbering
   resources, such as area codes, and a similar entity with assigning
   number blocks to carriers and other authorized entities, who in turn
   then assign numbers to customers.  Resellers with looser regulatory
   obligations can complicate the picture, and in many cases, it is
   difficult to distinguish the roles of enterprises from carriers.  In
   many countries, individual numbers are portable between carriers, at
   least within the same technology (e.g., wireline-to-wireline).
   Separate databases manage the mapping of numbers to switch
   identifiers, companies, and textual Caller ID information.

   As the PSTN transitions to using VoIP technologies, new assignment
   policies and management mechanisms are likely to emerge.  For
   example, it has been proposed that geography could play a smaller
   role in number assignments, that individual numbers could be assigned
   to end users directly rather than only to service providers, and that
   the assignment of numbers does not have to depend on providing actual
   call delivery services.

   Databases today already map telephone numbers to entities that have
   been assigned the number, e.g., through the LERG (Local Exchange
   Routing Guide) in the United States.  Thus, the transition to IP-
   based networks may offer an opportunity to integrate cryptographic
   bindings between numbers or number ranges and service providers into
   databases.

Top      ToC       Page 22 
7.  Basic Requirements

   This section describes only the high-level requirements of the STIR
   effort, which we expect will be further articulated as work
   continues:

   Generation:  Intermediaries as well as end systems must be able to
      generate the source identity information.

   Validation:  Intermediaries as well as end systems must be able to
      validate the source identity information.

   Usability:  Any validation mechanism must work without human
      intervention, for example, without mechanisms like CAPTCHA
      (Completely Automated Public Turing test to tell Computers and
      Humans Apart).

   Deployability:  Must survive transition of the call to the PSTN and
      the presence of B2BUAs.

   Reflecting existing authority:  Must stage credentials on existing
      national-level number delegations, without assuming the need for
      an international golden root on the Internet.

   Accommodating current practices:  Must allow number portability among
      carriers and must support legitimate usage of number spoofing
      (e.g., doctors' offices and call centers).

   Minimal payload overhead:  Must lead to minimal expansion of SIP
      header fields to avoid fragmentation in deployments that use UDP.

   Efficiency:  Must minimize RTTs for any network lookups and minimize
      any necessary cryptographic operations.

   Privacy:  A solution must minimize the amount of information that an
      unauthorized party can learn about what numbers have been called
      by a specific caller and what numbers have called a specific
      called party.

   Some requirements specifically outside the scope of the effort
   include:

   Display name:  This effort does not consider how the display name of
      the caller might be validated.

Top      ToC       Page 23 
   Response authentication:  This effort only considers the problem of
      providing secure telephone identity for requests, not for
      responses to requests; no solution is proposed for the problem of
      determining to which number a call has connected [RFC4916].

8.  Acknowledgments

   We would like to thank Sanjay Mishra, Fernando Mousinho, David
   Frankel, Penn Pfautz, Mike Hammer, Dan York, Andrew Allen, Philippe
   Fouquart, Hadriel Kaplan, Richard Shockey, Russ Housley, Alissa
   Cooper, Bernard Aboba, Sean Turner, Brian Rosen, Eric Burger, and
   Eric Rescorla for the discussion and input that contributed to this
   document.

9.  Security Considerations

   This document is about improving the security of call origin
   identification; security considerations for specific solutions will
   be discussed in solutions documents.

10.  Informative References

   [CONCERNS]   Rosenberg, J., "Concerns around the Applicability of RFC
                4474", Work in Progress, February 2008.

   [NEWS-HACK]  Wikipedia, "News International phone hacking scandal",
                June 2014,
                <http://en.wikipedia.org/w/index.php?title=News
                _International_phone_hacking_scandal&oldid=614607591>.

   [RFC3261]    Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
                A., Peterson, J., Sparks, R., Handley, M., and E.
                Schooler, "SIP: Session Initiation Protocol", RFC 3261,
                June 2002.

   [RFC3263]    Rosenberg, J. and H. Schulzrinne, "Session Initiation
                Protocol (SIP): Locating SIP Servers", RFC 3263, June
                2002.

   [RFC3324]    Watson, M., "Short Term Requirements for Network
                Asserted Identity", RFC 3324, November 2002.

   [RFC3325]    Jennings, C., Peterson, J., and M. Watson, "Private
                Extensions to the Session Initiation Protocol (SIP) for
                Asserted Identity within Trusted Networks", RFC 3325,
                November 2002.

Top      ToC       Page 24 
   [RFC3966]    Schulzrinne, H., "The tel URI for Telephone Numbers",
                RFC 3966, December 2004.

   [RFC4474]    Peterson, J. and C. Jennings, "Enhancements for
                Authenticated Identity Management in the Session
                Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC4916]    Elwell, J., "Connected Identity in the Session
                Initiation Protocol (SIP)", RFC 4916, June 2007.

   [RFC5039]    Rosenberg, J. and C. Jennings, "The Session Initiation
                Protocol (SIP) and Spam", RFC 5039, January 2008.

   [RFC5727]    Peterson, J., Jennings, C., and R. Sparks, "Change
                Process for the Session Initiation Protocol (SIP) and
                the Real- time Applications and Infrastructure Area",
                BCP 67, RFC 5727, March 2010.

   [RFC6698]    Hoffman, P. and J. Schlyter, "The DNS-Based
                Authentication of Named Entities (DANE) Transport Layer
                Security (TLS) Protocol: TLSA", RFC 6698, August 2012.

   [RFC7258]    Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is
                an Attack", BCP 188, RFC 7258, May 2014.

   [ROBOCALL-CHALLENGE]
                Federal Trade Commission (FTC), "FTC Robocall
                Challenge", <http://robocall.challenge.gov/>.

   [ROBOCALL-FCC]
                Federal Communications Commission (FCC), "Robocalls",
                April 2013, <http://www.fcc.gov/guides/robocalls>.

   [SECURE-ORIGIN]
                Cooper, A., Tschofenig, H., Peterson, J., and B. Aboba,
                "Secure Call Origin Identification", Work in Progress,
                November 2012.

   [SIP-SECURITY]
                Peterson, J., "Retargeting and Security in SIP: A
                Framework and Requirements", Work in Progress, February
                2005.

   [SWATTING]   The Federal Bureau of Investigation (FBI), "Don't Make
                the Call: The New Phenomenon of 'Swatting'", February
                2008, <http://www.fbi.gov/news/stories/2008/february/
                swatting020408>.

Top      ToC       Page 25 
   [TDOS]       Krebs, B., "DHS Warns of 'TDoS' Extortion Attacks on
                Public Emergency Networks", April 2013,
                <http://krebsonsecurity.com/2013/04/dhs-warns-of-tdos-
                extortion-attacks-on-public-emergency-networks/>.

   [VIPR-OVERVIEW]
                Barnes, M., Jennings, C., Rosenberg, J., and M. Petit-
                Huguenin, "Verification Involving PSTN Reachability:
                Requirements and Architecture Overview", Work in
                Progress, December 2013.

Authors' Addresses

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

   EMail: jon.peterson@neustar.biz


   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


   Hannes Tschofenig
   Hall, Tirol  6060
   Austria

   EMail: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at