Internet Engineering Task Force (IETF) J. Peterson
Request for Comments: 9475 Neustar
Category: Standards Track C. Wendt
ISSN: 2070-1721 Somos
December 2023
Messaging Use Cases and Extensions for Secure Telephone Identity
Revisited (STIR)
Abstract
Secure Telephone Identity Revisited (STIR) provides a means of
attesting the identity of a telephone caller via a signed token in
order to prevent impersonation of a calling party number, which is a
key enabler for illegal robocalling. Similar impersonation is
sometimes leveraged by bad actors in the text and multimedia
messaging space. This document explores the applicability of STIR's
Personal Assertion Token (PASSporT) and certificate issuance
framework to text and multimedia messaging use cases, including
support for both messages carried as a payload in SIP requests and
messages sent in sessions negotiated by SIP.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in 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/rfc9475.
Copyright Notice
Copyright (c) 2023 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
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Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Terminology
3. Applicability to Messaging Systems
3.1. Message Sessions
3.2. PASSporTs and Individual Messages
3.2.1. PASSporT Conveyance with Messaging
4. Certificates and Messaging
5. IANA Considerations
5.1. JSON Web Token Claims Registration
5.2. PASSporT Type Registration
6. Privacy Considerations
7. Security Considerations
8. References
8.1. Normative References
8.2. 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. [RFC8224] defines a SIP Identity header capable of
carrying PASSporT [RFC8225] objects 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 SIP cases, SIP
URI) associated with the originator of the call.
However, the problem of bulk, unsolicited commercial communications
is not limited to telephone calls. Spammers and fraudsters are
increasingly turning to messaging applications to deliver undesired
content to consumers. In some respects, mitigating these unwanted
messages resembles the email spam problem; for example, textual
analysis of the message contents can be used to fingerprint content
that is generated by spammers. However, encrypted messaging is
becoming more common, and analysis of message contents may no longer
be a reliable way to mitigate messaging spam in the future. As STIR
sees further deployment in the telephone network, the governance
structures put in place for securing telephone-network resources with
STIR could be repurposed to help secure the messaging ecosystem.
One of the more sensitive applications for message security is
emergency services. As next-generation emergency services
increasingly incorporate messaging as a mode of communication with
public safety personnel (see [RFC8876]), providing an identity
assurance could help to mitigate denial-of-service attacks and
ultimately help to identify the source of emergency communications in
general (including swatting attacks, see [RFC7340]).
Therefore, this specification explores how the PASSporT mechanism
defined for STIR could be applied in providing protection for textual
and multimedia messaging, but it focuses particularly on those
messages that use telephone numbers as the identity of the sender.
Moreover, it considers the reuse of existing STIR certificates, which
are beginning to see widespread deployment for signing PASSporTs that
protect messages. For that purpose, it defines a new PASSporT type
and an element that protects message integrity. It contains a
mixture of normative and informative guidance that specifies new
claims for use in PASSporTs as well as an overview of how STIR might
be applied to messaging in various environments.
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. Applicability to Messaging Systems
At a high level, PASSporT [RFC8225] claims provide similar value to
number-based messaging as they do to telephone calls. A signature
over the calling and called party numbers, along with a timestamp,
could already help to prevent impersonation in the mobile-messaging
ecosystem.
When it comes to protecting message contents, broadly, there are a
few ways that the PASSporT mechanism of STIR could apply to
messaging:
1. a PASSporT could be used to securely negotiate a session over
which messages will be exchanged (see Section 3.1), and
2. in sessionless scenarios, a PASSporT could be generated on a per-
message basis with its own built-in message security (see
Section 3.2).
3.1. Message Sessions
In the first case, SIP negotiates a session in which the media will
be text messages or MIME content, as, for example, with the Message
Session Relay Protocol (MSRP) [RFC4975]. This usage of STIR would
deviate little from [RFC8224]. An INVITE request sent with an
Identity header containing a PASSporT with the proper calling and
called party numbers would then negotiate an MSRP session the same
way that an INVITE for a telephone call would negotiate an audio
session. This could be applicable to MSRP sessions negotiated for
Rich Communication Suite (RCS) [RCC.07]. Note that, if TLS is used
to secure MSRP (per RCS [RCC.15]), fingerprints of those TLS keys
could be secured via the "mky" claim of PASSporT using the framework
described in [RFC8862]. Similar practices would apply to sessions
that negotiate real-time text over RTP ([RFC4103], [RFC5194]); any
that can operate over DTLS/SRTP (Secure Real-time Transport Protocol)
should work with the "mky" PASSporT claim. For the most basic use
cases, STIR for messaging should not require any further protocol
enhancements.
Current usage of [RFC8224] Identity is largely confined to INVITE
requests that initiate telephone calls. RCS-style applications would
require PASSporTs for all conversation participants, which could
become complex in multiparty conversations. Any solution in this
space would likely require the implementation of STIR-connected
identity [CONNECT-ID-STIR], but the specification of PASSporT-signed
session conferencing is outside the scope of this document.
Also note that the assurance offered by [RFC8862] is "end-to-end" in
the sense that it offers assurance between an authentication service
and verification service. If those are not implemented by the
endpoints themselves, there are still potential opportunities for
tampering before messages are signed and after they are verified.
However, for the most part, STIR does not intend to protect against
machine-in-the-middle attacks so much as spoofed origination; so the
protection offered may be sufficient to mitigate nuisance messaging.
3.2. PASSporTs and Individual Messages
In the second case described in Section 3, SIP also has a method for
sending messages in the body of a SIP request: the MESSAGE method
[RFC3428]. For example, MESSAGE is used in some North American
emergency services use cases. The interaction of STIR with MESSAGE
is not as straightforward as the potential use case with MSRP. An
Identity header could be added to any SIP MESSAGE request, but
without some extension to the PASSporT claims, the PASSporT would
offer no protection to the message content; it would potentially be
reusable for cut-and-paste attacks where the Identity header field
from a legitimate request for one user is reused in a request for a
different user. As the bodies of SIP requests are MIME encoded, S/
MIME [RFC8591] has been proposed as a means of providing integrity
for MESSAGE (and some MSRP cases as well). The use of Common
Presence and Instant Messaging (CPIM) [RFC3862] as a MIME body allows
the integrity of messages to withstand interworking with protocols
that are not SIP. The interaction of STIR certificates with S/MIME
(see [RFC8226]) for messaging applications would require further
specification; additionally, PASSporT can provide its own integrity
check for message contents through a new claim defined to provide a
hash over message contents.
In order to differentiate a PASSporT for an individual message from a
PASSporT used to secure a telephone call or message stream, this
document defines a new "msg" PASSporT type. "msg" PASSporTs may carry
a new optional JSON Web Token (JWT) [RFC7519] claim "msgi", which
provides a digest over a MIME body that contains a text or multimedia
message. Authentication services MUST NOT include "msgi" elements in
PASSporT types other than "msg", but "msgi" is OPTIONAL in "msg"
PASSporTs, as integrity for messages may be provided by some other
service (e.g. [RFC8591]). Verification services MUST ignore the
presence of "msgi" in non-"msg" PASSporT types.
The claim value of the "msgi" claim key is a string that defines the
crypto algorithm used to generate the digest concatenated by a hyphen
with a digest string. Implementations MUST support the hash
algorithms SHA-256, SHA-384, and SHA-512. These hash algorithms are
identified by "sha256", "sha384", and "sha512", respectively. SHA-
256, SHA-384, and SHA-512 are part of the SHA-2 set of cryptographic
hash functions [RFC6234] defined by the US National Institute of
Standards and Technology (NIST). [SHA2] implementations MAY support
additional recommended hash algorithms in the "COSE Algorithms"
registry (https://www.iana.org/assignments/cose); that is, the hash
algorithm has "Yes" in the "Recommended" column of the IANA registry.
Hash algorithm identifiers MUST use only lowercase letters, and they
MUST NOT contain hyphen characters. The character following the
algorithm string MUST be a hyphen character ("-" or ASCII character
45).
The subsequent characters in the claim value are the base64-encoded
[RFC4648] digest of a canonicalized and concatenated string or
binary-data-based MIME body of the message. An "msgi" message digest
is computed over the entirety of the MIME body (be it carried via SIP
or not); per [RFC3428], this may be any sort of MIME body, including
a multipart body in some cases, especially when multimedia content is
involved. Those MIME bodies may or may not contain encrypted content
or as the sender desires. The digest becomes the value of the JWT
"msgi" claim, as per this example:
"msgi" :
"sha256-H8BRh8j48O9oYatfu5AZzq6A9RINQZngK7T62em8MUt1FLm52t+eX6xO"
Per [RFC8224], this specification leaves it to local policy to
determine how messages are handled after verification succeeds or
fails. Broadly, if a SIP-based verification service wants to
communicate back to the sender that the "msgi" hash does not
correspond to the received message, using a SIP 438 response code
would be most appropriate.
Note that, in some CPIM environments, intermediaries may add or
consume CPIM headers used for metadata in messages. MIME-layer
integrity protection of "msgi" would be broken by a modification
along these lines. Any such environment would require a profile of
this specification that reduces the scope of protection only to the
CPIM payload, as discussed in Section 9.1 of [RFC8591].
Finally, note that messages may be subject to store-and-forward
treatment that differs from delivery expectations of SIP
transactions. In such cases, the expiry freshness window recommended
by [RFC8224] may be too strict, as routine behavior might dictate the
delivery of a MESSAGE minutes or hours after it was sent. The
potential for replay attacks can, however, be largely mitigated by
the timestamp in PASSporTs; duplicate messages are easily detected,
and the timestamp can be used to order messages displayed in the user
inbox in a way that precludes showing stale messages as fresh.
Relaxing the expiry timer would require support for such features on
the receiving side of the message.
3.2.1. PASSporT Conveyance with Messaging
If the message is being conveyed in SIP, via the MESSAGE method, then
the PASSporT could be conveyed in an Identity header in that request.
The authentication and verification service procedures for populating
that PASSporT would follow the guidance in [RFC8224], with the
addition of the "msgi" claim defined in Section 3.2.
In text messaging today, Multimedia Messaging Service (MMS) messages
are often conveyed with SMTP. Thus, there is a suite of additional
email security tools available in this environment for sender
authentication, such as "Domain-based Message Authentication,
Reporting, and Conformance (DMARC)" [RFC7489]. The interaction of
these mechanisms with STIR certificates and/or PASSporTs would
require further study and is outside the scope of this document.
For other cases where messages are conveyed by some protocol other
than SIP, that protocol itself might have some way of conveying
PASSporTs. There will surely be cases where legacy transmission of
messages will not permit an accompanying PASSporT; in such a
situation, something like out-of-band [RFC8816] conveyance would be
the only way to deliver the PASSporT. For example, this may be
necessary to support cases where legacy Short Message Peer-to-Peer
[SMPP] systems cannot be upgraded.
A MESSAGE request can be sent to multiple destinations in order to
support multiparty messaging. In those cases, the "dest" claim of
the PASSporT can accommodate the multiple targets of a MESSAGE
without the need to generate a PASSporT for each target of the
message. However, if the request is forked to multiple targets by an
intermediary later in the call flow, and the list of targets is not
available to the authentication service, then that forking
intermediary would need to use diversion PASSporTs [RFC8946] to sign
for its target set.
4. Certificates and Messaging
"Secure Telephone Identity Credentials: Certificates" [RFC8226]
defines a way to issue certificates that sign PASSporTs, which attest
through their TNAuthList a Service Provider Code (SPC) and/or a set
of one or more telephone numbers. This specification proposes that
the semantics of these certificates should suffice for signing for
messages from a telephone number without further modification.
Note that the certificate referenced by the "x5u" of a PASSporT can
change over time due to certificate expiry/rollover; in particular,
the use of short-lived certificates can entail rollover on a daily
basis or even more frequently. Thus, any store-and-forward messaging
system relying on PASSporTs must take into account the possibility
that the certificate that signed the PASSporT, though valid at the
time the PASSporT was generated, could expire before a user reads the
message. This might require:
* storing some indicator of the validity of the signature and
certificate at the time the message was received, or
* securely storing the certificate along with the PASSporT
so that the "iat" claim can be compared with the expiry freshness
window of the certificate prior to validation.
As the "orig" and "dest" claims of PASSporTs may contain URIs without
telephone numbers, the STIR for messaging mechanism contained in this
specification is not inherently restricted to the use of telephone
numbers. This specification offers no guidance on appropriate
certification authorities for designing "orig" values that do not
contain telephone numbers.
5. IANA Considerations
5.1. JSON Web Token Claims Registration
IANA has added one new claim to the "JSON Web Token Claims" registry
that was defined in [RFC7519].
Claim Name: msgi
Claim Description: Message Integrity Information
Change Controller: IETF
Specification Document(s): RFC 9475
5.2. PASSporT Type Registration
This specification defines one new PASSporT type for the "Personal
Assertion Token (PASSporT) Extensions" registry defined in [RFC8225].
ppt value: msg
Reference: Section 3.2 of RFC 9475
6. Privacy Considerations
Signing messages or message sessions with STIR has little direct
bearing on the privacy of messaging for SIP as described in [RFC3428]
or [RFC4975]. An authentication service signing a MESSAGE method may
compute the "msgi" hash over the message contents; if the message is
in cleartext, that will reveal its contents to the authentication
service, which might not otherwise be in the call path.
The implications for anonymity of STIR are discussed in [RFC8224],
and those considerations would apply equally here for anonymous
messaging. Creating an "msg" PASSporT does not add any additional
privacy protections to the original message content.
7. Security Considerations
This specification inherits the security considerations of [RFC8224].
The carriage of messages within SIP per Section 3.2 has a number of
security and privacy implications as documented in [RFC3428], which
are expanded in [RFC8591]; these considerations apply here as well.
The guidance about store-and-forward messaging and replay protection
in Section 3.2 should also be recognized by implementers.
Note that a variety of protocols that are not SIP, both those
integrated into the telephone network and those based on over-the-top
applications, are responsible for most of the messaging that is sent
to and from telephone numbers today. Introducing this capability for
SIP-based messaging will help to mitigate spoofing and nuisance
messaging for SIP-based platforms only.
8. References
8.1. 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>.
[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>.
[RFC3428] Campbell, B., Ed., Rosenberg, J., Schulzrinne, H.,
Huitema, C., and D. Gurle, "Session Initiation Protocol
(SIP) Extension for Instant Messaging", RFC 3428,
DOI 10.17487/RFC3428, December 2002,
<https://www.rfc-editor.org/info/rfc3428>.
[RFC3862] Klyne, G. and D. Atkins, "Common Presence and Instant
Messaging (CPIM): Message Format", RFC 3862,
DOI 10.17487/RFC3862, August 2004,
<https://www.rfc-editor.org/info/rfc3862>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.
[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>.
8.2. Informative References
[CONNECT-ID-STIR]
Peterson, J. and C. Wendt, "Connected Identity for STIR",
Work in Progress, Internet-Draft, draft-ietf-stir-rfc4916-
update-04, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-stir-
rfc4916-update-04>.
[RCC.07] GSMA, "Rich Communication Suite 8.0 Advanced
Communications Services and Client Specification", Version
9.0, May 2018, <https://www.gsma.com/futurenetworks/wp-
content/uploads/2019/09/RCC.07-v9.0.pdf>.
[RCC.15] GSMA, "IMS Device Configuration and Supporting Services",
Version 7.0, October 2019, <https://www.gsma.com/newsroom/
wp-content/uploads//RCC.15-v7.0.pdf>.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
<https://www.rfc-editor.org/info/rfc4103>.
[RFC4975] Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
"The Message Session Relay Protocol (MSRP)", RFC 4975,
DOI 10.17487/RFC4975, September 2007,
<https://www.rfc-editor.org/info/rfc4975>.
[RFC5194] van Wijk, A., Ed. and G. Gybels, Ed., "Framework for Real-
Time Text over IP Using the Session Initiation Protocol
(SIP)", RFC 5194, DOI 10.17487/RFC5194, June 2008,
<https://www.rfc-editor.org/info/rfc5194>.
[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>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC8591] Campbell, B. and R. Housley, "SIP-Based Messaging with S/
MIME", RFC 8591, DOI 10.17487/RFC8591, April 2019,
<https://www.rfc-editor.org/info/rfc8591>.
[RFC8816] Rescorla, E. and J. Peterson, "Secure Telephone Identity
Revisited (STIR) Out-of-Band Architecture and Use Cases",
RFC 8816, DOI 10.17487/RFC8816, February 2021,
<https://www.rfc-editor.org/info/rfc8816>.
[RFC8862] Peterson, J., Barnes, R., and R. Housley, "Best Practices
for Securing RTP Media Signaled with SIP", BCP 228,
RFC 8862, DOI 10.17487/RFC8862, January 2021,
<https://www.rfc-editor.org/info/rfc8862>.
[RFC8876] Rosen, B., Schulzrinne, H., Tschofenig, H., and R.
Gellens, "Non-interactive Emergency Calls", RFC 8876,
DOI 10.17487/RFC8876, September 2020,
<https://www.rfc-editor.org/info/rfc8876>.
[RFC8946] Peterson, J., "Personal Assertion Token (PASSporT)
Extension for Diverted Calls", RFC 8946,
DOI 10.17487/RFC8946, February 2021,
<https://www.rfc-editor.org/info/rfc8946>.
[SHA2] National Institute of Standards and Technology (NIST),
"Secure Hash Standard (SHS)", FIPS PUB 180-3, 2008,
<http://csrc.nist.gov/publications/fips/fips180-3/
fips180-3_final.pdf>.
[SMPP] SMS Forum, "Short Message Peer-to-Peer Protocol
Specification", Version 5.0, February 2003,
<https://smpp.org/SMPP_v5.pdf>.
Acknowledgments
We would like to thank Christer Holmberg, Brian Rosen, Ben Campbell,
Russ Housley, and Alex Bobotek for their contributions to this
specification.
Authors' Addresses
Jon Peterson
Neustar, Inc.
Email: jon.peterson@team.neustar