wdiff rfc9060.original rfc9060.txt

Network Working Group

Internet Engineering Task Force (IETF) J. Peterson
Internet-Draft
Request for Comments: 9060 Neustar
Intended status:
Category: Standards Track February 21, 2021
Expires: August 25, June 2021

STIR
ISSN: 2070-1721

Secure Telephone Identity Revisited (STIR) Certificate Delegation
draft-ietf-stir-cert-delegation-04

Abstract

The Secure Telephone Identity Revisited (STIR) certificate profile
provides a way to attest authority over telephone numbers and related
identifiers for the purpose of preventing telephone number spoofing.
This specification details how that authority can be delegated from a
parent certificate to a subordinate certificate. This supports a
number of use cases, including those where service providers grant
credentials to enterprises or other customers capable of signing
calls with STIR.

Status of This Memo

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This Internet-Draft will expire on August 25, 2021.
https://www.rfc-editor.org/info/rfc9060.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Delegation of STIR Certificates . . . . . . . . . . . . . . . 4
4.1. Scope of Delegation . . . . . . . . . . . . . . . . . . . 5
5. Authentication Services Service Signing with Delegate Certificates . 6
6. Verification Service Behavior for Delegate Certificate
Signatures . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Acquiring Multiple Certificates in STIR . . . . . . . . . . . 7
8. Certification Authorities and Service Providers . . . . . . . 8
8.1. ACME and Delegation . . . . . . . . . . . . . . . . . . . 9
8.2. Handling Multiple Certificates . . . . . . . . . . . . . 9
9. Alternative Solutions . . . . . . . . . . . . . . . . . . . . 10
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 10
12. Security Considerations . . . . . . . . . . . . . . . . . . . 11
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
14.1.
13.1. Normative References . . . . . . . . . . . . . . . . . . 12
14.2.
13.2. Informative References . . . . . . . . . . . . . . . . . 13
Acknowledgments
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14

1. Introduction

The STIR problem statement [RFC7340] reviews the difficulties facing
the telephone network that are enabled by impersonation, including
various forms of robocalling, voicemail hacking, and swatting
[RFC7375]. One of the most important components of a system to
prevent impersonation is the implementation of credentials which that
identify the parties who control telephone numbers. The STIR
certificates [RFC8226]
certificate specification [RFC8226] describes a credential system
based on [X.509] version 3 certificates [X.509] in accordance with [RFC5280]
for that purpose. Those credentials can then be used by STIR
authentication services [RFC8224] to sign PASSporT objects [RFC8225]
carried in SIP [RFC3261] requests.

[RFC8226] specifies an extension to X.509 that defines a Telephony
Number (TN) Authorization List that may be included by certification
authorities (CAs) in certificates. This extension provides
additional information that relying parties can use when validating
transactions with the certificate. When a SIP request, for example,
arrives at a terminating administrative domain, the calling number
attested by the SIP request can be compared to the TN Authorization
List of the certificate that signed the PASSporT to determine if the
caller is authorized to use that calling number.

Initial deployment of [RFC8226] has focused on the use of Service
Provider Codes (SPCs) to attest to the scope of authority of a
certificate. Typically, these codes are internal telephone network
identifiers such as the Operating Company Numbers (OCNs) assigned to
carriers in the United States. However, these network identifiers
are effectively unavailable to non-carrier entities, and this has
raised questions about how such entities might best participate in
STIR,
STIR when needed. Additionally, a carrier may sometimes operate
numbers that are formally assigned to another carrier. [RFC8226]
gave
gives an overview of a certificate enrollment model based on
"delegation,"
"delegation", whereby the holder of a certificate might allocate a
subset of that certificate's authority to another party. This
specification details how delegation of authority works for STIR
certificates.

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.

This specification also uses the following terms:

delegation: the The concept of STIR certificate delegation and its terms
are defined in [RFC8226].

legitimate spoofing: the The practice of selecting an alternative
presentation number for a telephone caller legitimately.

3. Motivation

The most pressing need for delegation in STIR arises in a set of use
cases where callers want to use a particular calling number, but for
whatever reason, their outbound calls will not pass through the
authentication service of the service provider that controls that
numbering resource.

One example would be an enterprise that places outbound calls through
a set of service providers, providers; for each call choosing call, a provider is chosen based
on a least-cost routing algorithm or similar local policy. The
enterprise was assigned a calling number by a particular service
provider, but some calls originating from that number will go out
through other service providers.

A user might also roam from their usual service provider to a
different network or administrative domain, domain for various reasons. Most
"legitimate spoofing" examples are of this form: form, where a user wants
to be able to use the main call-back callback number for their business as a
calling party number, even when the user is away from the business.

These sorts of use cases could be addressed if the carrier who
controls the numbering resource were able to delegate a credential
that could be used to sign calls regardless of which network or
administrative domain handles the outbound routing for the call. In
the absence of something like a delegation mechanism, outbound
carriers may be forced to sign calls with credentials that do not
cover the originating number in question. Unfortunately, that
practice would be difficult to distinguish from malicious spoofing,
and if it becomes widespread, it could erode trust in STIR overall.

4. Delegation of STIR Certificates

STIR delegate certificates are certificates containing a TNAuthList
object that have been signed with the private key of a parent
certificate that itself contains a TNAuthList object (either by-value by value
or by-reference, by reference; see Section 4.1). The parent certificate needs to
contain a basic constraints extension with the [RFC5280] cA boolean set to "true",
"true" [RFC5280], indicating that the subject can sign certificates.
Every STIR delegate certificate identifies its parent certificate
with a standard [RFC5280] Authority Key Identifier extension. extension [RFC5280].

The authority bestowed on the holder of the delegate certificate by
the parent certificate is recorded in the delegate certificate's
TNAuthList. Because STIR certificates use the TNAuthList object
rather than the Subject Name for indicating the scope of their
authority, traditional [RFC5280] name constraints [RFC5280] are not directly
applicable to STIR. In a manner similar to the RPKI Resource Public Key
Infrastructure (RPKI) [RFC6480] "encompassing" semantics, each
delegate certificate MUST have a TNAuthList scope that is equal to or
a subset of its parent certificate's scope: it must be "encompassed." "encompassed".
For example, a parent certificate with a TNAuthList that attested
authority for the numbering range +1-212-555-1000 through 1999 could
issue a certificate to one delegate attesting authority for the range
+1-212-555-1500 through 1599, and 1599 and, to another delegate delegate, a certificate
for the individual number +1-212-555-1824.

Delegate certificates MAY also contain a basic constraints extension
with the cA boolean set to "true", indicating that they can sign
subordinate certificates for further delegates. As only end-entity
certificates can actually sign PASSporTs, the holder of a STIR
certificate with a "true" cA boolean may create a separate end-entity
certificate either with either an identical TNAuthList to its parent, parent or
with a
subset of the parents parent's authority, that which would be used to sign
PASSporTs.

4.1. Scope of Delegation

The TNAuthList of a STIR certificate may contain one or more SPCs, or
one or more telephone number ranges, or even a mix of SPCs and
telephone number ranges. When delegating from a STIR certificate, a
child certificate may inherit from its parent either or both of the
above, and this specification explicitly permits SPC-only parent
certificates to delegate individual telephone numbers or ranges to a
child certificate, as this will be necessary in some operating
environments. Depending on the sort of numbering resources that a
delegate has been assigned, various syntaxes can be used to capture
the delegated resource.

Some non-carrier entities may be assigned large and complex
allocations of telephone numbers, which may be only partially
contiguous or entirely disparate. Allocations may also change
frequently,
frequently in minor or significant ways. These resources may be so
complex, dynamic, or extensive that listing them in a certificate is
prohibitively difficult. Section 10.1 of [RFC8226] describes one
potential way to address this, this: including the TNAuthList (specified in
[RFC8226]) in the certificate by-reference by reference rather than by value,
where a URL in the certificate points to a secure, dynamically- dynamically
updated list of the telephone numbers in the scope of authority of a
certificate. For entities that are carriers in all but name, another
alternative is the allocation of an SPC; this yields much the same
property, as the SPC is effectively a pointer to an external database
which
that dynamically tracks the numbers associated with the SPC. Either
of these approaches may make sense for a given deployment.
Certification path construction as detailed below treats by-reference
TNAuthLists in a certificate as if it they had been included by-value. by value.

Other non-carrier entities may have straightforward telephone number
assignments, such as enterprises receiving a set of a thousand blocks
from a carrier that may be kept for years or decades. Particular
freephone numbers may also have a long-term association with an
enterprise and its brand. For these sorts of assignments, assigning
an SPC may seem like overkill, and using the TN ranges of the
TNAuthList (by-value) (by value) is sufficient.

Whichever approach is taken to representing represent the delegated resource,
there are fundamental trade-offs regarding when and where in the
architecture a delegation is validated: validated -- that is, when the delegated
TNAuthList is checked and determined to be "encompassed" by the
TNAuthList of its parent. This might be performed at the time the
delegate certificate is issued, or at the time that a verification
service receives an inbound call, or potentially both. It is
generally desirable to offload as much of this as possible to the
certification process, process as verification occurs during call setup and thus setup; thus,
additional network dips could lead to perceptible delay, whereas
certification happens outside of call processing as a largely
administrative function. Ideally, if a delegate certificate can
supply a by-value TN range, then a verification service could
ascertain that an attested calling party number is within the scope
of the provided certificate without requiring any additional
transactions with a service. In practice, verification services may
already incorporate network queries into their processing (for
example, to dereference the "x5u" field of a PASSporT) that could
piggyback any additional information needed by the verification
service.

Note that the permission semantics of the [RFC8226] TNAuthList [RFC8226] are
additive: that is, the scope of a certificate is the superset of all
of the SPCs and telephone number ranges enumerated in the TNAuthList.
As SPCs themselves are effectively pointers to a set of telephone
number ranges, and a telephone number may belong to more than one
SPC, this may introduce some redundancy to the set of telephone
numbers specified as the scope of a certificate. The presence of one
or more SPCs and one or more sets of telephone number ranges are
similarly treated additively, even if the telephone number ranges
turn out to be redundant to the scope of an SPC.

5. Authentication Services Service Signing with Delegate Certificates

Authentication service behavior varies from [RFC8224] as follows,
although the same checks are performed by the authentication service
when comparing the calling party number attested in call signaling
with the scope of the authority of the signing certificate.
Authentication services SHOULD NOT use a delegate certificate without
validating that its scope of authority is encompassed by that of its
parent certificate, and if that certificate has its own parent, the
entire certification path SHOULD be validated.

This delegation architecture does not require that a non-carrier
entity act as its own authentication service. That function may be
performed by any authentication service that holds the private key
corresponding to the delegate certificate, including one run by an
outbound service provider, a third party in an enterprise's outbound
call path, or in the SIP User Agent itself.

Note that authentication services creating a PASSporT for a call
signed with a delegate certificate MUST provide an "x5u" link
corresponding to the entire certification path, path rather than just the
delegate certificate used to sign the call, as described in
Section 7.

6. Verification Service Behavior for Delegate Certificate Signatures

The responsibility of a verification service validating PASSporTs
signed with delegate certificates, while largely following baseline
specifications [RFC8224] and [RFC8225], requires some additional
procedures. When the verification service dereferences the "x5u"
parameter, it will acquire a certificate list rather than a single
certificate. It MUST then validate all of the credentials in the
list, identifying the parent certificate for each delegate through
its Authority Key Identifier extension.

While ordinarily, relying parties ordinarily have significant latitude in
certification path construction when validating a certification path,
STIR assumes a more rigid hierarchical subordination model, model rather
than one where relying parties may want to derive their own
certification path to particular trust anchors. If the certificates
acquired from the "x5u" element of a PASSporT do not lead to an
anchor that the verification service trusts, it treats the validation
no differently than it would when a non-delegated certificate was
issued by an untrusted root; in SIP, it MAY return a 437 "Unsupported
Credential" response if the call should be failed for lack of a valid
Identity header.

7. Acquiring Multiple Certificates in STIR

PASSporT [RFC8225] uses the "x5u" element to convey the URL where
verification services can acquire the certificate used to sign a
PASSporT. This value is mirrored by the "info" parameter of the
Identity header when a PASSporT is conveyed via SIP. Commonly, this
is an HTTPS URI.

When a STIR delegate certificate is used to sign a PASSporT, the
"x5u" element in the PASSporT will contain a URI indicating where a
certificate list is available. While the baseline JSON Web Signature
(JWS) also supports an "x5c" element specifically for certificate
chains, in operational practice, certification paths are already
being delivered in the STIR environment via the "x5u" element, so
this specification RECOMMENDS that implementations contain continue to use "x5u";
"x5u". "x5c" is OPTIONAL for environments where it is known to be
supported. That list will be a concatenation of PEM-encoded certificates encoded
with Privacy Enhanced Mail (PEM) of the type "application/pem-certificate-chain" "application/pem-
certificate-chain" defined in [RFC8555]. The certificate path
[RFC5280] ordering MUST be ordered from the signer to the trust
anchor. The list begins with the certificate used to sign the
PASSporT, followed by its parent, and then any subsequent
grandparents, great-grandparents, and so on. The key identifier in
the Authority Key Identifier extension in the first certificate MUST
appear in the Subject Key Identifier extension in the second
certificate. The key identifier pairing MUST match in this way
throughout the entire chain of certificates. Note that ACME Automatic
Certificate Management Environment (ACME) [RFC8555] requires the
first element in a pem-certificate-chain to be an end-entity
certificate.

8. Certification Authorities and Service Providers

Once a telephone service provider has received a CA certificate
attesting to their numbering resources, they may delegate resources
from it as they see fit. Note that the allocation to a service
provider of a certificate with a basic constraints extension with the
cA boolean set to "true" does not require that a service provider act
as a certification authority itself; serving as a certification
authority is a function requiring specialized expertise and
infrastructure. Certification authorities are are, for example example,
responsible for maintain maintaining certificate revocation lists and related
functions,
functions as well as publishing certification practice statements. A
third-party certification authority, including the same one that
issued the service provider its parent certificate, could act as the
CA that issues delegate certificates for the service provider, provider if the
necessary business relationships permit it. A service provider might
in this case act as a Token Authority (see Section 8.1) granting its
customers permissions to receive certificates from the CA.

Note that if the same CA that issued the parent certificate is also
issuing a delegate certificate, it may be possible to shorten the
certification path, which reduces the work required of verification
services. The trade-off here is that if the CA simply issued a non-
delegate certificate (whose parent is the CA's trust anchor) with the
proper TNAuthList value, relying parties might not be able to
ascertain which service provider owned those telephone numbers,
information which that might be used to make an authorization decision on
the terminating side. However, some additional object in the
certificate outside of the TNAuthList could preserve that
information; this is a potential area for future work, and longer
certification paths are the only mechanism currently defined.

All CAs must detail in their practices and policies a requirement to
validate that the "encompassing" of a delegate certificate is done by
its parent. Note that this requires that CAs have access to the
necessary industry databases to ascertain whether, for example, a
particular telephone number is encompassed by an SPC. Alternatively,
a CA may acquire an Authority Token (see Section 8.1) that affirms
that a delegation is in the proper scope. Exactly what operational
practices this entails may vary in different national telephone
administrations,
administrations and are thus left to the CP/CPS Certificate Policy /
Certification Practice Statement (CP/CPS) [RFC3647].

8.1. ACME and Delegation

STIR deployments commonly use ACME [RFC8555] for certificate
acquisition, and it is anticipated that delegate certificates as well will
also be acquired through an ACME interface. An entity can acquire a
certificate from a particular CA by requesting an Authority Token
[I-D.ietf-acme-authority-token]
[ACME-CHAL] from the parent with the desired TNAuthList [I-D.ietf-acme-authority-token-tnauthlist] [ACME-TOKEN]
object. Note that if the client intends to do further subdelegation
of its own, it should request a token with the "ca" Authority Token
flag set.

The entity then presents that Authority Token to a CA to acquire a
STIR delegate certificate. ACME returns an "application/pem-
certificate-chain" object, and that object would be suitable for
publishing
publication as an HTTPS resource for retrieval with the PASSporT
"x5u" mechanism as discussed in Section 7. If the CSR Certificate
Signing Request (CSR) presented to the ACME server is for a
certificate with the cA boolean set to "true", then the ACME server
makes a policy decision to determine whether or not it is appropriate
to issue that certificate to the requesting entity. That policy
decision will be reflected by the "ca" flag in the Authority Token.

Service providers that want the capability to rapidly age out
delegated certificates can rely on the ACME STAR [I-D.ietf-acme-star] Short-Term, Automatically
Renewed (STAR) [RFC8739] mechanism to automate the process of short-term short-
term certificate expiry.

8.2. Handling Multiple Certificates

In some deployments, non-carrier entities may receive telephone
numbers from several different carriers. This could lead to
enterprises needing to maintain a sort of STIR keyring, with
different certificates delegated to them from different providers, providers.
These certificates are potentially issued by different CAs, which they
enterprises choose between when signing a call. This could be the
case regardless of which syntax is used in the TNAuthList to
represent the scope of the delegation (see Section 4.1). As noted in
Section 8, if the parent certs use the same CA, it may be possible to
shorten the certification path.

For non-carrier entities handling a small number of certificates,
this is probably not a significant burden. For cases where it
becomes burdensome, a few potential approaches exist. A delegate
certificate could be cross-certified with another delegate
certificate via an Authority Information Access (AIA) field
containing the URL of a Certificate Authority Issuer, Issuer so that a signer
would only need to sign with a single certificate to inherit the
privileges of the other certificate(s) with which it has cross-certified with. cross-
certified. In very complex delegation cases, it might make more
sense to establish a bridge CA that cross-certifies with all of the
certificates held by the enterprise, enterprise rather than requiring a mesh of
cross-certification between a large number of certificates. Again,
this bridge CA function would likely be performed by some existing CA
in the STIR ecosystem. These procedures would however complicated would, however, complicate
the fairly straightforward certification path reconstruction approach
described in Section 7 and would require further specification.

9. Alternative Solutions

At the time this specification was written, STIR was only starting to
see deployment. In some future environment, the policies that govern
CAs may not permit them to issue intermediate certificates with a
TNAuthList object and a cA boolean set to "true" in the basic
constraints certificate extension [RFC5280]. Similar problems in the
web PKI space motivated the development of TLS subcerts
[I-D.ietf-tls-subcerts], [TLS-CRED],
which substitutes a signed "delegated credential" token for a
certificate for such environments. A comparable mechanism could be
developed for the STIR space, allowing which would allow STIR certificates to
sign a data object which that contains effectively the same data as the
delegate certificate specified here, including a public key that
could sign PASSporTs. The TLS subcerts system has
furthermore exploring further explored
leveraging ACME to issue short-lived certificates for temporary
delegation as a means of obviating the need for revocation.
Specification of a mechanism similar to TLS subcerts for STIR is
future work, work and will be undertaken only if the market require requires it.

10. IANA Considerations

This document contains has no actions for the IANA. IANA actions.

11. Privacy Considerations

Any STIR certificate that identifies a narrow range of telephone
numbers potentially exposes information about the entities that are
placing calls. As such a telephone number range is necessarily a necessary
superset of the calling party number that is openly signaled during
call setup, the privacy risks associated with this mechanism are not
substantially greater than baseline STIR. See [RFC8224] for guidance
on the use of anonymization mechanisms in STIR.

12. Security Considerations

This document is entirely about security. As delegation can allow
signing in
signing-in scenarios where unauthenticated "legitimate" spoofing
would otherwise be used, it the hope is hoped that delegation will improve the
overall security of the STIR ecosystem. For further information on
certificate security and practices, see [RFC5280], in particular particularly its
Security Considerations.
security considerations. Also see the Security Considerations security considerations of
[RFC8226] for general guidance on the implications of the use of
certificates in STIR, STIR and [RFC7375] for the STIR threat model.

Much of the security of delegation depends on the implementation of
the encompassing semantics described in Section 4. When delegating
from an SPC-based TNAuthList to a set of telephone number ranges,
understanding the encompassing semantics may require access to
industry databases that track the numbering assets of service
providers associated with a given SPC. In some operating
environments, such databases might not exist. How encompassing is
policed is therefore a matter outside the scope of this document, document and
specific to operational profiles of STIR.

The use of by-reference TNAuthLists as described in Section 4 entails means
that the TNAuthList associated with a certificate can change over
time; see the security considerations of [RFC3986] for more on the
implications of this property. It is considered a useful feature
here due to the potential dynamism of large lists of telephone
numbers, but this dynamism entails means that a relying party might once at one
point accept that a particular telephone number is associated with a
certificate,
certificate but later reject it for the same certificate as the
dynamic list changes. Also that note that if the HTTPS service housing
the by-reference telephone number list is improperly secured, that
too can lead to vulnerabilities. Ultimately, the CA that issued a
delegated certificate populates the URL in the AIA field, field and is
responsible for making a secure selection. Service providers acting
as CAs are directed to the cautionary words about running a CA in
Section 8 regarding the obligations this entails for certificate
revocation and so on.

13. Acknowledgments

We would like to thank Ines Robles, Richard Barnes, Chris Wendt, Dave
Hancock, Russ Housley, Benjamin Kaduk, and Sean Turner for key input
on this document.

14. References

14.1.

13.1. Normative References

[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>.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.

[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>.

[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/info/rfc8555>.

14.2.

13.2. Informative References

[I-D.ietf-acme-authority-token]

[ACME-CHAL]
Peterson, J., Barnes, M., Hancock, D., and C. Wendt, "ACME
Challenges Using an Authority Token", draft-ietf-acme-
authority-token-05 (work Work in progress), March 2020.

[I-D.ietf-acme-authority-token-tnauthlist] Progress,
July 2021, <https://datatracker.ietf.org/doc/html/draft-
ietf-acme-authority-token-06>.

[ACME-TOKEN]
Wendt, C., Hancock, D., Barnes, M., and J. Peterson,
"TNAuthList profile of ACME Authority Token", draft-ietf-
acme-authority-token-tnauthlist-06 (work Work in progress),
Progress, Internet-Draft, draft-ietf-acme-authority-token-
tnauthlist-08, 27 March 2020.

[I-D.ietf-acme-star]
Sheffer, Y., Lopez, D., Dios, O., Pastor, A., and T.
Fossati, "Support for Short-Term, Automatically-Renewed
(STAR) Certificates in Automated Certificate Management
Environment (ACME)", draft-ietf-acme-star-11 (work in
progress), October 2019.

[I-D.ietf-tls-subcerts]
Barnes, R., Iyengar, S., Sullivan, N., and E. Rescorla,
"Delegated Credentials for TLS", draft-ietf-tls-
subcerts-10 (work in progress), January 2021. 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-acme-
authority-token-tnauthlist-08>.

[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>.

[RFC3647] Chokhani, S., Ford, W., Sabett, R., Merrill, C., and S.
Wu, "Internet X.509 Public Key Infrastructure Certificate
Policy and Certification Practices Framework", RFC 3647,
DOI 10.17487/RFC3647, November 2003,
<https://www.rfc-editor.org/info/rfc3647>.

[RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support
Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
February 2012, <https://www.rfc-editor.org/info/rfc6480>.

[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>.

[RFC7375] Peterson, J., "Secure Telephone Identity Threat Model",
RFC 7375, DOI 10.17487/RFC7375, October 2014,
<https://www.rfc-editor.org/info/rfc7375>.

[RFC8739] Sheffer, Y., Lopez, D., Gonzalez de Dios, O., Pastor
Perales, A., and T. Fossati, "Support for Short-Term,
Automatically Renewed (STAR) Certificates in the Automated
Certificate Management Environment (ACME)", RFC 8739,
DOI 10.17487/RFC8739, March 2020,
<https://www.rfc-editor.org/info/rfc8739>.

[TLS-CRED] 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://datatracker.ietf.org/doc/html/draft-ietf-
tls-subcerts-10>.

[X.509] ITU-T Recommendation X.509 (10/2012) | ISO/IEC 9594-8, ITU-T, "Information technology - Open Systems
Interconnection - The Directory: Public-key and attribute
certificate frameworks", 2012. ITU-T Recommendation X.509,
October 2016, <https://www.itu.int/rec/T-REC-X.509>.

Acknowledgments

We would like to thank Ines Robles, Richard Barnes, Chris Wendt, Dave
Hancock, Russ Housley, Benjamin Kaduk, and Sean Turner for key input
on this document.

Author's Address

Jon Peterson
Neustar, Inc.

Email: jon.peterson@team.neustar