rfc9475v4.txt   rfc9475.txt 
Internet Engineering Task Force (IETF) J. Peterson Internet Engineering Task Force (IETF) J. Peterson
Request for Comments: 9475 Neustar Request for Comments: 9475 Neustar
Category: Standards Track C. Wendt Category: Standards Track C. Wendt
ISSN: 2070-1721 Somos ISSN: 2070-1721 Somos
October 2023 December 2023
Messaging Use Cases and Extensions for Secure Telephone Identity Messaging Use Cases and Extensions for Secure Telephone Identity
Revisited (STIR) Revisited (STIR)
Abstract Abstract
Secure Telephone Identity Revisited (STIR) provides a means of Secure Telephone Identity Revisited (STIR) provides a means of
attesting the identity of a telephone caller via a signed token in attesting the identity of a telephone caller via a signed token in
order to prevent impersonation of a calling party number, which is a order to prevent impersonation of a calling party number, which is a
key enabler for illegal robocalling. Similar impersonation is key enabler for illegal robocalling. Similar impersonation is
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The claim value of the "msgi" claim key is a string that defines the 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 crypto algorithm used to generate the digest concatenated by a hyphen
with a digest string. Implementations MUST support the hash with a digest string. Implementations MUST support the hash
algorithms SHA-256, SHA-384, and SHA-512. These hash algorithms are algorithms SHA-256, SHA-384, and SHA-512. These hash algorithms are
identified by "sha256", "sha384", and "sha512", respectively. SHA- identified by "sha256", "sha384", and "sha512", respectively. SHA-
256, SHA-384, and SHA-512 are part of the SHA-2 set of cryptographic 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 hash functions [RFC6234] defined by the US National Institute of
Standards and Technology (NIST). [SHA2] implementations MAY support Standards and Technology (NIST). [SHA2] implementations MAY support
additional recommended hash algorithms in the "COSE Algorithms" additional recommended hash algorithms in the "COSE Algorithms"
registry (https://www.iana.org/assignments/cose/ registry (https://www.iana.org/assignments/cose); that is, the hash
cose.xhtml#algorithms); that is, the hash algorithm has "Yes" in the algorithm has "Yes" in the "Recommended" column of the IANA registry.
"Recommended" column of the IANA registry. Hash algorithm Hash algorithm identifiers MUST use only lowercase letters, and they
identifiers MUST use only lowercase letters, and they MUST NOT MUST NOT contain hyphen characters. The character following the
contain hyphen characters. The character following the algorithm algorithm string MUST be a hyphen character ("-" or ASCII character
string MUST be a hyphen character ("-" or ASCII character 45). 45).
The subsequent characters in the claim value are the base64-encoded The subsequent characters in the claim value are the base64-encoded
[RFC4648] digest of a canonicalized and concatenated string or [RFC4648] digest of a canonicalized and concatenated string or
binary-data-based MIME body of the message. An "msgi" message digest 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 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 or not); per [RFC3428], this may be any sort of MIME body, including
a multipart body in some cases, especially when multimedia content is a multipart body in some cases, especially when multimedia content is
involved. Those MIME bodies may or may not contain encrypted content involved. Those MIME bodies may or may not contain encrypted content
or as the sender desires. The digest becomes the value of the JWT or as the sender desires. The digest becomes the value of the JWT
"msgi" claim, as per this example: "msgi" claim, as per this example:
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the PASSporT can accommodate the multiple targets of a MESSAGE the PASSporT can accommodate the multiple targets of a MESSAGE
without the need to generate a PASSporT for each target of the without the need to generate a PASSporT for each target of the
message. However, if the request is forked to multiple targets by an 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 intermediary later in the call flow, and the list of targets is not
available to the authentication service, then that forking available to the authentication service, then that forking
intermediary would need to use diversion PASSporTs [RFC8946] to sign intermediary would need to use diversion PASSporTs [RFC8946] to sign
for its target set. for its target set.
4. Certificates and Messaging 4. Certificates and Messaging
"Secure Telephone Identity Credentials: Certificates"[RFC8226] "Secure Telephone Identity Credentials: Certificates" [RFC8226]
defines a way to issue certificates that sign PASSporTs, which attest defines a way to issue certificates that sign PASSporTs, which attest
through their TNAuthList a Service Provider Code (SPC) and/or a set through their TNAuthList a Service Provider Code (SPC) and/or a set
of one or more telephone numbers. This specification proposes that of one or more telephone numbers. This specification proposes that
the semantics of these certificates should suffice for signing for the semantics of these certificates should suffice for signing for
messages from a telephone number without further modification. messages from a telephone number without further modification.
Note that the certificate referenced by the "x5u" of a PASSporT can Note that the certificate referenced by the "x5u" of a PASSporT can
change over time due to certificate expiry/rollover; in particular, change over time due to certificate expiry/rollover; in particular,
the use of short-lived certificates can entail rollover on a daily the use of short-lived certificates can entail rollover on a daily
basis or even more frequently. Thus, any store-and-forward messaging basis or even more frequently. Thus, any store-and-forward messaging
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