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<rfc category="std" docName="draft-ietf-stir-messaging-08"
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<!-- ***** FRONT MATTER ***** --> <rfc xmlns:xi="http://www.w3.org/2001/XInclude" docName="draft-ietf-stir-messagi ng-08" number="9475" submissionType="IETF" category="std" consensus="true" ipr=" trust200902" obsoletes="" updates="" xml:lang="en" tocInclude="true" tocDepth="4 " symRefs="true" sortRefs="true" version="3">
<front> <front>
<!-- The abbreviated title is used in the page header - it is only necessary
if the
full title is longer than 39 characters -->
<title abbrev="STIR Messaging">Messaging Use Cases and Extensions for STIR</ <title abbrev="STIR Messaging">Messaging Use Cases and Extensions for Secure
title> Telephone Identity Revisited (STIR)</title>
<seriesInfo name="RFC" value="9475"/>
<author initials="J." surname="Peterson" fullname="Jon Peterson"> <author initials="J." surname="Peterson" fullname="Jon Peterson">
<organization abbrev="Neustar">Neustar, Inc.</organization> <organization abbrev="Neustar">Neustar, Inc.</organization>
<address> <address>
<email>jon.peterson@team.neustar</email> <email>jon.peterson@team.neustar</email>
</address> </address>
</author> </author>
<author fullname="Chris Wendt" initials="C." surname="Wendt">
<author fullname="Chris Wendt" initials=
"C." surname="Wendt">
<organization>Somos</organization> <organization>Somos</organization>
<address> <address>
<email>chris-ietf@chriswendt.net</email> <email>chris-ietf@chriswendt.net</email>
</address> </address>
</author> </author>
<date year="2023" month="December"/>
<date year="2023" /> <area>art</area>
<workgroup>stir</workgroup>
<!-- <area>
ART
</area>-->
<keyword>SIP</keyword> <keyword>SIP</keyword>
<abstract> <abstract>
<t> <t>
Secure Telephone Identity Revisited (STIR) provides a means of attesti Secure Telephone Identity Revisited (STIR) provides a means of attesti
ng the identity of a telephone caller via a signed token in order to prevent imp ng the identity of a telephone caller via a signed token in order to prevent imp
ersonation of a calling party number, which is a key enabler for illegal robocal ersonation of a calling party number, which is a key enabler for illegal robocal
ling. Similar impersonation is sometimes leveraged by bad actors in the text and ling. Similar impersonation is sometimes leveraged by bad actors in the text and
multimedia messaging space. This document explores the applicability of STIR's multimedia messaging space. This document explores the applicability of STIR's
Personal Assertion Token (PASSporT) and certificate issuance framework to text a Personal Assertion Token (PASSporT) and certificate issuance framework to text a
nd multimedia messaging use cases, including support both for messages carried a nd multimedia messaging use cases, including support for both messages carried a
s a payload in SIP requests and for messages sent in sessions negotiated by SIP. s a payload in SIP requests and messages sent in sessions negotiated by SIP.
</t> </t>
</abstract> </abstract>
</front> </front>
<middle> <middle>
<section title="Introduction"> <section numbered="true" toc="default">
<t> <name>Introduction</name>
The STIR problem statement <xref target="RFC7340"/> describes widespread <t>
problems enabled by impersonation in the telephone network, including illegal ro The STIR problem statement <xref target="RFC7340" format="default"/> desc
bocalling, voicemail hacking, and swatting. ribes widespread problems enabled by impersonation in the telephone network, inc
As telephone services are increasingly migrating onto the Internet and us luding illegal robocalling, voicemail hacking, and swatting.
ing Voice over IP (VoIP) protocols such as <xref target="RFC3261">SIP</xref>, it As telephone services are increasingly migrating onto the Internet and us
is necessary for these protocols ing Voice over IP (VoIP) protocols such as <xref target="RFC3261" format="defaul
to support stronger identity mechanisms to prevent impersonation. <xref t t">SIP</xref>, it is necessary for these protocols
arget="RFC8224"/> defines a SIP Identity header capable of carrying <xref target to support stronger identity mechanisms to prevent impersonation. <xref t
="RFC8225">PASSporT</xref> objects in SIP as a means to cryptographically attest arget="RFC8224" format="default"/> defines a SIP Identity header capable of carr
that the originator of a telephone call is authorized to use the calling party ying <xref target="RFC8225" format="default">PASSporT</xref> objects in SIP as a
number (or, for native SIP cases, SIP URI) associated with the originator of the means to cryptographically attest that the originator of a telephone call is au
call. thorized to use the calling party number (or, for SIP cases, SIP URI) associated
</t><t> with the originator of the call.
The problem of bulk, unsolicited commercial communications is not, howeve </t>
r, limited to telephone calls. Spammers and fraudsters are increasingly turning <t>
to messaging applications to deliver undesired content to consumers. In some res However, the problem of bulk, unsolicited commercial communications is no
pects, mitigating these unwanted messages resembles the email spam problem: text t limited to telephone calls. Spammers and fraudsters are increasingly turning t
ual analysis of the message contents can be used to fingerprint content that is o messaging applications to deliver undesired content to consumers. In some resp
generated by spammers, for example. However, encrypted messaging is becoming mor ects, mitigating these unwanted messages resembles the email spam problem; for e
e common, and analysis of message contents may no longer be a reliable way to mi xample, textual analysis of the message contents can be used to fingerprint cont
tigate messaging spam in the future. And as STIR sees further deployment in the ent that is generated by spammers. However, encrypted messaging is becoming more
telephone network, the governance structures put in place for securing telephone common, and analysis of message contents may no longer be a reliable way to mit
network resources with STIR could be repurposed to help secure the messaging ec igate messaging spam in the future. As STIR sees further deployment in the telep
osystem. hone network, the governance structures put in place for securing telephone-netw
</t><t> ork resources with STIR could be repurposed to help secure the messaging ecosyst
One of the more sensitive applications for message security is emergency em.
services. As next-generation emergency services increasingly incorporate messagi </t>
ng as a mode of communication with public safety personnel (see <xref target="RF <t>
C8876"/>), providing an identity assurance could help to mitigate denial-of-serv One of the more sensitive applications for message security is emergency
ice attacks, as well as ultimately helping to identify the source of emergency c services. As next-generation emergency services increasingly incorporate messagi
ommunications in general (including swatting attacks, see <xref target="RFC7340" ng as a mode of communication with public safety personnel (see <xref target="RF
/>). C8876" format="default"/>), providing an identity assurance could help to mitiga
</t><t> te denial-of-service attacks and ultimately help to identify the source of emerg
This specification therefore explores how the PASSporT mechanism defined ency communications in general (including swatting attacks, see <xref target="RF
for STIR could be applied to providing protection for textual and multimedia mes C7340" format="default"/>).
saging, but focuses particularly on those messages that use telephone numbers as </t>
the identity of the sender. It moreover considers the reuse of existing STIR ce <t>
rtificates, which are beginning to see widespread deployment, for signing PASSpo Therefore, this specification explores how the PASSporT mechanism defined
rTs that protect messages. For that purpose it defines a new PASSporT type and a for STIR could be applied in providing protection for textual and multimedia me
n element that protects message integrity. It contains a mixture of normative an ssaging, but it focuses particularly on those messages that use telephone number
d informative guidance that specifies new fields for use in PASSporTs as well as s as the identity of the sender. Moreover, it considers the reuse of existing ST
an overview of how STIR might be applied to messaging in various environemnts. IR certificates, which are beginning to see widespread deployment for signing PA
</t> SSporTs that protect messages. For that purpose, it defines a new PASSporT type
and an element that protects message integrity. It contains a mixture of normati
ve and informative guidance that specifies new claims for use in PASSporTs as we
ll as an overview of how STIR might be applied to messaging in various environme
nts.
</t>
</section> </section>
<section numbered="true" toc="default">
<section title="Terminology"> <name>Terminology</name>
<t>
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQU
"SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this d IRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
ocument NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>
are to be interpreted as described in BCP 14 <xref target="RFC2119"/> <xref targ RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
et="RFC8174"/> when, and only when, they appear in all capitals, as shown here.< "<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to
/t> be interpreted as
described in BCP&nbsp;14 <xref target="RFC2119"/> <xref target="RFC8174"/>
when, and only when, they appear in all capitals, as shown here.
</t>
</section> </section>
<section anchor="applic" numbered="true" toc="default">
<name>Applicability to Messaging Systems</name>
<t>
<section anchor="applic" title="Applicability to Messaging Systems"> At a high level, <xref target="RFC8225" format="default">PASSporT</xref> c
<t> laims provide similar value to number-based messaging as they do to telephone ca
At a high level, baseline <xref target="RFC8225">PASSporT</xref> claim lls. A signature over the calling and called party numbers, along with a timesta
s provide similar value to number-based messaging as they do to traditional tele mp, could already help to prevent impersonation in the mobile-messaging ecosyste
phone calls. A signature over the calling and called party numbers, along with a m.</t>
timestamp, could already help to prevent impersonation in the mobile messaging <t>When it comes to protecting message contents, broadly, there are a few
ecosystem. When it comes to protecting message contents, broadly, there are a fe ways that the PASSporT mechanism of STIR could apply to messaging:</t>
w ways that the PASSporT mechanism of STIR could apply to messaging: first, a PA <ol><li>a PASSporT could be used to securely negotiate a session over whi
SSporT could be used to securely negotiate a session over which messages will be ch messages will be exchanged (see <xref target="session"/>), and</li>
exchanged; and second, in sessionless scenarios, a PASSporT could be generated <li>in sessionless scenarios, a PASSporT could be generated on a per-mess
on a per-message basis with its own built-in message security. age basis with its own built-in message security (see <xref target="message"/>).
</t> </li></ol>
<section anchor="session" title="Message Sessions"> <section anchor="session" numbered="true" toc="default">
<t> <name>Message Sessions</name>
For the first case, where SIP negotiates a session where the media will <t>
be text messages or MIME content, as, for example, with the <xref target="RFC49
75">Message Session Relay Protocol (MSRP)</xref>, the usage of STIR would deviat
e little from <xref target="RFC8224"/>. An INVITE request sent with an Identity
header containing a PASSporT with the proper calling and called party numbers wo
uld then negotiate an MSRP session the same way that an INVITE for a telephone c
all would negotiate an audio session. This could be applicable to MSRP sessions
negotiated for <xref target="RCC.07">RCS</xref>. Note that if TLS is used to se
cure MSRP (per RCS <xref target="RCC.15"/>), fingerprints of those TLS keys coul
d be secured via the "mky" claim of PASSporT using the <xref target="RFC8862"/>
framework. Similar practices would apply to sessions that negotiate real-time te
xt over RTP (<xref target="RFC4103"/>, <xref target="RFC5194"/>); any that can o
perate over DTLS/SRTP should work with the "mky" PASSporT claim. For the most ba
sic use cases, STIR for messaging should not require any further protocol enhanc
ements.
</t><t>
Current usage of baseline <xref target="RFC8224"/> Identity is largely
confined to INVITE requests that initiate telephone calls. RCS-style application
s would require PASSporTs for all conversation participants, which could become
complex in multi-party conversations. Any solution in this space would likely re
quire the implementation of STIR <xref target="I-D.peterson-stir-rfc4916-update"
>connected identity</xref>, but the specification of PASSporT-signed session con
ferencing is outside the scope of this document.
</t><t>
Also note that the assurance offered by <xref target="RFC8862"/> is "en
d-to-end" in the sense that it offers assurance between an authentication servic
e and verification service. If those are not implemented by the endpoints themse
lves, there are still potential opportunities for tampering before messages are
signed and after they are verified. For the most part, STIR does not intend to p
rotect against machine-in-the-middle attacks so much as spoofed origination, how
ever, so the protection offered may be sufficient to mitigate nuisance messaging
.
</t>
</section>
<section anchor="message" title="PASSporTs and Individual Messages"> In the first case, SIP negotiates a session in which the media will be
<t> text messages or MIME content, as, for example, with the <xref target="RFC4975"
In the second case, SIP also has a method for sending messages in the b format="default">Message Session Relay Protocol (MSRP)</xref>. This usage of ST
ody of a SIP request: the <xref target="RFC3428">MESSAGE</xref> method. MESSAGE IR would deviate little from <xref target="RFC8224" format="default"/>. An INVIT
is used for example in some North American emergency services use cases. The int E request sent with an Identity header containing a PASSporT with the proper cal
eraction of STIR with MESSAGE is not as straightforward as the potential use cas ling and called party numbers would then negotiate an MSRP session the same way
e with MSRP. An Identity header could be added to any SIP MESSAGE request, but w that an INVITE for a telephone call would negotiate an audio session. This coul
ithout some extension to the PASSporT claims, the PASSporT would offer no protec d be applicable to MSRP sessions negotiated for <xref target="RCC.07" format="de
tion to the message content, and potentially be reusable for cut-and-paste attac fault">Rich Communication Suite (RCS)</xref>. Note that, if TLS is used to secur
ks where the Identity header field from a legitimate request for one user is reu e MSRP (per RCS <xref target="RCC.15" format="default"/>), fingerprints of those
sed in a request for a different user. As the bodies of SIP requests are MIME en TLS keys could be secured via the "mky" claim of PASSporT using the framework d
coded, <xref target="RFC8591">S/MIME</xref> has been proposed as a means of prov escribed in <xref target="RFC8862" format="default"/>. Similar practices would a
iding integrity for MESSAGE (and some MSRP cases as well). The use of <xref targ pply to sessions that negotiate real-time text over RTP (<xref target="RFC4103"
et="RFC3862">CPIM</xref> as a MIME body allows the integrity of messages to with format="default"/>, <xref target="RFC5194" format="default"/>); any that can ope
stand interworking with non-SIP protocols. The interaction of <xref target="RFC8 rate over DTLS/SRTP (Secure Real-time Transport Protocol) should work with the
226"/> STIR certificates with S/MIME for messaging applications would require fu "mky" PASSporT claim. For the most basic use cases, STIR for messaging should no
rther specification; and additionally, PASSporT can provide its own integrity ch t require any further protocol enhancements.
eck for message contents through a new claim defined to provide a hash over mess </t>
age contents. <t>
</t> Current usage of <xref target="RFC8224" format="default"/> Identity is
<t> largely confined to INVITE requests that initiate telephone calls. RCS-style app
In order to differentiate a PASSporT for an individual message from a P lications would require PASSporTs for all conversation participants, which could
ASSporT used to secure a telephone call or message stream, this document defines become complex in multiparty conversations. Any solution in this space would li
a new "msg" PASSporT Type. "msg" PASSporTs may carry a new optional JWT <xref t kely require the implementation of <xref target="I-D.ietf-stir-rfc4916-update" f
arget="RFC7519"/> claim "msgi" which provides a digest over a MIME body that con ormat="default">STIR-connected identity</xref>, but the specification of PASSpor
tains a text or multimedia message. Authentication services MUST NOT include "ms T-signed session conferencing is outside the scope of this document.
gi" elements in PASSporT types other than "msg", but "msgi" is OPTIONAL in "msg" </t>
PASSporTs, as integrity for messages may be provided by some other service (e.g <t>
. <xref target="RFC8591"/>). Verification services MUST ignore the presence of " Also note that the assurance offered by <xref target="RFC8862" format="
msgi" in non-"msg" PASSporT types. default"/> is "end-to-end" in the sense that it offers assurance between an auth
</t><t> entication service and verification service. If those are not implemented by the
The claim value of "msgi" claim key is a string that defines the crypto endpoints themselves, there are still potential opportunities for tampering bef
algorithm used to generate the digest concatenated by a hyphen with a digest st ore messages are signed and after they are verified. However, for the most part,
ring. Implementations MUST support the hash algorithms SHA-256, SHA-384, STIR does not intend to protect against machine-in-the-middle attacks so much a
s spoofed origination; so the protection offered may be sufficient to mitigate n
uisance messaging.
</t>
</section>
<section anchor="message" numbered="true" toc="default">
<name>PASSporTs and Individual Messages</name>
<t>
In the second case described in <xref target="applic"/>, SIP also has a
method for sending messages in the body of a SIP request: the <xref target="RFC
3428" format="default">MESSAGE method</xref>. For example, MESSAGE is used in so
me North American emergency services use cases. The interaction of STIR with MES
SAGE 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 conte
nt; it would potentially be reusable for cut-and-paste attacks where the Identit
y 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, <xref target=
"RFC8591" format="default">S/MIME</xref> has been proposed as a means of providi
ng integrity for MESSAGE (and some MSRP cases as well). The use of <xref target=
"RFC3862" format="default">Common Presence and Instant Messaging (CPIM)</xref> a
s a MIME body allows the integrity of messages to withstand interworking with pr
otocols that are not SIP. The interaction of STIR certificates with S/MIME (see
<xref target="RFC8226" format="default"/>) for messaging applications would requ
ire further specification; additionally, PASSporT can provide its own integrity
check for message contents through a new claim defined to provide a hash over me
ssage contents.
</t>
<t>
In order to differentiate a PASSporT for an individual message from a P
ASSporT 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 To
ken (JWT) <xref target="RFC7519" format="default"/> claim "msgi", which provides
a digest over a MIME body that contains a text or multimedia message. Authentic
ation services <bcp14>MUST NOT</bcp14> include "msgi" elements in PASSporT types
other than "msg", but "msgi" is <bcp14>OPTIONAL</bcp14> in "msg" PASSporTs, as
integrity for messages may be provided by some other service (e.g. <xref target=
"RFC8591" format="default"/>). Verification services <bcp14>MUST</bcp14> ignore
the presence of "msgi" in non-"msg" PASSporT types.
</t>
<t>
The claim value of the "msgi" claim key is a string that defines the cr
ypto algorithm used to generate the digest concatenated by a hyphen with a diges
t string. Implementations <bcp14>MUST</bcp14> support the hash algorithms SHA-2
56, SHA-384,
and SHA-512. These hash algorithms are identified by "sha256", "sha384", and SHA-512. These hash algorithms are identified by "sha256", "sha384",
and "sha512", respectively. SHA-256, SHA-384, and SHA-512 are part of and "sha512", respectively. SHA-256, SHA-384, and SHA-512 are part of
the SHA-2 set of cryptographic hash functions <xref target="RFC6234"/> def the SHA-2 set of cryptographic hash functions <xref target="RFC6234" forma
ined by the t="default"/> defined by the
US National Institute of Standards and Technology (NIST). <xref target="SH US National Institute of Standards and Technology (NIST).
A2"/> Implementations <xref target="SHA2" format="default"/> implementations
MAY support additional recommended hash algorithms in <eref tar <bcp14>MAY</bcp14> support additional recommended hash algorithms in the
get=" <eref target="https://www.iana.org/assignments/cose">"COSE Algorithms" registry<
https://www.iana.org/assignments/cose/cose.xhtml#algo /eref>;
rithms">
[IANA-COSE-ALG]
</eref>;
that is, the hash algorithm has "Yes" in the "Recommended" column of that is, the hash algorithm has "Yes" in the "Recommended" column of
the IANA registry. Hash algorithm identifiers MUST use only lowercase the IANA registry. Hash algorithm identifiers <bcp14>MUST</bcp14> use onl
letters, and they MUST NOT contain hyphen characters. The character follow y lowercase
ing the algorithm string MUST be a hyphen character, "-", or ASCII 45. letters, and they <bcp14>MUST NOT</bcp14> contain hyphen characters. The c
</t><t> haracter following the algorithm string <bcp14>MUST</bcp14> be a hyphen characte
The subsequent characters in the claim value are the base64 encoded [RFC46 r ("-" or ASCII character 45).
48] digest of a canonicalized and concatenated string or binary data based MIME </t>
body of the message. <t>
A "msgi" message digest is computed over the entirety of the MIME body The subsequent characters in the claim value are the base64-encoded <xref
(be it carried via SIP or no), which per <xref target="RFC3428"/> may be any sor target="RFC4648"/> digest of a canonicalized and concatenated string or binary-d
t of MIME body, including a multipart body in some cases, especially when multim ata-based MIME body of the message.
edia content is involved. Those MIME bodies contain encrypted content or not as An "msgi" message digest is computed over the entirety of the MIME body
the sender desires. (be it carried via SIP or not); per <xref target="RFC3428" format="default"/>,
this may be any sort of MIME body, including a multipart body in some cases, esp
ecially when multimedia content is involved. Those MIME bodies may or may not co
ntain encrypted content or as the sender desires.
The digest becomes the value of the JWT "msgi" claim, as per this examp le: The digest becomes the value of the JWT "msgi" claim, as per this examp le:
</t><t> </t>
<t>
"msgi" : "sha256-H8BRh8j48O9oYatfu5AZzq6A9RINQZngK7T62em8MUt1FLm52t+eX6 xO" "msgi" : "sha256-H8BRh8j48O9oYatfu5AZzq6A9RINQZngK7T62em8MUt1FLm52t+eX6 xO"
</t><t> </t>
Per baseline <xref target="RFC8224"/>, this specifications leaves it to <t>
local policy to determine how messages are handled after verification succeeds Per <xref target="RFC8224" format="default"/>, this specification leave
or fails. Broadly, if a SIP-based verification service wants to communicate back s it to local policy to determine how messages are handled after verification su
to the sender that the "msgi" hash does not correspond to the received message, cceeds or fails. Broadly, if a SIP-based verification service wants to communica
using a SIP 438 response code would be most appropriate. te back to the sender that the "msgi" hash does not correspond to the received m
</t><t> essage, using a SIP 438 response code would be most appropriate.
Note that in some CPIM environments, intermediaries may add or consume </t>
CPIM headers used for metadata in messages. MIME-layer integrity protection of " <t>
msgi" would be broken by a modification along these lines. Any such environment Note that, in some CPIM environments, intermediaries may add or consume
would require a profile of this specification that reduces the scope of protecti CPIM headers used for metadata in messages. MIME-layer integrity protection of
on only to the CPIM payload, as discussed in <xref target="RFC8591"/> Section 9. "msgi" would be broken by a modification along these lines. Any such environment
1. would require a profile of this specification that reduces the scope of protect
</t><t> ion only to the CPIM payload, as discussed in <xref target="RFC8591" sectionForm
Finally, note that messages may be subject to store-and-forward treatme at="of" section="9.1"/>.
nt that differs from traditional delivery expectations of SIP transactions. In s </t>
uch cases, the expiry freshness window recommended by <xref target="RFC8224"/> m
ay be too strict, as routine behavior might dictate the delivery of a MESSAGE mi
nutes or hours after it was sent. The potential for replay attacks can, however,
be largely mitigated by the timestamp in PASSporTs; duplicate messages are easi
ly detected, and the timestamp can order messages displayed to 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.
</t>
<section anchor="convey" title="PASSporT Conveyance with Messaging">
<t>
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 a
uthentication and verification service procedures for populating that PASSporT w
ould follow <xref target="RFC8224"/>, with the addition of the "msgi" claim defi
ned in <xref target="message"/>.
</t><t>
In text messaging today, multimedia message system (MMS) messages
are often conveyed with SMTP. There are thus a suite of additional email securi
ty tools available in this environment for sender authentication, such as <xref
target="RFC7489">DMARC</xref>. The interaction of these mechanisms with STIR cer
tificates and/or PASSporTs would require further study and is outside the scope
of this document.
</t><t>
For other cases where messages are conveyed by some protocol othe
r than SIP, that protocol might itself have some way of conveying PASSporTs. But
there will surely be cases where legacy transmission of messages will not permi
t an accompanying PASSporT, in which case something like out-of-band <xref targe
t="RFC8816"/> conveyance would be the only way to deliver the PASSporT. This may
be necessary to support cases where legacy Short Message Peer-to-Peer <xref tar
get="SMPP"/> systems cannot be upgraded, for example.
</t><t>
A MESSAGE request can be sent to multiple destinations in order t
o support multiparty messaging. In those cases, the "dest" field of the PASSporT
can accommodate the multiple targets of a MESSAGE without the need to generate
a PASSporT for each target of the message. If however the request is forked to m
ultiple targets by an intermediary later in the call flow, and the list of targe
ts is not available to the authentication service, then that forking intermediar
y would need to use <xref target="RFC8946">diversion</xref> PASSporTs to sign fo
r its target set.
</t>
</section>
</section>
</section>
<section anchor="certs" title="Certificates and Messaging"> <t>
<t> Finally, note that messages may be subject to store-and-forward treatme
The <xref target="RFC8226"/> STIR certificate profiles defines a way to nt that differs from delivery expectations of SIP transactions. In such cases, t
issue certificates that sign PASSporTs, which attest through their TNAuthList a he expiry freshness window recommended by <xref target="RFC8224" format="default
Service Provider Code (SPC) and/or a set of one or more telephone numbers. This "/> may be too strict, as routine behavior might dictate the delivery of a MESSA
specification proposes that the semantics of these certificates should suffice GE minutes or hours after it was sent. The potential for replay attacks can, how
for signing for messages from a telephone number without further modification. ever, be largely mitigated by the timestamp in PASSporTs; duplicate messages are
</t><t> easily detected, and the timestamp can be
Note that the certificate referenced by the "x5u" of a PASSporT can cha used to order messages displayed in the user inbox in a way that
nge over time, due to certificate expiry/rollover; in particular the use of shor precludes showing stale messages as fresh. Relaxing the expiry timer would
t-lived certificates can entail rollover on a daily basis, or even more frequent require support for such features on the receiving side of the message.
ly. Thus any store-and-forward messaging system relying on PASSporTs must take i </t>
nto account the possibility that the certificate that signed the PASSporT, thoug <section anchor="convey" numbered="true" toc="default">
h valid at the time the PASSporT was generated, could expire before a user reads <name>PASSporT Conveyance with Messaging</name>
the message. This might require storing some indicator of the validity of the s <t>
ignature and certificate at the time the message was received, or securely stori If the message is being conveyed in SIP, via the MESSAGE method,
ng the certificate along with the PASSporT, so that the "iat" field can be compa then the PASSporT could be conveyed in an Identity header in that request. The a
red with the expiry freshness window of the certificate prior to validation. uthentication and verification service procedures for populating that PASSporT w
</t><t> ould follow the guidance in <xref target="RFC8224" format="default"/>, with the
As the "orig" and "dest" field of PASSporTs may contain URIs containing addition of the "msgi" claim defined in <xref target="message" format="default"/
SIP URIs without telephone numbers, the STIR for messaging mechanism contained >.
in this specification is not inherently restricted to the use of telephone numbe </t>
rs. This specification offers no guidance on certification authorities who are a
ppropriate to sign for non-telephone number "orig" values.
</t> <t>
In text messaging today, Multimedia Messaging Service (MMS) messa
ges are often conveyed with SMTP. Thus, there is a suite of additional email sec
urity tools available in this environment for sender authentication, such as "<x
ref target="RFC7489" format="title" />" <xref target="RFC7489" format="default"/
>. The interaction of these mechanisms with STIR certificates and/or PASSporTs w
ould require further study and is outside the scope of this document.
</t>
<t>
For other cases where messages are conveyed by some protocol othe
r than SIP, that protocol itself might have some way of conveying PASSporTs. The
re will surely be cases where legacy transmission of messages will not permit an
accompanying PASSporT; in such a situation, something like out-of-band <xref ta
rget="RFC8816" format="default"/> conveyance would be the only way to deliver th
e PASSporT. For example, this may be necessary to support cases where legacy Sho
rt Message Peer-to-Peer <xref target="SMPP" format="default"/> systems cannot be
upgraded.
</t>
<t>
A MESSAGE request can be sent to multiple destinations in order t
o 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 targ
ets is not available to the authentication service, then that forking intermedia
ry would need to use <xref target="RFC8946" format="default">diversion PASSporTs
</xref> to sign for its target set.
</t>
</section>
</section>
</section> </section>
<section anchor="certs" numbered="true" toc="default">
<name>Certificates and Messaging</name>
<t>
"<xref target="RFC8226" format="title"/>" <xref target="RFC8226" format
="default"/> defines a way to issue certificates that sign PASSporTs, which att
est through their TNAuthList a Service Provider Code (SPC) and/or a set of one o
r more telephone numbers. This specification proposes that the semantics of thes
e certificates should suffice for signing for messages from a telephone number w
ithout further modification.
</t>
<t>
Note that the certificate referenced by the "x5u" of a PASSporT can chang
e 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 int
o 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 t
he message. This might require:</t>
<ul>
<li>storing some indicator of the validity of the signature and certifica
te at the time the message was received, or</li>
<li>securely storing the certificate along with the PASSporT</li></ul>
<t>so that the "iat" claim can be compared with the expiry freshness wind
ow of the certificate prior to validation.</t>
<section anchor="Acknowledgments" title="Acknowledgments"> <t>
<t>We would like to thank Christer Holmberg, Brian Rosen, Ben Campbell, Ru As the "orig" and "dest" claims of PASSporTs may contain URIs without t
ss Housley, and Alex Bobotek for their contributions to this specification.</t> elephone numbers, the STIR for messaging mechanism contained in this specificati
</section> on is not inherently restricted to the use of telephone numbers. This specificat
ion offers no guidance on appropriate certification authorities for designing "o
rig" values that do not contain telephone numbers.
<section anchor="IANA" title="IANA Considerations"> </t>
<section title="JSON Web Token Claim
s Registration">
<t>This specification requests that the IANA add one new claim to the JSON
Web Token Claims registry as defined in <xref target="RFC7519"/>.</t>
<t>
Claim Name: "msgi"
</t><t>
Claim Description: Message Integrity Information
</t><t>
Change Controller: IESG
</t><t>
Specification Document(s): [RFCThis]
</t>
</section>
<section title="PASSporT Type Registration">
<t>This specification defines one new PASSporT type for the PASSport Exten
sions Registry defined in <xref target="RFC8225"/>, which resides at https://www
.iana.org/assignments/passport/passport.xhtml#passport-extensions.</t>
<t>
ppt value: "msg"
</t><t>
Reference: [RFCThis] <xref target="message"/>
</t>
</section>
</section> </section>
<section anchor="Privacy" title="Privacy Considerations"> <section anchor="IANA" numbered="true" toc="default">
<name>IANA Considerations</name>
<section numbered="true" toc="default">
<name>JSON Web Token Claims Registration</name>
<t>IANA has added one new claim to the "JSON Web Token Claims" registry
that was defined in <xref target="RFC7519" format="default"/>.</t>
<dl>
<dt>Claim Name:</dt><dd>msgi</dd>
<dt>Claim Description:</dt><dd>Message Integrity Information</dd>
<dt>Change Controller:</dt><dd>IETF</dd>
<dt>Specification Document(s):</dt><dd>RFC 9475</dd>
</dl>
</section>
<section numbered="true" toc="default">
<name>PASSporT Type Registration</name>
<t>This specification defines one new PASSporT type for the "Personal As
sertion Token (PASSporT) Extensions" registry defined in <xref target="RFC8225"
format="default"/>.</t>
<dl>
<dt>ppt value:</dt><dd>msg</dd>
<dt>Reference:</dt><dd><xref target="message" format="default"/> of RFC 9475</dd
>
</dl>
</section>
</section>
<section anchor="Privacy" numbered="true" toc="default">
<name>Privacy Considerations</name>
<t> <t>
Signing messages or message sessions with STIR has little direct bearin Signing messages or message sessions with STIR has little direct bearin
g on the privacy of messaging for SIP as described in <xref target="RFC3428"/> o g on the privacy of messaging for SIP as described in <xref target="RFC3428" for
r <xref target="RFC4975"/>. An authentication service signing a MESSAGE method m mat="default"/> or <xref target="RFC4975" format="default"/>. An authentication
ay compute the "msgi" hash over the message contents; if the message is in clear service signing a MESSAGE method may compute the "msgi" hash over the message co
text, that will reveal its contents to the authentication service, which might n ntents; if the message is in cleartext, that will reveal its contents to the aut
ot otherwise be in the call path. hentication service, which might not otherwise be in the call path.
</t><t> </t>
The implications for anonymity of STIR are discussed in <xref target="R <t>
FC8224"/>, and those considerations would apply equally here for anonymous messa The implications for anonymity of STIR are discussed in <xref target="R
ging. Creating a "msg" PASSporT does not add any additional privacy FC8224" format="default"/>, and those considerations would apply equally here fo
r anonymous messaging. Creating an "msg" PASSporT does not add any additional pr
ivacy
protections to the original message content. protections to the original message content.
</t> </t>
</section> </section>
<section anchor="Security" numbered="true" toc="default">
<section anchor="Security" title="Security Considerations"> <name>Security Considerations</name>
<t> <t>
This specification inherits the security considerations of <xref target This specification inherits the security considerations of <xref target
="RFC8224"/>. The carriage of messages within SIP per <xref target="message"/> h ="RFC8224" format="default"/>. The carriage of messages within SIP per <xref tar
as a number of security and privacy implications as documented in <xref target=" get="message" format="default"/> has a number of security and privacy implicatio
RFC3428"/>, which are expanded in <xref target="RFC8591"/>; these considerations ns as documented in <xref target="RFC3428" format="default"/>, which are expande
apply here well. The guidance about store-and-forward messaging and replay prot d in <xref target="RFC8591" format="default"/>; these considerations apply here
ection in <xref target="message"/> should also be recognized by implementers. as well. The guidance about store-and-forward messaging and replay protection in
</t><t> <xref target="message" format="default"/> should also be recognized by implemen
Note that a variety of non-SIP protocols, both those integrated into th ters.
e traditional telephone network and those based on over-the-top applications, ar </t>
e responsible for most of the messaging that is sent to and from telephone numbe <t>
rs today. Introducing this capability for SIP-based messaging will help to mitig Note that a variety of protocols that are not SIP, both those integrate
ate spoofing and nuisance messaging for SIP-based platforms only. d into the telephone network and those based on over-the-top applications, are r
</t> esponsible 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.
</t>
</section> </section>
</middle> </middle>
<!-- *****BACK MATTER ***** -->
<back> <back>
<!-- References split into informative and normative -->
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s: <references>
1. define an ENTITY at the top, and use "ampersand character"RFC2629; here ( <name>References</name>
as shown) <references>
2. simply use a PI "less than character"?rfc include="reference.RFC.2119.xml <name>Normative References</name>
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<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3
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<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
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<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
225.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
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<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3
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<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3
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<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6
234.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.46
48.xml"/>
</references>
<references>
<name>Informative References</name>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
340.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4
975.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
591.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4
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<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
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<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
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<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
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<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
876.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
816.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
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<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5
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Both are cited textually in the same manner: by using xref elements. <!-- [I-D.peterson-stir-rfc4916-update] Replaced by [I-D.ietf-stir-rfc491
If you use the PI option, xml2rfc will, by default, try to find included fil 6-update] IESG state I-D Exists -->
es in the same
directory as the including file. You can also define the XML_LIBRARY environ
ment variable
with a value containing a set of directories to search. These can be either
in the local
filing system or remote ones accessed by http (http://domain/dir/... ).-->
<references title="Normative References">
&RFC2119; <xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D
&RFC8174; .ietf-stir-rfc4916-update.xml"/>
&RFC3261;
&RFC8224;
&RFC8225;
&RFC8226;
&RFC3428;
&RFC3862;
&RFC6234;
</references>
<references title="Informative References">
&RFC7340;
&RFC4975;
&RFC8591;
&RFC4103;
&RFC7519;
&RFC8862;
&RFC7489;
&RFC8876;
&RFC8816;
&RFC8946;
&RFC5194;
&I-D.peterson-stir-rfc4916-update;
<reference anchor='SHA2'> <reference anchor="SHA2" target="http://csrc.nist.gov/publications/fips/
<front> fips180-3/fips180-3_final.pdf">
<title>Secure Hash Standard (SHS)</title> <front>
<author> <title>Secure Hash Standard (SHS)</title>
<organization> <author>
National Institute of Standards and Technology FIPS PUB 180-3. http:/ <organization>
/csrc.nist.gov/publications/fips/fips180-3/ National Institute of Standards and Technology (NIST)
fips180-3_final.pdf </organization>
</organization> </author>
</author> <date year="2008"/>
<date year='2018' /> </front>
</front> <seriesInfo name="FIPS PUB" value="180-3"/>
</reference> </reference>
<reference anchor='RCC.07'> <reference anchor="RCC.07" target="https://www.gsma.com/futurenetworks/w
<front> p-content/uploads/2019/09/RCC.07-v9.0.pdf">
<front>
<title>Rich Communication Suite 8.0 Advanced Communications Services and Client Specification</title> <title>Rich Communication Suite 8.0 Advanced Communications Services and Client Specification</title>
<author> <author>
<organization> <organization>GSMA
GSMA RCC.07 v9.0 | 16 May 2018 </organization>
</organization>
</author> </author>
<date year='2018' /> <date month="May" year="2018"/>
</front> </front>
</reference> <refcontent>Version 9.0</refcontent>
</reference>
<reference anchor='RCC.15'> <reference anchor="RCC.15" target="https://www.gsma.com/newsroom/wp-cont
<front> ent/uploads//RCC.15-v7.0.pdf">
<front>
<title>IMS Device Configuration and Supporting Services</title> <title>IMS Device Configuration and Supporting Services</title>
<author> <author>
<organization> <organization>GSMA</organization>
GSMA PRD-RCC.15 v5.0 | 16 May 2018
</organization>
</author> </author>
<date year='2018' /> <date month="October" year="2019"/>
</front> </front>
</reference> <refcontent>Version 7.0</refcontent>
</reference>
<reference anchor='SMPP'> <reference anchor="SMPP" target="https://smpp.org/SMPP_v5.pdf">
<front> <front>
<title>Short Message Peer to Peer Protocol Specification</title> <title>Short Message Peer-to-Peer Protocol Specification</title>
<author> <author>
<organization> <organization>SMS Forum</organization>
SMS Forum v5.0 | 19 February 2003
</organization>
</author> </author>
<date year='2003' /> <date month="February" year="2003"/>
</front> </front>
</reference> <refcontent>Version 5.0</refcontent>
</reference>
</references>
</references> </references>
</back> <section anchor="Acknowledgments" numbered="false">
<name>Acknowledgments</name>
<t>We would like to thank <contact fullname="Christer Holmberg"/>,
<contact fullname="Brian Rosen"/>, <contact fullname="Ben Campbell"/>,
<contact fullname="Russ Housley"/>, and <contact fullname="Alex
Bobotek"/> for their contributions to this specification.</t>
</section>
</back>
</rfc> </rfc>
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