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Versions: (draft-rescorla-stir-fallback) 00 01

Network Working Group                                        E. Rescorla
Internet-Draft                                                   Mozilla
Intended status: Standards Track                             J. Peterson
Expires: May 2, 2018                                             Neustar
                                                        October 29, 2017


              STIR Out-of-Band Architecture and Use Cases
                       draft-ietf-stir-oob-01.txt

Abstract

   The PASSporT format defines a token that can be carried by signaling
   protocols, including SIP, to cryptographically attest the identify of
   callers.  Not all telephone calls use Internet signaling protocols,
   however, and some calls use them for only part of their signaling
   path.  This document describes use cases that require the delivery of
   PASSporT objects outside of the signaling path, and defines
   architectures and semantics to provide this functionality.

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
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   This Internet-Draft will expire on May 2, 2018.

Copyright Notice

   Copyright (c) 2017 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
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   (https://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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Operating Environments  . . . . . . . . . . . . . . . . . . .   4
   4.  Dataflows . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Case 1: VoIP to PSTN Call . . . . . . . . . . . . . . . .   6
     5.2.  Case 2: Two Smart PSTN endpoints  . . . . . . . . . . . .   6
     5.3.  Case 3: PSTN to VoIP Call . . . . . . . . . . . . . . . .   7
     5.4.  Case 4: Gateway Out-of-band . . . . . . . . . . . . . . .   7
   6.  Storing and Retrieving PASSporTs  . . . . . . . . . . . . . .   8
     6.1.  Storage . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Retrieval . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Solution Architecture . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Credentials and Phone Numbers . . . . . . . . . . . . . .  12
     7.2.  Call Flow . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.3.  Security Analysis . . . . . . . . . . . . . . . . . . . .  13
     7.4.  Substitution Attacks  . . . . . . . . . . . . . . . . . .  13
   8.  Call Placement Service Discovery  . . . . . . . . . . . . . .  14
   9.  Credential Lookup . . . . . . . . . . . . . . . . . . . . . .  15
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   13. Informative References  . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The STIR problem statement [RFC7340] describes widespread problems
   enabled by impersonation in the telephone network, including illegal
   robocalling, voicemail hacking, and swatting.  As telephone services
   are increasingly migrating onto the Internet, and using Voice over IP
   (VoIP) protocols such as SIP [RFC3261], it is necessary for these
   protocols to support stronger identity mechanisms to prevent
   impersonation.  For example, [I-D.ietf-stir-rfc4474bis] defines an
   Identity header of SIP requests capable of carrying a PASSporT
   [I-D.ietf-stir-passport] object in SIP as a means to
   cryptographically attest that the originator of a telephone call is
   authorized to use the calling party number (or, for native SIP cases,
   SIP URI) associated with the originator of the call.  of the request.

   Not all telephone calls use SIP today, however; and even those that
   do use SIP do not always carry SIP signaling end-to-end.  Most calls



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   from telephone numbers still traverse the Public Switched Telephone
   Network (PSTN) at some point.  Broadly, calls fall into one of three
   categories:

   1.  One or both of the endpoints is actually a PSTN endpoint.

   2.  Both of the endpoints are non-PSTN (SIP, Jingle, ...) but the
       call transits the PSTN at some point.

   3.  Non-PSTN calls which do not transit the PSTN at all (such as
       native SIP end-to-end calls).

   The first two categories represent the majority of telephone calls
   associated with problems like illegal robocalling: many robocalls
   today originate on the Internet but terminate at PSTN endpoints.
   However, the core network elements that operate the PSTN are legacy
   devices that are unlikely to be upgradable at this point to support
   an in-band authentication system.  As such, those devices largely
   cannot be modified to pass signatures originating on the Internet--or
   indeed any inband signaling data--intact.  Even if fields for
   tunneling arbtirary data can be found in traditional PSTN signaling,
   in some cases legacy elements would strip the signatures from those
   fields; in others, they might damage them to the point where they
   cannot be verified.  For those first two categories above, any in-
   band authentication scheme does not seem practical in the current
   environment.

   But while the core network of the PSTN remains fixed, the endpoints
   of the telephone network are becoming increasingly programmable and
   sophisticated.  Landline "plain old telephone service" deployments,
   especially in the developed world, are shrinking, and increasingly
   being replaced by three classes of intelligent devices: smart phones,
   IP PBXs, and terminal adapters.  All three are general purpose
   computers, and typically all three have Internet access as well as
   access to the PSTN.  Additionally, various kinds of gateways
   increasingly front for legacy equipment.  All of this provides a
   potential avenue for building an authentication system that
   implements stronger identity while leaving PSTN systems intact.

   This capability also provides an ideal transitional technology while
   in-band STIR adoption is ramping up.  It permits early adopters to
   use the technology even when intervening network elements are not yet
   STIR-aware, and through various kinds of gateways it may allow
   providers with a significant PSTN investment to still secure their
   calls with STIR.

   This specification therefore builds on the PASSporT
   [I-D.ietf-stir-passport] mechanism and the work of



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   [I-D.ietf-stir-rfc4474bis] to define a way that a PASSporT object
   created in the originating network of a call can reach the
   terminating network even when it cannot be carried end-to-end in-band
   in the call signaling.  This relies on a new service defined in this
   document that permits the PASSporT object to be stored during call
   processing and retrieved for verification purposes.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

3.  Operating Environments

   This section describes the environments in which the proposed
   mechanism is intended to operate.  In the simplest setting, Alice is
   calling Bob through some set of gateways and/or the PSTN.  Both Alice
   and Bob have smart devices which can be modified, but they do not
   have a clear connection between them: Alice cannot inject any data
   into signaling which Bob can read, with the exception of the asserted
   destination and origination E.164 numbers.  The calling party number
   might originate from her own device or from the network.  These
   numbers are effectively the only data that can be used for
   coordination between the endpoints.

                                 +---------+
                                /           \
                            +---             +---+
       +----------+        /                      \        +----------+
       |          |       |        Gateways        |       |          |
       |   Alice  |<----->|         and/or         |<----->|    Bob   |
       | (caller) |       |          PSTN          |       | (callee) |
       +----------+        \                      /        +----------+
                            +---             +---+
                                \           /
                                 +---------+

   In a more complicated setting, Alice and/or Bob may not have a smart
   or programmable device, but one or both of them are behind a STIR-
   aware gateway that can participate in out-of-band coordination, as
   shown below:








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                                 +---------+
                                /           \
                            +---             +---+
      +----------+  +--+   /                      \   +--+  +----------+
      |          |  |  |  |        Gateways        |  |  |  |          |
      |   Alice  |<-|GW|->|         and/or         |<-|GW|->|    Bob   |
      | (caller) |  |  |  |          PSTN          |  |  |  | (callee) |
      +----------+  +--+   \                      /   +--+  +----------+
                            +---             +---+
                                \           /
                                 +---------+

   In such a case, Alice might have an analog connection to her gateway/
   switch which is responsible for her identity.  Similarly, the gateway
   would verify Alice's identity, generate the right calling party
   number information and provide that number to Bob using ordinary POTS
   mechanisms.

4.  Dataflows

   Because in these operating environments endpoints cannot pass
   cryptographic information to one another directly through signaling,
   any solution must involve some rendezvous mechanism to allow
   endpoints to communicate.  We call this rendezvous service a "call
   placement service" (CPS), a service where a record of call placement,
   in this case a PASSporT, can be stored for future retrieval.  In
   principle this service could communicate any information, but
   minimally we expect it to include a full-form PASSporT that attests
   the caller, callee, and the time of the call.  The callee can use the
   existence of a PASSporT for a given incoming call as rough validation
   of the asserted origin of that call.  (See Section 9 for limitations
   of this design.)

   There are roughly two plausible dataflow architectures for the CPS:

      The callee registers with the CPS.  When the caller wishes to
      place a call to the callee, it sends the PASSporT to the CPS,
      which immediately forwards it to the callee.

      The caller stores the PASSporT with the CPS at the time of call
      placement.  When the callee receives the call, it contacts the CPS
      and retrieves the PASSporT.

   While the first architecture is roughly isomorphic to current VoIP
   protocols, it shares their drawbacks.  Specifically, the callee must
   maintain a full-time connection to the CPS to serve as a notification
   channel.  This comes with the usual networking costs to the callee
   and is especially problematic for mobile endpoints.  Indeed, if the



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   endpoints had the capabilities to implement such an architecture,
   they could surely just use SIP or some other protocol to set up a
   secure session; even if the media were going through the traditional
   PSTN, a "shadow" SIP session could convey the PASSporT.  Thus, we
   focus on the second architecture in which the PSTN incoming call
   serves as the notification channel and the callee can then contact
   the CPS to retrieve the PASSporT.

5.  Use Cases

   The following are the motivating use cases for this mechanism.  Bear
   in mind that just as in [I-D.ietf-stir-rfc4474bis] there may be
   multiple Identity headers in a single SIP INVITE, so there may be
   multiple PASSporTs in this out-of-band mechanism associated with a
   single call.  For example, a SIP user agent might create a PASSporT
   for a call with an end user credential, and as the call exits the
   originating administrative domain the network authentication service
   might create its own PASSporT for the same call.  As such, these use
   cases may overlap in the processing of a single call.

5.1.  Case 1: VoIP to PSTN Call

   A call originates in the SIP world in a STIR-aware administrative
   domain.  The local authentication service for that administrative
   domain creates a PASSporT which is carried in band in the call per
   [I-D.ietf-stir-rfc4474bis].  The call is routed out of the
   originating administrative domain and reaches a gateway to the PSTN.
   Eventually, the call will terminate on a mobile smartphone that
   supports this out-of-band mechanism.

   In this use case, the originating authentication service can store
   the PASSporT with the appropriate CPS for the target telephone number
   as a fallback in case SIP signaling will not reach end-to-end.  When
   the destination mobile smartphone receives the call over the PSTN, it
   consults the CPS and discovers a PASSporT from the originating
   telephone number waiting for it.  It uses this PASSporT to verify the
   calling party number.

5.2.  Case 2: Two Smart PSTN endpoints

   A call originates with an enterprise PBX that has both Internet
   access and a built-in gateway to the PSTN.  It will immediately drop
   its call to the PSTN, but before it does, it provisions a PASSporT on
   the CPS associated with the target telephone number.

   After normal PSTN routing, the call lands on a smart mobile handset
   that supports the STIR out-of-band mechanism.  It queries the
   appropriate CPS over the Internet to determine if a call has been



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   placed to it by a STIR-aware device.  It finds the PASSporT
   provisioned by the enterprise PBX and uses it to verify the calling
   party number.

5.3.  Case 3: PSTN to VoIP Call

   A call originates with an enterprise PBX that has both Internet
   access and a built-in gateway to the PSTN.  It will immediate drop
   the call to the PSTN, but before it does, it provisions a PASSporT
   with the CPS associated with the target telephone number.  However,
   it turns out that the call will eventually route through the PSTN to
   an Internet gateway, which will translate this into a SIP call and
   deliver it to an administrative domain with a STIR verification
   service.

   In this case, there are two subcases for how the PASSporT might be
   retrieved.  In subcase 1, the Internet gateway that receives the call
   from the PSTN could query the appropriate CPS to determine if the
   original caller created and provisioned a PASSporT for this call.  If
   so, it can retrieve the PASSporT and, when it creates a SIP INVITE
   for this call, add a corresponding Identity header per
   [I-D.ietf-stir-rfc4474bis].  When the SIP INVITE reaches the
   destination administrative domain, it will be able to verify the
   PASSporT normally.  Note that to avoid discrepancies with the Date
   header field value, only full-form PASSporT should be used for this
   purpose.  In subcase 2, the gateway does not retrieve the PASSporT
   itself, but instead the verification service at the destination
   administrative domain does so.  Subcase 1 would perhaps be valuable
   for deployments where the destination administrative domain supports
   in-band STIR but not out-of-band STIR.

5.4.  Case 4: Gateway Out-of-band

   A call originates in the SIP world in a STIR-aware administrative
   domain.  The local authentication service for that administrative
   domain creates a PASSporT which is carried in band in the call per
   [I-D.ietf-stir-rfc4474bis].  The call is routed out of the
   originating administrative domain and eventually reaches a gateway to
   the PSTN.

   In this case, the originating authentication service does not support
   the out-of-band mechanism, so instead the gateway to the PSTN
   extracts the PASSporT from the SIP request and provisions it to the
   CPS.  (When the call reaches the gateway to the PSTN, the gateway
   might first check the CPS to see if a PASSporT object had already
   been provisioned for this call, and only provision a PASSporT if none
   is present).




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   Ultimately, the call may terminate on the PSTN, or be routed back to
   the IP world.  In the former case, perhaps the destination endpoints
   queries the CPS to retrieve the PASSporT provisioned by the first
   gateway.  Or if the call ultimately returns to the IP world, it might
   be the gateway from the PSTN back to the Internet that retrieves the
   PASSporT from the CPS and attaches it to the new SIP INVITE it
   creates, or it might be the terminating administrative domain's
   verification service that checks the CPS when an INVITE arrives with
   no Identity header field.  Either way the PASSporT can survive the
   gap in SIP coverage caused by the PSTN leg of the call.

6.  Storing and Retrieving PASSporTs

   The use cases show a variety of entities accessing the CPS to store
   and retrieve PASSporTs.  The question of how the CPS authorizes the
   storage and retrieval of PASSporT is thus a key design decision in
   the architecture.  Broadly, the architecture described here is one
   focused on permitting any entity to store encrypted PASSporTs at the
   CPS, indexed under the caller number.  PASSporTs will be encrypted
   with associated with the called number, so these PASSporTs may also
   be retrieved by any entity, as only holders of the corresponding
   private key will be able to decrypt the PASSporT.  This also prevents
   the CPS itself from learning the contents of PASSporTs, and thus
   metadata about calls in progress, which would make the CPS a less
   attractive target for pervasive monitoring (see [RFC7258]).  Ho
   bolster the privacy story, prevent denial-of-service flooding of the
   CPS, and to complicate traffic analysis, a few additional mechanisms
   are also recommended.

   The STIR architecture assumes that service providers and in some
   cases end user devices will have credentials suitable for attesting
   authority over telephone numbers per [I-D.ietf-stir-certificates].
   These credentials provide the most obvious way that a CPS can
   authorize the storage and retrieval of PASSporTs.  However, as use
   cases 3 and 4 in Section 5 show, it may sometimes make sense for the
   entity storing or retrieving PASSporTs to be an intermediary rather
   than a device associated with either the originating or terminating
   side of a call, and those intermediaries often would not have access
   to STIR credentials covering the telephone numbers in question.
   Requiring authorization based on a credential to store PASSporTs is
   therefore undesirable, though potentially acceptible if sufficient
   steps are taken to mitigate the privacy risk as described in the next
   section.

   Furthermore, it is an explicit design goal of this mechanism to
   minimize the potential privacy exposure of using a CPS.  Ideally, the
   out-of-band mechanism should not result in a worse privacy situation
   than in-band [I-D.ietf-stir-rfc4474bis] STIR: for in-band, we might



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   say that a SIP entity is authorized to receive a PASSporT if it is an
   intermediate or final target of the routing of a SIP request.  As the
   originator of a call cannot necessarily predict the routing path a
   call will follow, an out-of-band mechanism could conceivably even
   improve on the privacy story.  As a first step, transport-level
   security can provide confidentiality from eavesdroppers for both the
   storage and retrieval of PASSporTs.

6.1.  Storage

   For authorizing the storage of PASSporTs, the architecture can permit
   some flexibility.  Note that in this architecture a CPS has no way to
   tell if a PASSporT is valid; it simply conveys encrypted blocks that
   it cannot access itself.  In that architecture, it does not matter
   whether the CPS received a PASSporT from the authentication service
   that created it or from an intermediary gateway downstream in the
   routing path as in case 4.

   Note that this architecture requires clients that stores PASSporTs to
   have access to a public key associated with the intended called party
   to be used to encrypt the PASSporT.  Discovering this key requires
   some new service that does not exist today; depending on how the CPS
   is architected, however, some kind of key store or repository could
   be implemented adjacent to it, and perhaps even incorporated into its
   operation.  Key discovery is made more complicated by the fact that
   there can potentially be multiple entities that have authority over a
   telephone number: a carrier, a reseller, an enterprise, and an end
   user might all have credentials permitting them to attest that they
   are allowed to originate calls from a number, say.  PASSporTs
   therefore might need to be encrypted with multiple keys in the hopes
   that one will be decipherable by the relying party.

   However, if literally anyone can store PASSporTs in the CPS, an
   attacker could easily flood the CPS with millions of bogus PASSporTs
   indexed under a target number, and thereby prevent that called party
   from finding a valid PASSporT for an incoming call buried in a
   haystack of fake entries.  A CPS must therefore implement some sort
   of traffic control system to prevent flooding.  Preferably, this
   should not require authenticating the source, as this will reveal to
   the CPS both ths source and destination of traffic.

   In order to do this, we propose the use of "blind signatures".  A
   sender will initially authenticate to the CPS, and acquire a signed
   token for the CPS that will be presented later when storing a
   PASSporT.  The flow looks as follows:






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       Sender                                 CPS

       Authenticate to CPS --------------------->
       Blinded(K_temp) ------------------------->
       <------------- Sign(K_cps, Blinded(K_temp))
       [Disconnect]


       Sign(K_cps, K_temp))
       Sign(K_temp, E(K_receiver, PASSporT)) --->

   At an initial time when no call is yet in progress, a potential
   client connects to the CPS, authenticates, and sends a blinded
   version of a freshly generated public key.  The CPS returns a signed
   version of that blinded key.  The sender can then unblind the key and
   gets a signature on K_temp from the CPS

   Then later, when a client wants to store a PASSporT, it connects to
   the CPS anonymously (preferably over a network connection that cannot
   be correlated with the token acquisition) and sends both the signed
   K_temp and its own signature over the encrypted PASSporT.  The CPS
   verifies both signatures and if they verify, stores the encrypted
   passport (discarding the signatures).

   This design lets the CPS rate limit how many PASSporTs a given sender
   can store just by counting how many times K_temp appears; perhaps CPS
   policy might reject storage attempts and require acqusition of a new
   K_temp after storing more than a certain number of PASSporTs indexed
   under the same destination number in a short interval.  This does not
   of course allow the CPS to tell when bogus data is being provisioned
   by an attacker, simply the rate at which data is being provisioned.
   Potentially, feedback mechanisms could be developed that would allow
   the called parties to tell the CPS when they are receiving unusual or
   bogus PASSporTs.

   This architecture also assumes that the CPS will age out PASSporTs.
   A CPS SHOULD NOT keep any stored PASSporT for more than sixty
   seconds.  Any reduction in this window makes substitution attacks
   (see Section 7.4) harder to mount, but making the window too small
   might conceivably age PASSporTs out while a heavily redirected call
   is still alerting.  harder to mount

6.2.  Retrieval

   For retrieval of PASSporTs, this architecture assumes that clients
   contact the CPS to send requests of the form:





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      Are there any current PASSporTs for calls destined to
      2.222.222.2222?

   As all PASSporTs stored at the CPS are encrypted with a key belonging
   to the intended destination, then potentially the CPS could allow
   anyone to download PASSporTs for a called number without much fear of
   compromising private information about calls in progress - provided
   that the CPS always provides at least one encrypted blob in response
   to a request, even if there was no call in progress.  Otherwise,
   entities could poll the CPS constantly, or eavesdrop on traffic, to
   learn whether or not calls were in progress.  The CPS MUST generate
   at least one unique and plausible encrypted response to all retrieval
   requests, and these dummy encrypted PASSporTs MUST NOT be repeated
   for later calls.

   Because the entity placing a call may discover multiple keys
   associated with the called party number, multiple valid PASSporTs may
   be stored in the CPS.  A particular called party who retrieves
   PASSporTs from the CPS may have access to only one of those keys.
   Thus, the presence of one or more PASSporTs that the called party
   cannot decrypt - which would be indistinguishable from the "dummy"
   PASSporTS created by the CPS when no calls are in progress - does not
   entail that there is no call in progress.  A retriever likely will
   need decrypt all PASSporTs retrieved from the CPS, and may find only
   one that is valid.

   Note that in call forwarding cases, the difficulties in managing the
   relationship between PASSporTs with the diversion extension
   [I-D.ietf-stir-passport-divert] become more serious.  The originating
   authentication service would encrypt the PASSporT with the public key
   of the intended destination, but when a call is forwarded, it may go
   to a destination that does not possess the corresponding private key.
   This requires special behavior on the part of the retargeting entity,
   and probably the CPS as well, to accommodate encrypted PASSporTs that
   show a secure chain of diversion.  A storer could for example notify
   the CPS that the divert PASSporT it is storing relates to a specific
   PASSporT already in the CPS, but in so doing, the storer will
   inevitably reveal more metadata to the CPS.

7.  Solution Architecture

   In this section, we discuss a strawman architecture for providing the
   service described in the previous sections.  This discussion is
   deliberately sketchy, focusing on broad concepts and skipping over
   details.  The intent here is merely to provide an overall
   architecture, not an implementable specification.





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7.1.  Credentials and Phone Numbers

   We start from the premise of the STIR problem statement [RFC7340]
   that phone numbers can be associated with credentials which can be
   used to attest ownership of numbers.  For purposes of exposition, we
   will assume that ownership is associated with the endpoint (e.g., a
   smartphone) but it might well be associated with a provider or
   gateway acting for the endpoint instead.  It might be the case that
   multiple entities are able to act for a given number, provided that
   they have the appropriate authority.  [I-D.ietf-stir-certificates]
   describes a credentials system suitable for this purpose; the
   question of how an entity is determined to have control of a given
   number is out of scope for the current document.

7.2.  Call Flow

   An overview of the basic calling and verification process is shown
   below.  In this diagram, we assume that Alice has the number
   +1.111.111.1111 and Bob has the number +2.222.222.2222.

Alice                       Call Placement Service                  Bob
-----------------------------------------------------------------------

Store PASSporT for 2.222.222.2222-->

Call from 1.111.111.1111 --------------------------------------------->


                                     <------------- Retrieve PASSporT(s)
                                                    for 2.222.222.2222?

                                    Encrypted PASSporT
                                    -(2.222.222.2222,1.111.111.1111)-->

                                              [Ring phone with callerid
                                                      = 1.111.111.1111]

   When Alice wishes to make a call to Bob, she contacts the CPS and
   stores an encrypted PASSporT on the CPS indexed under Bob's number.
   The CPS then awaits retrievals for that number.

   Once Alice has stored the PASSporT, she then places the call to Bob
   as usual.  At this point, Bob's phone would usually ring and display
   Alice's number (+1.111.111.1111), which is informed by the existing
   PSTN mechanisms for relying a calling party number (i.e., the CIN
   field of the IAM).  Instead, Bob's phone transparently contacts the
   CPS and requests any current PASSporTs for calls to his number.  The
   CPS responds with any such PASSporTs (assuming they exist).  If such



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   a PASSpoRT exists, and the verification service in Bob's phone
   decrypts it using his private key, validates it, then Bob's phone can
   then present the calling party number information as valid.
   Otherwise, the call is unverifiable.  Note that this does not
   necessarily mean that the call is bogus; because we expect
   incremental deployment many legitimate calls will be unverifiable.

7.3.  Security Analysis

   The primary attack we seek to prevent is an attacker convincing the
   callee that a given call is from some other caller C.  There are two
   scenarios to be concerned with:

      The attacker wishes to impersonate a target when no call from that
      target is in progress.

      The attacker wishes to substitute himself for an existing call
      setup as described in Section 7.4.

   If an attacker can inject fake PASSporT into the CPS or in the
   communication from the CPS to the callee, he can mount either attack.
   As PASSporTs should be digitally signed by an appropriate authority
   for the number and verified by the callee (see Section 7.1), this
   should not arise in ordinary operations.  For privacy and robustness
   reasons, using TLS on the originating side when storing the PASSporT
   at the CPS is recommended.

   The entire system depends on the security of the credential
   infrastructure.  If the authentication credentials for a given number
   are compromised, then an attacker can impersonate calls from that
   number.  However, that is no different from in-band
   [I-D.ietf-stir-rfc4474bis] STIR.

7.4.  Substitution Attacks

   All that receipt of the PASSporT from the CPS proves to the called
   party is that Alice is trying to call Bob (or at least was as of very
   recently) - it does not prove that any particular incoming call is
   from Alice.  Consider the scenario in which we have a service which
   provides an automatic callback to a user-provided number.  In that
   case, the attacker can try to arrange for a false caller-id value, as
   shown below:









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 Attacker            Callback Service              CPS               Bob
 -----------------------------------------------------------------------
 Place call to Bob ---------->

                             Store PASSporT for
                             CS:Bob -------------->

 Call from CS (forged caller-id info)  -------------------------------->

                             Call from CS ---------------------------> X


                                                <----- Retrieve PASSporT
                                                              for CS:Bob

                        PASSporT for CS:Bob --------------------------->

                                         [Ring phone with callerid = CS]

   In order to mount this attack, the attacker contacts the Callback
   Service (CS) and provides it with Bob's number.  This causes the CS
   to initiate a call to Bob. As before, the CS contacts the CPS to
   insert an appropriate PASSporT and then initiates a call to Bob.
   Because it is a valid CS injecting the PASSporT, none of the security
   checks mentioned above help.  However, the attacker simultaneously
   initiates a call to Bob using forged caller-id information
   corresponding to the CS.  If he wins the race with the CS, then Bob's
   phone will attempt to verify the attacker's call (and succeed since
   they are indistinguishable) and the CS's call will go to busy/voice
   mail/call waiting.  Note: in a SIP environment, the callee might
   notice that there were multiple INVITEs and thus detect this attack.

8.  Call Placement Service Discovery

   In order for the two ends of the out-of-band dataflow to coordinate,
   they must agree on a way to discover a CPS and retrieve PASSporT
   objects from it based solely on the rendezvous information available:
   the calling party number and the called number.  Because the storage
   of PASSporTs in this architecture is indexed by the called party
   number, it makes sense to discover a CPS based on the called party
   number as well.  There are a number of potential service discovery
   mechanisms that could be used for this purpose.  The means of service
   discovery may vary by use case.

   Although the discussion above is written in terms of a single CPS,
   having a significant fraction of all telephone calls result in
   storing and retrieving PASSporTs at a single monolithic CPS has
   obvious scaling problems, and would as well allow the CPS to gather



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   metadata about a very wide set of callers and callees.  These issues
   can be alleviated by operational models with a federated CPS; any
   service discovery mechanism for out-of-band STIR should enable
   federation of the CPS function.

   Some service discovery possibilities under consideration include the
   following:

      If a credential lookup service is already available (see
      Section 9), the CPS location can also be recorded in the callee's
      credentials; an extension to [I-D.ietf-stir-certificates] could
      for example provide a link to the location of the CPS where
      PASSporTs should be stored for a destination.

      There exist a number of common directory systems that might be
      used to translate telephone numbers into the URIs of a CPS.  ENUM
      [RFC6116] is commonly implemented, though no "golden root" central
      ENUM administration exists that could be easily reused today to
      help the endpoints discover a common CPS.  Other protocols
      associated with queries for telephone numbers, such as the TeRI
      [I-D.peterson-modern-teri] protocol, could also serve for this
      application.

      Another possibility is to use a single distributed service for
      this function.  VIPR [I-D.rosenberg-dispatch-vipr-overview]
      proposed a RELOAD [RFC6940] usage for telephone numbers to help
      direct calls to enterprises on the Internet.  It would be possible
      to describe a similar RELOAD usage to identify the CPS where calls
      for a particular telephone number should be stored.  One advantage
      that the STIR architecture has over VIPR is that it assumes a
      credential system that proves authority over telephone numbers;
      those credentials could be used to determine whether or not a CPS
      could legitimately claim to be the proper store for a given
      telephone number.

   Future versions of this specification will identify suitable service
   discovery mechanisms for out-of-band STIR.

9.  Credential Lookup

   In order to encrypt a PASSporT (see Section 6.1), the caller needs
   access to the callee's credentials (specifically their public key).
   This requires some sort of directory/lookup system.  This document
   does not specify any particular scheme, but a list of requirements
   would be something like:

   Obviously, if there is a single central database and the caller and
   callee each contact it in real time to determine the other's



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   credentials, then this represents a real privacy risk, as the central
   database learns about each call.  A number of mechanisms are
   potentially available to mitigate this:

      Have endpoints pre-fetch credentials for potential counterparties
      (e.g., their address book or the entire database).

      Have caching servers in the user's network that proxy their
      fetches and thus conceal the relationship between the user and the
      credentials they are fetching.

   Clearly, there is a privacy/timeliness tradeoff in that getting up-
   to-date knowledge about credential validity requires contacting the
   credential directory in real-time (e.g., via OCSP).  This is somewhat
   mitigated for the caller's credentials in that he can get short-term
   credentials right before placing a call which only reveals his
   calling rate, but not who he is calling.  Alternately, the CPS can
   verify the caller's credentials via OCSP, though of course this
   requires the callee to trust the CPS's verification.  This approach
   does not work as well for the callee's credentials, but the risk
   there is more modest since an attacker would need to both have the
   callee's credentials and regularly poll the database for every
   potential caller.

   We consider the exact best point in the tradeoff space to be an open
   issue.

10.  Acknowledgments

   The ideas in this document come out of discussions with Richard
   Barnes and Cullen Jennings.  We'd also like to thank Robert Sparks
   for helpful suggestions.

11.  IANA Considerations

   This memo includes no request to IANA.

12.  Security Considerations

   This entire document is about security, but the detailed security
   properties depend on having a single concrete scheme to analyze.

13.  Informative References

   [I-D.ietf-stir-certificates]
              Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", draft-ietf-stir-
              certificates-14 (work in progress), May 2017.



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   [I-D.ietf-stir-passport]
              Wendt, C. and J. Peterson, "Personal Assertion Token
              (PASSporT)", draft-ietf-stir-passport-11 (work in
              progress), February 2017.

   [I-D.ietf-stir-passport-divert]
              Peterson, J., "PASSporT Extension for Diverted Calls",
              draft-ietf-stir-passport-divert-00 (work in progress),
              July 2017.

   [I-D.ietf-stir-rfc4474bis]
              Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-16
              (work in progress), February 2017.

   [I-D.peterson-modern-teri]
              Peterson, J., "An Architecture and Information Model for
              Telephone-Related Information (TeRI)", draft-peterson-
              modern-teri-03 (work in progress), July 2017.

   [I-D.rosenberg-dispatch-vipr-overview]
              Rosenberg, J., Jennings, C., and M. Petit-Huguenin,
              "Verification Involving PSTN Reachability: Requirements
              and Architecture Overview", draft-rosenberg-dispatch-vipr-
              overview-04 (work in progress), October 2010.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC6116]  Bradner, S., Conroy, L., and K. Fujiwara, "The E.164 to
              Uniform Resource Identifiers (URI) Dynamic Delegation
              Discovery System (DDDS) Application (ENUM)", RFC 6116,
              DOI 10.17487/RFC6116, March 2011,
              <https://www.rfc-editor.org/info/rfc6116>.

   [RFC6940]  Jennings, C., Lowekamp, B., Ed., Rescorla, E., Baset, S.,
              and H. Schulzrinne, "REsource LOcation And Discovery
              (RELOAD) Base Protocol", RFC 6940, DOI 10.17487/RFC6940,
              January 2014, <https://www.rfc-editor.org/info/rfc6940>.



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   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,
              <https://www.rfc-editor.org/info/rfc7340>.

Authors' Addresses

   Eric Rescorla
   Mozilla

   Email: ekr@rtfm.com


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

   Email: jon.peterson@neustar.biz



























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