Internet Engineering Task Force (IETF) E. Rescorla

Internet Engineering Task Force (IETF) E. Rescorla
Request for Comments: 8816 Mozilla
Category: Informational J. Peterson
ISSN: 2070-1721 Neustar
February 2021

Secure Telephone Identity Revisited (STIR) Out-of-Band Architecture and
Use Cases

Abstract

The Personal Assertion Token (PASSporT) format defines a token that
can be carried by signaling protocols, including SIP, to
cryptographically attest the identity of callers. However, not all
telephone calls use Internet signaling protocols, and some calls use
them for only part of their signaling path, while some cannot
reliably deliver SIP header fields end-to-end. 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 document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8816.

Copyright Notice

Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(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
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. Terminology
3. Operating Environments
4. Dataflows
5. Use Cases
5.1. Case 1: VoIP to PSTN Call
5.2. Case 2: Two Smart PSTN Endpoints
5.3. Case 3: PSTN to VoIP Call
5.4. Case 4: Gateway Out-of-Band
5.5. Case 5: Enterprise Call Center
6. Storing and Retrieving PASSporTs
6.1. Storage
6.2. Retrieval
7. Solution Architecture
7.1. Credentials and Phone Numbers
7.2. Call Flow
7.3. Security Analysis
7.4. Substitution Attacks
7.5. Rate Control for CPS Storage
8. Authentication and Verification Service Behavior for
Out-of-Band
8.1. Authentication Service (AS)
8.2. Verification Service (VS)
8.3. Gateway Placement Services
9. Example HTTPS Interface to the CPS
10. CPS Discovery
11. Encryption Key Lookup
12. IANA Considerations
13. Privacy Considerations
14. Security Considerations
15. Informative References
Acknowledgments
Authors' Addresses

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 a SIP Identity header
field capable of carrying PASSporT objects [RFC8225] 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.

Not all telephone calls use SIP today, however, and even those that
do use SIP do not always carry SIP signaling end-to-end. Calls from
telephone numbers still routinely 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, etc.) but the
call transits the PSTN at some point.

3. Non-PSTN calls that 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 in-band signaling data -- intact. Even if fields for
tunneling arbitrary 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.

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 Private Branch Exchanges (PBXs), and terminal adapters. All three
are general purpose computers, and typically all three have Internet
access as well as access to the PSTN; they may be used for
residential, mobile, or enterprise telephone services. Additionally,
various kinds of gateways increasingly front for deployments of
legacy PBX and PSTN switches. 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.

The techniques described in this document therefore build on the
PASSporT [RFC8225] mechanism and the work of [RFC8224] to describe 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 called a Call Placement Service
(CPS) that permits the PASSporT object to be stored during call
processing and retrieved for verification purposes.

Potential implementors should note that this document merely defines
the operating environments in which this out-of-band STIR mechanism
is intended to operate. It provides use cases, gives a broad
description of the components, and a potential solution architecture.
Various environments may have their own security requirements: a
public deployment of out-of-band STIR faces far greater challenges
than a constrained intra-network deployment. To flesh out the
storage and retrieval of PASSporTs in the CPS within this context,
this document includes a strawman protocol suitable for that purpose.
Deploying this framework in any given environment would require
additional specification outside the scope of this document.

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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Operating Environments

This section describes the environments in which the proposed out-of-
band STIR mechanism is intended to operate. In the simplest setting,
Alice calls Bob, and her call is routed through some set of gateways
and/or the PSTN that do not support end-to-end delivery of STIR.
Both Alice and Bob have smart devices that can access the Internet
(perhaps enterprise devices, or even end-user ones), but they do not
have a clear telephone signaling connection between them: Alice
cannot inject any data into signaling that 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 instead just a traditional telephone.
However, one or both of them are behind a STIR-aware gateway that can
participate in out-of-band coordination, as shown below:

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

In such a case, Alice might have an analog (e.g., PSTN) connection to
her gateway or switch that 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 Plain Old Telephone Service (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.)

This architecture does not mandate that any particular sort of entity
operate a CPS or mandate any means to discover a CPS. A CPS could be
run internally within a network or made publicly available. One or
more CPSes could be run by a carrier, as repositories for PASSporTs
for calls sent to its customers, or a CPS could be built into an
enterprise PBX or even a smartphone. To the degree possible, it is
specified here generically as an idea that may have applicability to
a variety of STIR deployments.

There are roughly two plausible dataflow architectures for the CPS:

1. 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.

2. 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
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. In specialized environments, for
example, a call center that receives a large volume of incoming calls
that originated in the PSTN, the notification channel approach might
be viable.

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
header fields 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 a SIP environment in a STIR-aware administrative
domain. The local authentication service for that administrative
domain creates a PASSporT that 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 (per the practices of
Section 10) 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, which communicates through
traditional telephone signaling protocols. The PBX immediately
routes the 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
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 immediately route
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 field 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 a 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 that 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 a
SIP environment. In the former case, perhaps the destination
endpoint queries the CPS to retrieve the PASSporT provisioned by the
first gateway. If the call ultimately returns to a SIP environment,
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.

5.5. Case 5: Enterprise Call Center

A call originates from a mobile user, and a STIR authentication
service operated by their carrier creates a PASSporT for the call.
As the carrier forwards the call via SIP, it attaches the PASSporT to
the SIP call with an Identity header field. As a fallback in case
the call will not go end-to-end over SIP, the carrier also stores the
PASSporT in a CPS.

The call is then routed over SIP for a time, before it transitions to
the PSTN and ultimately is handled by a legacy PBX at a high-volume
call center. The call center supports the out-of-band service, and
has a high-volume interface to a CPS to retrieve PASSporTs for
incoming calls; agents at the call center use a general purpose
computer to manage inbound calls and can receive STIR notifications
through it. When the PASSporT arrives at the CPS, it is sent through
a subscription/notification interface to a system that can correlate
incoming calls with valid PASSporTs. The call center agent sees that
a valid call from the originating number has arrived.

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 PASSporTs is thus a key design decision in
the architecture. 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, 4, and 5 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; 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 acceptable if
sufficient steps are taken to mitigate any privacy risk of leaking
data.

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
STIR [RFC8224]: 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 follow, an
out-of-band mechanism could conceivably even improve on the privacy
story.

Broadly, the architecture recommended here thus is one focused on
permitting any entity to store encrypted PASSporTs at the CPS,
indexed under the called number. PASSporTs will be encrypted with a
public key associated with the called number, so these PASSporTs may
safely be retrieved by any entity because 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 makes the CPS a less
attractive target for pervasive monitoring (see [RFC7258]). As a
first step, transport-level security can provide confidentiality from
eavesdroppers for both the storing and retrieval of PASSporTs. To
bolster the privacy story, to prevent denial-of-service flooding of
the CPS, and to complicate traffic analysis, a few additional
mechanisms are also recommended below.

6.1. Storage

There are a few dimensions to authorizing the storage of PASSporTs.
Encrypting PASSporTs prior to storage entails that a CPS has no way
to tell if a PASSporT is valid; it simply conveys encrypted blocks
that it cannot access itself and can make no authorization decision
based on the PASSporT contents. There is certainly no prospect for
the CPS to verify the PASSporTs itself.

Note that this architecture requires clients that store PASSporTs to
have access to an encryption key associated with the intended called
party to be used to encrypt the PASSporT. Discovering this key
requires the existence of a key lookup service (see Section 11),
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 for out-of-band use therefore might
need to be encrypted with multiple keys in the hopes that one will be
decipherable by the relying party.

Again, the most obvious way to authorize storage is to require the
originator to authenticate themselves to the CPS with their STIR
credential. However, since the call is indexed at the CPS under the
called number, this can weaken the privacy story of the architecture,
as it reveals to the CPS both the identity of the caller and the
callee. Moreover, it does not work for the gateway use cases
described above; to support those use cases, we must effectively
allow any entity to store PASSporTs at a CPS. This does not degrade
the anti-impersonation security of STIR, because entities who do not
possess the necessary credentials to sign the PASSporT will not be
able to create PASSporTs that will be treated as valid by verifiers.
In this 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
above. 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 calling number, and thereby prevent the
called party from finding a valid PASSporT for an incoming call
buried in a haystack of fake entries.

The solution architecture must therefore include 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 the source and destination of traffic. A potential solution is
discussed below in Section 7.5.

6.2. Retrieval

For retrieval of PASSporTs, this architecture assumes that clients
will contact the CPS through some sort of polling or notification
interface to receive all current PASSporTs for calls destined to a
particular telephone number, or block of numbers.

As PASSporTs stored at the CPS are encrypted with a key belonging to
the intended destination, the CPS can safely 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 returns 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. An encryption scheme needs to be carefully chosen to make
messages look indistinguishable from random when encrypted, so that
information about the called party is not discoverable from
legitimate encrypted PASSporTs.

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 to decrypt all PASSporTs retrieved from the CPS, and may find
only one that is valid.

In order to prevent the CPS from learning the numbers that a callee
controls, callees might also request PASSporTs for numbers that they
do not own, that they have no hope of decrypting. Implementations
could even allow a callee to request PASSporTs for a range or prefix
of numbers: a trade-off where that callee is willing to sift through
bulk quantities of undecryptable PASSporTs for the sake of hiding
from the CPS which numbers it controls.

Note that in out-of-band call forwarding cases, special behavior is
required to manage the relationship between PASSporTs using the
diversion extension [PASSPORT-DIVERT]. The originating
authentication service encrypts the initial PASSporT with the public
encryption 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 generate encrypted
PASSporTs that show a secure chain of diversion: a retargeting storer
SHOULD use the "div-o" PASSporT type, with its "opt" extension, as
specified in [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.

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 that 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
credential 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 this 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
+1.111.555.1111 and Bob has the number +2.222.555.2222.

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

Store Encrypted PASSporT for 2.222.555.2222 ->

Call from 1.111.555.1111 ------------------------------------------>

<-------------- Request PASSporT(s)
for 2.222.555.2222

Obtain Encrypted PASSporT -------->
(2.222.555.2222, 1.111.555.1111)

[Ring phone with verified callerid
= 1.111.555.1111]

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.

When Alice places the call, Bob's phone would usually ring and
display Alice's number (+1.111.555.1111), which is informed by the
existing PSTN mechanisms for relaying a calling party number (e.g.,
the Calling Party's Number (CIN) field of the Initial Address Message
(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 (or dummy PASSporTs if no relevant
ones are currently stored). If such a PASSporT exists, and the
verification service in Bob's phone decrypts it using his private
key, validates it, then Bob's phone can 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:

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

2. The attacker wishes to substitute himself for an existing call
setup.

If an attacker can inject fake PASSporTs 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. Any attacker who is aware
of calls in progress can attempt to mount a race to substitute
themselves as described in Section 7.4. For privacy and robustness
reasons, using TLS [RFC8446] 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 STIR [RFC8224].

A secondary attack we must also prevent is denial-of-service against
the CPS, which requires some form of rate control solution that will
not degrade the privacy properties of the architecture.

7.4. Substitution Attacks

All that the 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
that 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:

Attacker Callback Service CPS Bob
--------------------------------------------------------------------
Place call to Bob ---------->
(from 111.555.1111)
Store PASSporT for
CS:Bob ------------->

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

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

<-- Retrieve PASSporT
for CS:Bob

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

[Ring phone with callerid =
111.555.1111]

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.

In order to prevent a passive attacker from using traffic analysis or
similar means to learn precisely when a call is placed, it is
essential that the connection between the caller and the CPS be
encrypted as recommended above. Authentication services could store
dummy PASSporTs at the CPS at random intervals in order to make it
more difficult for an eavesdropper to use traffic analysis to
determine that a call was about to be placed.

Note that in a SIP environment, the callee might notice that there
were multiple INVITEs and thus detect this attack, but in some PSTN
interworking scenarios, or highly intermediated networks, only one
call setup attempt will reach the target. Also note that the success
of this substitution attack depends on the attacker landing their
call within the narrow window that the PASSporT is retained in the
CPS, so shortening that window will reduce the opportunity for the
attack. Finally, smart endpoints could implement some sort of state
coordination to ensure that both sides believe the call is in
progress, though methods of supporting that are outside the scope of
this document.

7.5. Rate Control for CPS Storage

In order to prevent the flooding of a CPS with bogus PASSporTs, we
propose the use of "blind signatures" (see [RFC5636]). A sender will
initially authenticate to the CPS using its STIR credentials and
acquire a signed token from the CPS that will be presented later when
storing a PASSporT. The flow looks as follows:

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
get 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 acquisition 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 longer than the
recommended freshness policy for the "iat" value as described in
[RFC8224] (i.e., sixty seconds) unless some local policy for a CPS
deployment requires a longer or shorter interval. 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.

An alternative potential approach to blind signatures would be the
use of verifiable oblivious pseudorandom functions (VOPRFs, per
[PRIVACY-PASS]), which may prove faster.

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 (AS)

Out-of-band authentication services perform steps similar to those
defined in [RFC8224] with some exceptions:

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 some 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 into the calling device, for example, this is the same check
performed in Step 1: does the calling device hold a private key, one
corresponding to a STIR certificate, that can sign for the
originating identity?

Step 3: The authentication service MUST acquire the public encryption
key of the destination, which will be used to encrypt the PASSporT
(see Section 11). It MUST also discover (see Section 10) the CPS
associated with the destination. The authentication service may
already have the encryption key and destination CPS cached, or may
need to query a service to acquire the key. Note that per
Section 7.5, 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 encryption 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 be 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 a cleartext version rather than an encrypted version sent to
the CPS. In that case, out-of-band would serve as a fallback
mechanism if the request was not conveyed over SIP end-to-end. Also,
note 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 (VS)

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; or alternatively it may
receive PASSporTs through a push interface from the CPS in some
deployments. 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 calling party 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 [RFC8225].

Finally, the verification service will end up with one or more valid
PASSporTs corresponding to the call it has received. In keeping with
baseline STIR, this document does not dictate any particular
treatment of calls that have valid PASSporTs associated with them;
the handling of the call 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 intermediary implementations, or
changing how the user is alerted or how identity is rendered in user
agent implementations.

8.3. Gateway Placement Services

The STIR 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 field, extract a
PASSporT from the Identity header field, 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 of
Section 8.1, 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 a rough example, we show a CPS implementation here that uses a
Representational State Transfer (REST) API [REST] 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 2.222.555.2222 (note that
these are dummy values):

eyJhbGciOiJFUzI1NiIsInR5cCI6InBhc3Nwb3J0IiwieDV1IjoiaHR0cHM6Ly9
jZXJ0LmV4YW1wbGUub3JnL3Bhc3Nwb3J0LmNlciJ9.eyJkZXN0Ijp7InRuIjpbI
jIyMjI1NTUyMjIyIl19LCJpYXQiOiIxNTgzMjUxODEwIiwib3JpZyI6eyJ0biI6
IjExMTE1NTUxMTExIn19.pnij4IlLHoR4vxID0u3CT1e9Hq4xLngZUTv45Vbxmd
3IVyZug4KOSa378yfP4x6twY0KTdiDypsereS438ZHaQ

Through some discovery mechanism (see Section 10), the authentication
service discovers the network location of a web service that acts as
the CPS for 2.222.555.2222. Through the same mechanism, we will say
that it has also discovered one public encryption key for that
destination. It uses that encryption 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/2.222.555.2222/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 contacts
its CPS to ask for the set of incoming calls for its telephone number
(2.222.222.2222).

GET /cps/2.222.555.2222/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/2.222.555.2222/ppts/ppt1/ which yields:

HTTP/1.1 200 OK
Content-Type: application/passport
Link: <https://cps.example.com/cps/2.222.555.2222/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 largely 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. Likely models include ones where a
carrier operates one or more CPS instances on behalf of its
customers, an enterprise runs a CPS instance on behalf of its PBX
users, or a third-party service provider offers a CPS as a cloud
service.

Some service discovery possibilities under consideration include the
following:

For some deployments in closed (e.g., intra-network) environments,
the CPS location can simply be provisioned in implementations,
obviating the need for a discovery protocol.

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
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
Telephone-Related Information (TeRI) protocol [MODERN-TERI], could
also serve for this application.

Another possibility is to use a single distributed service for
this function. Verification Involving PSTN Reachability (VIPR)
[VIPR-OVERVIEW] proposed a REsource LOcation And Discovery
(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 the location of the CPS at the authentication service and
verification service.

11. Encryption Key Lookup

In order to encrypt a PASSporT (see Section 6.1), the caller needs
access to the callee's public encryption key. Note that because STIR
uses the Elliptic Curve Digital Signature Algorithm (ECDSA) for
signing PASSporTs, the public key used to verify PASSporTs is not
suitable for this function, and thus the encryption key must be
discovered separately. This requires some sort of directory/lookup
system.

Some initial STIR deployments have fielded certificate repositories
so that verification services can acquire the signing credentials for
PASSporTs, which are linked through a URI in the "x5u" element of the
PASSporT. These certificate repositories could clearly be repurposed
for allowing authentication services to download the public
encryption key for the called party -- provided they can be
discovered by calling parties. This document does not specify any
particular discovery scheme, but instead offers some general guidance
about potential approaches.

It is a desirable property that the public encryption key for a given
party be linked to their STIR credential. An Elliptic Curve
Diffie-Hellman (ECDH) [RFC7748] public-private key pair might be
generated for a subcert [TLS-SUBCERTS] of the STIR credential. That
subcert could be looked up along with the STIR credential of the
called party. Further details of this subcert, and the exact lookup
mechanism involved, are deferred for future protocol work.

Obviously, if there is a single central database that the caller and
callee each access in real time to download the other's keys, then
this represents a real privacy risk, as the central key database
learns about each call. A number of mechanisms are potentially
available to mitigate this:

Have endpoints pre-fetch keys 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
keys they are fetching.

Clearly, there is a privacy/timeliness trade-off in that getting up-
to-date knowledge about credential validity requires contacting the
credential directory in real-time (e.g., via the Online Certificate
Status Protocol (OCSP) [RFC6960]). 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 trade-off space to be an open
issue.

12. IANA Considerations

This document has no IANA actions.

13. Privacy Considerations

Delivering PASSporTs out-of-band offers a different set of privacy
properties than traditional in-band STIR. In-band operations convey
PASSporTs as headers in SIP messages in cleartext, which any
forwarding intermediaries can potentially inspect. By contrast, out-
of-band STIR stores these PASSporTs at a service after encrypting
them as described in Section 6, effectively creating a path between
the authentication and verification service in which the CPS is the
sole intermediary, but the CPS cannot read the PASSporTs.
Potentially, out-of-band PASSporT delivery could thus improve on the
privacy story of STIR.

The principle actors in the operation of out-of-band are the AS, VS,
and CPS. The AS and VS functions differ from baseline behavior
[RFC8224], in that they interact with a CPS over a non-SIP interface,
of which the REST interface in Section 9 serves as an example. Some
out-of-band deployments may also require a discovery service for the
CPS itself (Section 10) and/or encryption keys (Section 11). Even
with encrypted PASSporTs, the network interactions by which the AS
and VS interact with the CPS, and to a lesser extent any discovery
services, thus create potential opportunities for data leakage about
calling and called parties.

The process of storing and retrieving PASSporTs at a CPS can itself
reveal information about calls being placed. The mechanism takes
care not to require that the AS authenticate itself to the CPS,
relying instead on a blind signature mechanism for flood control
prevention. Section 7.4 discusses the practice of storing "dummy"
PASSporTs at random intervals to thwart traffic analysis, and as
Section 8.2 notes, a CPS is required to return a dummy PASSporT even
if there is no PASSporT indexed for that calling number, which
similarly enables the retrieval side to randomly request PASSporTs
when there are no calls in progress. Note that the caller's IP
address itself leaks information about the caller. Proxying the
storage of the CPS through some third party could help prevent this
attack. It might also be possible to use a more sophisticated system
such as Riposte [RIPOSTE]. These measures can help to mitigate
information disclosure in the system. In implementations that
require service discovery (see Section 10), perhaps through key
discovery (Section 11), similar measures could be used to make sure
that service discovery does not itself disclose information about
calls.

Ultimately, this document only provides a framework for future
implementation of out-of-band systems, and the privacy properties of
a given implementation will depend on architectural assumptions made
in those environments. More closed systems for intranet operations
may adopt a weaker security posture but otherwise mitigate the risks
of information disclosure, whereas more open environments will
require careful implementation of the practices described here.

For general privacy risks associated with the operations of STIR,
also see the privacy considerations covered in Section 11 of
[RFC8224].

14. Security Considerations

This entire document is about security, but the detailed security
properties will vary depending on how the framework is applied and
deployed. General guidance for dealing with the most obvious
security challenges posed by this framework is given in Sections 7.3
and 7.4, along proposed solutions for problems like denial-of-service
attacks or traffic analysis against the CPS.

Although there are considerable security challenges associated with
widespread deployment of a public CPS, those must be weighed against
the potential usefulness of a service that delivers a STIR assurance
without requiring the passage of end-to-end SIP. Ultimately, the
security properties of this mechanism are at least comparable to in-
band STIR: the substitution attack documented in Section 7.4 could be
implemented by any in-band SIP intermediary or eavesdropper who
happened to see the PASSporT in transit, say, and launched its own
call with a copy of that PASSporT to race against the original to the
destination.

15. Informative References

[MODERN-TERI]
Peterson, J., "An Architecture and Information Model for
Telephone-Related Information (TeRI)", Work in Progress,
Internet-Draft, draft-ietf-modern-teri-00, 2 July 2018,
<https://tools.ietf.org/html/draft-ietf-modern-teri-00>.

[PASSPORT-DIVERT]
Peterson, J., "PASSporT Extension for Diverted Calls",
Work in Progress, Internet-Draft, draft-ietf-stir-
passport-divert-09, 13 July 2020,
<https://tools.ietf.org/html/draft-ietf-stir-passport-
divert-09>.

[PRIVACY-PASS]
Davidson, A. and N. Sullivan, "The Privacy Pass Protocol",
Work in Progress, Internet-Draft, draft-privacy-pass-00, 3
November 2019,
<https://tools.ietf.org/html/draft-privacy-pass-00>.

[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures, Chapter 5:
Representational State Transfer", Ph.D.
Dissertation, University of California, Irvine, 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>.

[RFC5636] Park, S., Park, H., Won, Y., Lee, J., and S. Kent,
"Traceable Anonymous Certificate", RFC 5636,
DOI 10.17487/RFC5636, August 2009,
<https://www.rfc-editor.org/info/rfc5636>.

[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>.

[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/info/rfc6960>.

[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>.

[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[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>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[RIPOSTE] Corrigan-Gibbs, H., Boneh, D., and D. Mazières, "Riposte:
An Anonymous Messaging System Handling Millions of Users",
May 2015, <https://people.csail.mit.edu/henrycg/pubs/
oakland15riposte/>.

[TLS-SUBCERTS]
Barnes, R., Iyengar, S., Sullivan, N., and E. Rescorla,
"Delegated Credentials for TLS", Work in Progress,
Internet-Draft, draft-ietf-tls-subcerts-10, 24 January
2021,
<https://tools.ietf.org/html/draft-ietf-tls-subcerts-10>.

[VIPR-OVERVIEW]
Barnes, M., Jennings, C., Rosenberg, J., and M. Petit-
Huguenin, "Verification Involving PSTN Reachability:
Requirements and Architecture Overview", Work in Progress,
Internet-Draft, draft-jennings-vipr-overview-06, 9
December 2013, <https://tools.ietf.org/html/draft-
jennings-vipr-overview-06>.

Acknowledgments

The ideas in this document came out of discussions with Richard
Barnes and Cullen Jennings. We'd also like to thank Russ Housley,
Chris Wendt, Eric Burger, Mary Barnes, Ben Campbell, Ted Huang,
Jonathan Rosenberg, and Robert Sparks for helpful suggestions.

Authors' Addresses

Eric Rescorla
Mozilla

Email: [email protected]

Jon Peterson
Neustar, Inc.
1800 Sutter St Suite 570
Concord, CA 94520
United States of America

Email: [email protected]