Network Working Group E. Rescorla
Internet-Draft Mozilla
Intended status: Standards Track J. Peterson
Expires: January 3, 2019 Neustar
July 2, 2018
STIR Out-of-Band Architecture and Use Cases
draft-ietf-stir-oob-03.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
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This Internet-Draft will expire on January 3, 2019.
Copyright Notice
Copyright (c) 2018 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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include Simplified BSD License text as described in Section 4.e of
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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. Authentication and Verification Service Behavior for Out-of-
Band . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Authentication Service . . . . . . . . . . . . . . . . . 14
8.2. Verification Service . . . . . . . . . . . . . . . . . . 16
8.3. Gateway Placement Services . . . . . . . . . . . . . . . 17
9. Example HTTPS Interface to the CPS . . . . . . . . . . . . . 17
10. CPS Discovery . . . . . . . . . . . . . . . . . . . . . . . . 19
11. Credential Lookup . . . . . . . . . . . . . . . . . . . . . . 20
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
14. Security Considerations . . . . . . . . . . . . . . . . . . . 21
15. Informative References . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
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, [RFC8224] defines an Identity header of
SIP requests capable of carrying a PASSporT [RFC8225] object in SIP
as a means to cryptographically attest that the originator of a
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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
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
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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 [RFC8225]
mechanism and the work of [RFC8224] 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 11 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 [RFC8224] 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
[RFC8224]. 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 [RFC8224].
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
[RFC8224]. 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).
Ultimately, the call may terminate on the PSTN, or be routed back to
the IP world. In the former case, perhaps the destination endpoints
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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 [RFC8226]. 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 [RFC8224] STIR: for in-band, we might 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
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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 out-of-band call forwarding cases, special behavior is
required to manage the relationship between PASSporTs using the
diversion extension [I-D.ietf-stir-passport-divert]. The originating
authentication service would encrypt the initial PASSporT with the
public key of the intended destination, but once a call is forwarded,
it may go to a destination that does not possess the corresponding
private key and thus could not decrypt the original PASSporT. This
requires the retargeting entity to generated encrypted PASSporTs that
show a secure chain of diversion: a retargeting storer SHOULD use the
"opt" extension to "div" specified in [I-D.ietf-stir-passport-divert]
in order to nest the original PASSporT within the encrypted diversion
PASSporT.
7. Solution Architecture
In this section, we discuss a high-level 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. A more concrete
example of how this might be specified is given in Section 9.
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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. [RFC8226] 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 [RFC8224] 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. Authentication and Verification Service Behavior for Out-of-Band
[RFC8224] defines an authentication service and a verification
service as functions that act in the context of SIP requests and
responses. This specification thus provides a more generic
description of authentication service and verification service
behavior that might or might not involve any SIP transactions, but
depends only on placing a request for communications from an
originating identity to one or more destination identities.
8.1. Authentication Service
Out-of-band authentication services perform steps similar to those
defined in [RFC8224] with some exceptions:
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Step 1: The authentication service MUST determine whether it is
authoritative for the identity of the originator of the request, that
is, the identity it will populate in the "orig" claim of the
PASSporT. It can do so only if it possesses the private key of one
or more credentials that can be used to sign for that identity, be it
a domain or a telephone number or something other identifier. For
example, the authentication service could hold the private key
associated with a STIR certificate [RFC8225].
Step 2: The authentication service MUST determine that the originator
of communications can claim the originating identity. This is a
policy decision made by the authentication service that depends on
its relationship to the originator. For an out-of-band application
built in to the calling device, for example, this is the same check
performed in Step 1: does the calling device have a private key, such
one corresponding to a STIR certificate, that can sign for the
originating identity?
Step 3: The authentication service MUST acquire the public key of the
destination, which will be used to encrypt the PASSporT. It must
also discover (see Section 10) the CPS associated with the
destination. The authentication service may already have the key and
destination CPS cached, or may need to query a service to acquire the
key. Note that per Section 6.1 the authentication service may also
need to acquire a token for PASSporT storage from the CPS upon CPS
discovery. It is anticipated that the discovery mechanism (see
Section 10) used to find the appropriate CPS will also find the
proper key server for the public key of the destination. In some
cases, a destination may have multiple public keys associated with
it. In that case, the authentication service MUST collect all of
those keys.
Step 4: The authentication service MUST create the PASSporT object.
This includes acquiring the system time to populate the "iat" claim,
and populating the "orig" and "dest" claims as described in
[RFC8225]. The authentication service MUST then encrypt the
PASSporT. If in Step 3 the authentication service discovered
multiple public keys for the destination, it MUST create one
encrypted copy for each public key it discovered.
Finally, the authentication service stores the encrypted PASSporT(s)
at the CPS discovered in Step 3. Only after that is completed should
any call initiated. Note that a call might be initiated over SIP,
and the authentication service would place the same PASSporT in the
Identity header field value of the SIP request - though SIP would
carry cleartext version rather than an encrypted version sent to the
CPS. In that case, out-of-band would serve as a fallback mechanism
in case the request was not conveyed over SIP end-to-end. Also, note
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that the authentication service MAY use a compact form of the
PASSporT for a SIP request, whereas the version stored at the CPS
MUST always be a full form PASSporT.
8.2. Verification Service
When a call arrives, an out-of-band verification service performs
steps similar to those defined in [RFC8224] with some exceptions:
Step 1: The verification service contacts the CPS and requests all
current PASSporTs for its destination number. The verification
service MUST then decrypt all PASSporTs using its private key. Some
PASSporTs may not be decryptable for any number of reasons: they may
be intended for a different verification service, or they may be
"dummy" values inserted by the CPS for privacy purposes. The next
few steps will narrow down the set of PASSporTs that the verification
service will examine from that initial decryptable set.
Step 2: The verification service MUST determine if any "ppt"
extensions in the PASSporTs are unsupported. It takes only the set
of supported PASSporTs and applies the next step to them.
Step 3: The verification service MUST determine if there is an
overlap between the called party number number presented in call
signaling and the "orig" field of any decrypted PASSporTs. It takes
the set of matching PASSporTs and applies the next step to them.
Step 4: The verification service MUST determine if the credentials
that signed each PASSporT are valid, and if the verification service
trusts the CA that issued the credentials. It takes the set of
trusted PASSporTs to the next step.
Step 5: The verification service MUST check the freshness of the
"iat" claim of each PASSporT. The exact interval of time that
determines freshness is left to local policy. It takes the set of
fresh PASSporTs to the next step.
Step 6: The verification service MUST check the validity of the
signature over each PASSporT, as described in described in [RFC8225].
Finally, the verification service will end up with one or more valid
PASSporTs corresponding to the call it has received. This document
does not prescribe any particular treatment of calls that have valid
PASSporTs associated with them. The handling of the message after
the verification process depends on how the verification service is
implemented and on local policy. However, it is anticipated that
local policies could involve making different forwarding decisions in
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intermediary implementations, or changing how the user is alerted or
how identity is rendered in UA implementations.
8.3. Gateway Placement Services
The out-of-band mechanism also supports the presence of gateway
placement services, which do not create PASSporTs themselves, but
instead take PASSporTs out of signaling protocols and store them at a
CPS before gatewaying to a protocol that cannot carry PASSporTs
itself. For example, a SIP gateway that sends calls to the PSTN
could receive a call with an Identity header, extract a PASSporT from
the Identity header, and store that PASSporT at a CPS.
To place a PASSporT at a CPS, a gateway MUST perform Step 3 of
Section 8.1 above: that is, it must discover the CPS and public key
associated with the destination of the call, and may need to acquire
a PASSporT storage token (see Section 6.1). Per Step 3 this may
entail discovering several keys. The gateway then collects the in-
band PASSporT(s) from the in-band signaling, encrypts the
PASSporT(s), and stores them at the CPS.
A similar service could be performed by a gateway that retrieves
PASSporTs from a CPS and inserts them into signaling protocols that
support carrying PASSporTS in-band. This behavior may be defined by
future specifications.
9. Example HTTPS Interface to the CPS
As an rough example, we should a Call Placement Service
implementation here which uses a REST API to store and retrieve
objects at the CPS. The calling party stores the PASSporT at the CPS
prior to initiating the call; the PASSporT is stored at a location at
the CPS that corresponds to the called number. Note that it is
possible for multiple parties to be calling a number at the same
time, and that for called numbers such as large call centers, many
PASSporTs could legitimately be stored simultaneously, and it might
prove difficult to correlate these with incoming calls.
Assume that an authentication service has created the following
PASSporT for a call to the telephone number 2.222.222.2222 (note that
these are dummy values):
eyJhbGciOiJFUzI1NiIsInR5cCI6InBhc3Nwb3J0IiwieDV1IjoiaHR0cHM6Ly9j
ZXJ0LmV4YW1wbGUub3JnL3Bhc3Nwb3J0LmNlciJ9.eyJkZXN0Ijp7InVyaSI6WyJz
aXA6YWxpY2VAZXhhbXBsZS5jb20iXX0sImlhdCI6IjE0NDMyMDgzNDUiLCJvcmlnI
jp7InRuIjoiMTIxNTU1NTEyMTIifX0.rq3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1
VOgFWSjHBr8Qjpjlk-cpFYpFYsojNCpTzO3QfPOlckGaS6hEck7w
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Through some discovery mechanism (see Section 10), the authentication
service discovers the network location of a web service that acts as
the CPS for 2.222.222.2222. Through the same mechanism, we will say
that it has also discovered one public key for that destination. It
uses that public key to encrypt the PASSporT, resulting in the
encrypted PASSporT:
rlWuoTpvBvWSHmV1AvVfVaE5pPV6VaOup3Ajo3W0VvjvrQI1VwbvnUE0pUZ6Yl9w
MKW0YzI4LJ1joTHho3WaY3Oup3Ajo3W0YzAypvW9rlWxMKA0Vwc7VaIlnFV6JlWm
nKN6LJkcL2INMKuuoKOfMF5wo20vKK0fVzyuqPV6VwR0AQZlZQtmAQHvYPWipzyaV
wc7VaEhVwbvZGVkAGH1AGRlZGVvsK0ed3cwG1ubEjnxRTwUPaJFjHafuq0-mW6S1
IBtSJFwUOe8Dwcwyx-pcSLcSLfbwAPcGmB3DsCBypxTnF6uRpx7j
Having concluded the numbered steps in Section 8.1, including
acquiring any token (per Section 6.1) needed to store the PASSporT at
the CPS, the authentication service then stores the encrypted
PASSporT:
POST /cps/2.222.222.2222/ppts HTTP/1.1
Host: cps.example.com
Content-Type: application/passport
rlWuoTpvBvWSHmV1AvVfVaE5pPV6VaOup3Ajo3W0VvjvrQI1VwbvnUE0pUZ6Yl9w
MKW0YzI4LJ1joTHho3WaY3Oup3Ajo3W0YzAypvW9rlWxMKA0Vwc7VaIlnFV6JlWm
nKN6LJkcL2INMKuuoKOfMF5wo20vKK0fVzyuqPV6VwR0AQZlZQtmAQHvYPWipzyaV
wc7VaEhVwbvZGVkAGH1AGRlZGVvsK0ed3cwG1ubEjnxRTwUPaJFjHafuq0-mW6S1
IBtSJFwUOe8Dwcwyx-pcSLcSLfbwAPcGmB3DsCBypxTnF6uRpx7j
The web service assigns a new location for this encrypted PASSporT in
the collection, returning a 201 OK with the location of
/cps/2.222.222.2222/ppts/ppt1. Now the authentication service can
place the call, which may be signaled by various protocols. Once the
call arrives at the terminating side, a verification service
interrogates its CPS to ask for the set of incoming calls for its
telephone number (2.222.222.2222).
GET /cps/2.222.222.2222/ppts
Host: cps.example.com
This returns to the verification service a list of the PASSporTs
currently in the collection, which currently consists of only
/cps/2.222.222.2222/ppts/ppt1. The verification service then sends a
new GET for /cps/2.222.222.2222/ppts/ppt1/ which yields:
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HTTP/1.1 200 OK
Content-Type: application/passport
Link: <https://cps.example.com/cps/2.222.222.2222/ppts>
rlWuoTpvBvWSHmV1AvVfVaE5pPV6VaOup3Ajo3W0VvjvrQI1VwbvnUE0pUZ6Yl9w
MKW0YzI4LJ1joTHho3WaY3Oup3Ajo3W0YzAypvW9rlWxMKA0Vwc7VaIlnFV6JlWm
nKN6LJkcL2INMKuuoKOfMF5wo20vKK0fVzyuqPV6VwR0AQZlZQtmAQHvYPWipzyaV
wc7VaEhVwbvZGVkAGH1AGRlZGVvsK0ed3cwG1ubEjnxRTwUPaJFjHafuq0-mW6S1
IBtSJFwUOe8Dwcwyx-pcSLcSLfbwAPcGmB3DsCBypxTnF6uRpx7j
That concludes Step 1 of Section 8.2; the verification service then
goes on to the next step, processing that PASSporT through its
various checks. A complete protocol description for CPS interactions
is left to future work.
10. CPS 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
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 11), the CPS location can also be recorded in the callee's
credentials; an extension to [RFC8226] 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
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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.
This document does not prescribe any single way to do service
discovery for a CPS; it is envisioned that initial deployments will
provision at the AS and VS.
11. 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
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
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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.
12. 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.
13. IANA Considerations
This memo includes no request to IANA.
14. Security Considerations
This entire document is about security, but the detailed security
properties depend on having a single concrete scheme to analyze.
15. Informative References
[I-D.ietf-stir-passport-divert]
Peterson, J., "PASSporT Extension for Diverted Calls",
draft-ietf-stir-passport-divert-02 (work in progress),
March 2018.
[I-D.peterson-modern-teri]
Peterson, J., "An Architecture and Information Model for
Telephone-Related Information (TeRI)", draft-peterson-
modern-teri-04 (work in progress), March 2018.
[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>.
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[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>.
[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>.
[RFC8224] Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 8224,
DOI 10.17487/RFC8224, February 2018,
<https://www.rfc-editor.org/info/rfc8224>.
[RFC8225] Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
<https://www.rfc-editor.org/info/rfc8225>.
[RFC8226] Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", RFC 8226,
DOI 10.17487/RFC8226, February 2018,
<https://www.rfc-editor.org/info/rfc8226>.
Authors' Addresses
Eric Rescorla
Mozilla
Email: ekr@rtfm.com
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Jon Peterson
Neustar, Inc.
1800 Sutter St Suite 570
Concord, CA 94520
US
Email: jon.peterson@neustar.biz
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