draft-ietf-stir-rfc4474bis-09.txt   draft-ietf-stir-rfc4474bis-10.txt 
Network Working Group J. Peterson Network Working Group J. Peterson
Internet-Draft NeuStar Internet-Draft NeuStar
Intended status: Standards Track C. Jennings Intended status: Standards Track C. Jennings
Expires: November 26, 2016 Cisco Expires: January 8, 2017 Cisco
E. Rescorla E. Rescorla
RTFM, Inc. RTFM, Inc.
C. Wendt C. Wendt
Comcast Comcast
May 25, 2016 July 7, 2016
Authenticated Identity Management in the Session Initiation Protocol Authenticated Identity Management in the Session Initiation Protocol
(SIP) (SIP)
draft-ietf-stir-rfc4474bis-09.txt draft-ietf-stir-rfc4474bis-10.txt
Abstract Abstract
The baseline security mechanisms in the Session Initiation Protocol The baseline security mechanisms in the Session Initiation Protocol
(SIP) are inadequate for cryptographically assuring the identity of (SIP) are inadequate for cryptographically assuring the identity of
the end users that originate SIP requests, especially in an the end users that originate SIP requests, especially in an
interdomain context. This document defines a mechanism for securely interdomain context. This document defines a mechanism for securely
identifying originators of SIP requests. It does so by defining a identifying originators of SIP requests. It does so by defining a
SIP header field for conveying a signature used for validating the SIP header field for conveying a signature used for validating the
identity, and for conveying a reference to the credentials of the identity, and for conveying a reference to the credentials of the
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 26, 2016. This Internet-Draft will expire on January 8, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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5. Signature Generation and Validation . . . . . . . . . . . . . 7 5. Signature Generation and Validation . . . . . . . . . . . . . 7
5.1. Authentication Service Behavior . . . . . . . . . . . . . 7 5.1. Authentication Service Behavior . . . . . . . . . . . . . 7
5.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 10 5.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 10
5.2.1. Handling 'canon' parameters . . . . . . . . . . . . . 12 5.2.1. Handling 'canon' parameters . . . . . . . . . . . . . 12
6. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 13 6. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. Credential Use by the Authentication Service . . . . . . 13 6.1. Credential Use by the Authentication Service . . . . . . 13
6.2. Credential Use by the Verification Service . . . . . . . 14 6.2. Credential Use by the Verification Service . . . . . . . 14
6.3. Handling 'info' parameter URIs . . . . . . . . . . . . . 15 6.3. Handling 'info' parameter URIs . . . . . . . . . . . . . 15
6.4. Credential System Requirements . . . . . . . . . . . . . 15 6.4. Credential System Requirements . . . . . . . . . . . . . 15
7. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 16 7. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 16
7.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 16 7.1. Authority for Telephone Numbers . . . . . . . . . . . . . 18
7.1.1. Canonicalization Procedures . . . . . . . . . . . . . 17 7.2. Telephone Number Canonicalization Procedures . . . . . . 18
7.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . 19 7.3. Authority for Domain Names . . . . . . . . . . . . . . . 19
8. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 20 7.4. URI Normalization . . . . . . . . . . . . . . . . . . . . 20
9. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 23 8. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 21
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 24 9. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 24
11. Security Considerations . . . . . . . . . . . . . . . . . . . 26 10. Backwards Compatibililty with RFC4474 . . . . . . . . . . . . 25
11.1. Protected Request Fields . . . . . . . . . . . . . . . . 26 11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 25
11.1.1. Protection of the To Header and Retargeting . . . . 28 12. Security Considerations . . . . . . . . . . . . . . . . . . . 27
11.2. Unprotected Request Fields . . . . . . . . . . . . . . . 28 12.1. Protected Request Fields . . . . . . . . . . . . . . . . 27
11.3. Malicious Removal of Identity Headers . . . . . . . . . 29 12.1.1. Protection of the To Header and Retargeting . . . . 29
11.4. Securing the Connection to the Authentication Service . 29 12.2. Unprotected Request Fields . . . . . . . . . . . . . . . 30
11.5. Authorization and Transitional Strategies . . . . . . . 30 12.3. Malicious Removal of Identity Headers . . . . . . . . . 30
11.6. Display-Names and Identity . . . . . . . . . . . . . . . 31 12.4. Securing the Connection to the Authentication Service . 31
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32 12.5. Authorization and Transitional Strategies . . . . . . . 32
12.1. Identity-Info Parameters . . . . . . . . . . . . . . . . 32 12.6. Display-Names and Identity . . . . . . . . . . . . . . . 33
12.2. Identity-Info Algorithm Parameter Values . . . . . . . . 32 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
12.3. Response Codes defined in RFC4474 . . . . . . . . . . . 32 13.1. Identity-Info Parameters . . . . . . . . . . . . . . . . 33
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33 13.2. Identity-Info Algorithm Parameter Values . . . . . . . . 34
14. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 34 13.3. Response Codes defined in RFC4474 . . . . . . . . . . . 34
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35
15.1. Normative References . . . . . . . . . . . . . . . . . . 34 15. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 35
15.2. Informative References . . . . . . . . . . . . . . . . . 35 16. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37 16.1. Normative References . . . . . . . . . . . . . . . . . . 36
16.2. Informative References . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
1. Introduction 1. Introduction
This document provides enhancements to the existing mechanisms for This document provides enhancements to the existing mechanisms for
authenticated identity management in the Session Initiation Protocol authenticated identity management in the Session Initiation Protocol
(SIP, [RFC3261]). An identity, for the purposes of this document, is (SIP, [RFC3261]). An identity, for the purposes of this document, is
defined as either a SIP URI, commonly a canonical address-of-record defined as either a SIP URI, commonly a canonical address-of-record
(AoR) employed to reach a user (such as (AoR) employed to reach a user (such as
'sip:alice@atlanta.example.com'), or a telephone number, which can be 'sip:alice@atlanta.example.com'), or a telephone number, which can be
represented as either a TEL URI [RFC3966] or as the user portion of a represented as either a TEL URI [RFC3966] or as the user portion of a
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user portion is not a telephone number, the authentication service user portion is not a telephone number, the authentication service
MUST extract the hostname portion of the identity field and compare MUST extract the hostname portion of the identity field and compare
it to the domain(s) for which it is responsible (following the it to the domain(s) for which it is responsible (following the
procedures in RFC 3261 [RFC3261], Section 16.4). If the identity procedures in RFC 3261 [RFC3261], Section 16.4). If the identity
field uses the TEL URI scheme [RFC3966], or the identity field is a field uses the TEL URI scheme [RFC3966], or the identity field is a
SIP or SIPS URI with a telephone number in the user portion, the SIP or SIPS URI with a telephone number in the user portion, the
authentication service determines whether or not it is responsible authentication service determines whether or not it is responsible
for this telephone number; see Section 7.1 for more information. An for this telephone number; see Section 7.1 for more information. An
authentication service proceeding with a signature over a telephone authentication service proceeding with a signature over a telephone
number MUST then follow the canonicalization procedures described in number MUST then follow the canonicalization procedures described in
Section 7.1.1. If the authentication service is not authoritative Section 7.2. If the authentication service is not authoritative for
for the identity in question, it SHOULD process and forward the the identity in question, it SHOULD process and forward the request
request normally unless the local policy is to block such requests. normally unless the local policy is to block such requests. The
The authentication service MUST NOT add an Identity header if the authentication service MUST NOT add an Identity header if the
authentication service does not have the authority to make the claim authentication service does not have the authority to make the claim
it asserts. it asserts.
Step 2: Step 2:
The authentication service MUST then determine whether or not the The authentication service MUST then determine whether or not the
sender of the request is authorized to claim the identity given in sender of the request is authorized to claim the identity given in
the identity field. In order to do so, the authentication service the identity field. In order to do so, the authentication service
MUST authenticate the sender of the message. Some possible ways in MUST authenticate the sender of the message. Some possible ways in
which this authentication might be performed include: which this authentication might be performed include:
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to the user agent. to the user agent.
Authorization of the use of a particular username or telephone number Authorization of the use of a particular username or telephone number
in the user part of the From header field is a matter of local policy in the user part of the From header field is a matter of local policy
for the authentication service; see Section 6.1 for more information. for the authentication service; see Section 6.1 for more information.
Note that this check is performed only on the addr-spec in the Note that this check is performed only on the addr-spec in the
identity field (e.g., the URI of the sender, like identity field (e.g., the URI of the sender, like
'sip:alice@atlanta.example.com'); it does not convert the display- 'sip:alice@atlanta.example.com'); it does not convert the display-
name portion of the From header field (e.g., 'Alice Atlanta'). For name portion of the From header field (e.g., 'Alice Atlanta'). For
more information, see Section 11.6. more information, see Section 12.6.
Step 3: Step 3:
An authentication service MUST add a Date header field to SIP An authentication service MUST add a Date header field to SIP
requests that do not have one. The authentication service MUST requests that do not have one. The authentication service MUST
ensure that any preexisting Date header in the request is accurate. ensure that any preexisting Date header in the request is accurate.
Local policy can dictate precisely how accurate the Date must be; a Local policy can dictate precisely how accurate the Date must be; a
RECOMMENDED maximum discrepancy of sixty seconds will ensure that the RECOMMENDED maximum discrepancy of sixty seconds will ensure that the
request is unlikely to upset any verifiers. If the Date header request is unlikely to upset any verifiers. If the Date header
contains a time different by more than one minute from the current contains a time different by more than one minute from the current
time noted by the authentication service, the authentication service time noted by the authentication service, the authentication service
SHOULD reject the request. This behavior is not mandatory because a SHOULD reject the request. This behavior is not mandatory because a
user agent client (UAC) could only exploit the Date header in order user agent client (UAC) could only exploit the Date header in order
to cause a request to fail verification; the Identity header is not to cause a request to fail verification; the Identity header is not
intended to provide a source of non-repudiation or a perfect record intended to provide a source of non-repudiation or a perfect record
of when messages are processed. Finally, the authentication service of when messages are processed. Finally, the authentication service
MUST verify that both the Date header and the current time fall MUST verify that both the Date header and the current time fall
within the validity period of its credential. within the validity period of its credential.
See Section 11 for information on how the Date header field assists See Section 12 for information on how the Date header field assists
verifiers. verifiers.
Step 4: Step 4:
Subsequently, the authentication service MUST form a PASSporT object Subsequently, the authentication service MUST form a PASSporT object
and add a corresponding an Identity header to the request containing and add a corresponding an Identity header to the request containing
this signature. For baseline PASSporT objects headers (without an this signature. For baseline PASSporT objects headers (without an
Identity header "ppt" parameter), this follows the procedures in Identity header "ppt" parameter), this follows the procedures in
Section 8; if the authentication service is using an alternative Section 8; if the authentication service is using an alternative
"ppt" format, it MUST add an appropriate "ppt" parameter and follow "ppt" format, it MUST add an appropriate "ppt" parameter and follow
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Also, in some cases, a request signed by an authentication service Also, in some cases, a request signed by an authentication service
will be rejected by the verification service on the receiving side, will be rejected by the verification service on the receiving side,
and the authentication service will receive a SIP 4xx status code in and the authentication service will receive a SIP 4xx status code in
the backwards direction, such as a 438 indicating a verification the backwards direction, such as a 438 indicating a verification
failure. If the authentication service did not originally send the failure. If the authentication service did not originally send the
Identity header with the "canon" parameter, it SHOULD retry a request Identity header with the "canon" parameter, it SHOULD retry a request
once after receiving a 438 response, this time including the "canon". once after receiving a 438 response, this time including the "canon".
The information in "canon" is useful on the verification side for The information in "canon" is useful on the verification side for
debugging errors, and there are some known causes of verification debugging errors, and there are some known causes of verification
failures (such as the Date header changing in transit, see failures (such as the Date header changing in transit, see
Section 11.1 for more information) that can be resolved by the Section 12.1 for more information) that can be resolved by the
inclusion of "canon". inclusion of "canon".
Finally, the authentication service MUST forward the message Finally, the authentication service MUST forward the message
normally. normally.
5.2. Verifier Behavior 5.2. Verifier Behavior
This document specifies a logical role for SIP entities called a This document specifies a logical role for SIP entities called a
verification service, or verifier. When a verifier receives a SIP verification service, or verifier. When a verifier receives a SIP
message containing one or more Identity headers, it inspects the message containing one or more Identity headers, it inspects the
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A SIP request may contain zero, one, or more Identity headers. A A SIP request may contain zero, one, or more Identity headers. A
verification service performs the procedures above on each Identity verification service performs the procedures above on each Identity
header that appears in a request. If the verifier does not support header that appears in a request. If the verifier does not support
an Identity header present in a request due to the presence of an an Identity header present in a request due to the presence of an
unsupported "ppt" parameter, or if no Identity header is present, and unsupported "ppt" parameter, or if no Identity header is present, and
the presence of an Identity header is required by local policy (for the presence of an Identity header is required by local policy (for
example, based on a per-sending-domain policy, or a per-sending-user example, based on a per-sending-domain policy, or a per-sending-user
policy), then a 428 'Use Identity Header' response MUST be sent in policy), then a 428 'Use Identity Header' response MUST be sent in
the backwards direction. For more on this and other failure the backwards direction. For more on this and other failure
responses, see Section 12.3. responses, see Section 13.3.
In order to verify an Identity header in a message, an entity acting In order to verify an Identity header in a message, an entity acting
as a verifier MUST perform the following steps, in the order here as a verifier MUST perform the following steps, in the order here
specified. Note that when an Identity header contains the optional specified. Note that when an Identity header contains the optional
"canon" parameter, the verifier MUST follow the additional procedures "canon" parameter, the verifier MUST follow the additional procedures
in Section 5.2.1. in Section 5.2.1.
Step 1: Step 1:
The verifier MUST inspect any optional "ppt" parameter appearing the The verifier MUST inspect any optional "ppt" parameter appearing the
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present, and the verifier does not support it, it MUST ignore the present, and the verifier does not support it, it MUST ignore the
Identity header. If a supported "ppt" parameter value is present, Identity header. If a supported "ppt" parameter value is present,
the verifier follows the procedures below, including the variations the verifier follows the procedures below, including the variations
described in Step 5. described in Step 5.
Step 2: Step 2:
In order to determine whether the signature for the identity field In order to determine whether the signature for the identity field
should be over the entire identity field URI or just a canonicalized should be over the entire identity field URI or just a canonicalized
telephone number, the verification service MUST follow the telephone number, the verification service MUST follow the
canonicalization process described in Section 7.1.1. That section canonicalization process described in Section 7.2. That section also
also describes the procedures the verification service MUST follow to describes the procedures the verification service MUST follow to
determine if the signer is authoritative for a telephone number. For determine if the signer is authoritative for a telephone number. For
domains, the verifier MUST follow the process described in domains, the verifier MUST follow the process described in
Section 7.2 to determine if the signer is authoritative for the Section 7.3 to determine if the signer is authoritative for the
identity field. identity field.
Step 3: Step 3:
The verifier must first ensure that it possesses the proper keying The verifier must first ensure that it possesses the proper keying
material to validate the signature in the Identity header field, material to validate the signature in the Identity header field,
which usually involves dereferencing a URI in the "info" parameter of which usually involves dereferencing a URI in the "info" parameter of
the Identity header. See Section 6.2 for more information on these the Identity header. See Section 6.2 for more information on these
procedures. If the verifier does not support the credential procedures. If the verifier does not support the credential
described in the "info" parameter, then it should consider the described in the "info" parameter, then it should consider the
credential for this header unsupported. If a SIP request contains no credential for this header unsupported. If a SIP request contains no
Identity headers with a supported credential, then the verifier MUST Identity headers with a supported credential, then the verifier MUST
return a 437 "Unsupported Credential" response. return a 437 "Unsupported Credential" response.
Step 4: Step 4:
The verifier MUST furthermore ensure that the value of the Date The verifier MUST furthermore ensure that the value of the Date
header of the request meets local policy for freshness (usually, header of the request meets local policy for freshness (usually,
within sixty seconds) and that it falls within the validity period of within sixty seconds) and that it falls within the validity period of
the credential used to sign the Identity header. For more on the the credential used to sign the Identity header. For more on the
attacks this prevents, see Section 11.1. If the "canon" parameter is attacks this prevents, see Section 12.1. If the "canon" parameter is
present, the verifier should follow the Date-related behavior in present, the verifier should follow the Date-related behavior in
Section 5.2.1. Section 5.2.1.
Step 5: Step 5:
The verifier MUST validate the signature in the Identity header field The verifier MUST validate the signature in the Identity header field
over the PASSporT object. For baseline PASSporT objects (with no over the PASSporT object. For baseline PASSporT objects (with no
Identity header "ppt" parameter) the verifier MUST follow the Identity header "ppt" parameter) the verifier MUST follow the
procedures for generating the signature over a PASSporT object procedures for generating the signature over a PASSporT object
described in Section 8. If a "ppt" parameter is present (and per described in Section 8. If a "ppt" parameter is present (and per
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forwarding decisions in intermediary implementations, or changing how forwarding decisions in intermediary implementations, or changing how
the user is alerted, or how identity is rendered, in user agent the user is alerted, or how identity is rendered, in user agent
implementations. implementations.
5.2.1. Handling 'canon' parameters 5.2.1. Handling 'canon' parameters
If the optional "canon" parameter of the Identity header is present, If the optional "canon" parameter of the Identity header is present,
it contains a base64 encoding of the header and claim component of it contains a base64 encoding of the header and claim component of
the PASSporT object constructed by the authentication service, and the PASSporT object constructed by the authentication service, and
this it conveys any canonical telephone number formats created by the this it conveys any canonical telephone number formats created by the
authentication service (see Section 7.1.1), as well as an "iat" claim authentication service (see Section 7.2), as well as an "iat" claim
corresponding to the Date header that the authentication service corresponding to the Date header that the authentication service
used. The "canon" is provided purely as an optimization and used. The "canon" is provided purely as an optimization and
debugging mechanism for the verification service. debugging mechanism for the verification service.
When "canon" is present, the verification service MAY compute its own When "canon" is present, the verification service MAY compute its own
canonicalization of the numbers and compare them to the values in the canonicalization of the numbers and compare them to the values in the
"canon" parameter before performing any cryptographic functions in "canon" parameter before performing any cryptographic functions in
order to ascertain whether or not the two ends agree on the canonical order to ascertain whether or not the two ends agree on the canonical
number form. Also, when "canon" is present, during Step 4 the number form. Also, when "canon" is present, during Step 4 the
verification service SHOULD compare the "iat" value in the "canon" to verification service SHOULD compare the "iat" value in the "canon" to
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Identity-Info URI before a verifier evaluates a request signed by an Identity-Info URI before a verifier evaluates a request signed by an
authentication service, this would cause obvious verifier failures. authentication service, this would cause obvious verifier failures.
When a rollover occurs, authentication services SHOULD thus provide When a rollover occurs, authentication services SHOULD thus provide
new Identity-Info URIs for each new credential, and SHOULD continue new Identity-Info URIs for each new credential, and SHOULD continue
to make older key acquisition URIs available for a duration longer to make older key acquisition URIs available for a duration longer
than the plausible lifetime of a SIP transaction (a minute would most than the plausible lifetime of a SIP transaction (a minute would most
likely suffice). likely suffice).
7. Identity Types 7. Identity Types
7.1. Telephone Numbers This specification focuses primarily on cases where the called and
calling parties identified in the To and From header field values use
telephone numbers, as this remains the dominant use case in the
deployment of SIP. However, this specification also works with
"greenfield" identifiers (of the form "sip:user@host"), and
potentially other identifiers when SIP interworks with another
protocol.
Since many SIP applications provide a Voice over IP (VoIP) service, The guidance in this section also applies to extracting the URI
telephone numbers are commonly used as identities in SIP deployments. containing the originator's identity from the P-Asserted-Identity
In order for telephone numbers to be used with the mechanism header field value instead of the From header field value. In some
described in this document, authentication services must enroll with environments, the P-Asserted-Identity header field is used in lieu of
an authority that issues credentials for telephone numbers or the From header field to convey the address-of-record or telephone
telephone number ranges, and verification services must trust the number of the sender of a request; while it is not envisioned that
authority employed by the authentication service that signs a many of those networks would or should make use of the Identity
request. Enrollment procedures and credential management are outside mechanism described in this specification, where they do, local
the scope of this document. policy might therefore dictate that the canonical identity derive
from the P-Asserted-Identity header field rather than the From.
In the longer term, it is possible that some directory or other Ultimately, in any case where local policy canonicalizes the idenity
discovery mechanism may provide a way to determine which into a form different from how it appears in the From header field,
administrative domain is responsible for a telephone number, and this the use of the "canon" parameter by authentication services is
may aid in the signing and verification of SIP identities that RECOMMENDED, but because "canon" itself could then divulge
contain telephone numbers. This is a subject for future work. information about users or networks, implementers should be mindful
of the guidelines in Section 11.
In order to work with any such authorities, authentication and It may not be trivial to tell if a given URI contains a telephone
verification services must be able to identify when a request should number. In order to determine whether or not the user portion of a
be signed by an authority for a telephone number, and when it should SIP URI is a telephone number, authentication services and
be signed by an authority for a domain. Telephone numbers most verification services MUST perform the following procedure on any SIP
commonly appear in SIP header field values in the username portion of URI they inspect which contains a numeric user part. Note that the
a SIP URI (e.g., 'sip:+17005551008@chicago.example.com;user=phone'). same procedures are followed for creating the canonical form of URIs
The user part of that URI conforms to the syntax of the TEL URI found in the From header field as they are in the To header field or
scheme (RFC 3966 [RFC3966]). It is also possible for a TEL URI to the P-Asserted-Identity header field.
appear in the SIP To or From header field outside the context of a
SIP or SIPS URI (e.g., 'tel:+17005551008'). In both of these cases,
it's clear that the signer must have authority over the telephone
number, not the domain name of the SIP URI. It is also possible,
however, for requests to contain a URI like
'sip:7005551000@chicago.example.com'. It may be non-trivial for a
service to ascertain in this case whether the URI contains a
telephone number or not.
7.1.1. Canonicalization Procedures First, implementations must look for obvious indications that the
user-portion of the URI constitutes a telephone number. Telephone
numbers most commonly appear in SIP header field values in the
username portion of a SIP URI (e.g.,
'sip:+17005551008@chicago.example.com;user=phone'). The user part of
that URI conforms to the syntax of the TEL URI scheme (RFC 3966
[RFC3966]). It is also possible for a TEL URI to appear in the SIP
To or From header field outside the context of a SIP or SIPS URI
(e.g., 'tel:+17005551008'). Thus, in some environments, numbers will
be explicitly labeled by the use of TEL URIs or the 'user=phone'
parameter, or implicitly by the presence of the '+' indicator at the
start of the user-portion. Absent these indications, if there are
numbers present in the user-portion, implementations may also detect
that the user-portion of the URI contains a telephone number by
determining whether or not those numbers would be dialable or
routable in the local environment -- bearing in mind that the
telephone number may be a valid E.164 number, a nationally-specific
number, or even a private branch exchange number. Once a telephone
number has been detected, implementations should follow the
procedures in Section 7.2.
In order to determine whether or not the user portion of a SIP URI is If the URI field does not contain a telephone number, URI
a telephone number, authentication services and verification services normalization procedures are invoked to canonicalize the URI before
must perform the following canonicalization procedure on any SIP URI it is included in a PASSporT object in, for example, an "uri" claim.
they inspect which contains a wholly numeric user part. Note that See Section 7.4 for that behavior.
the same procedures are followed for creating the canonical form of
URIs found in both the From and To header field values; this section
also describes procedures for extracting the URI containing the
telephone number from the P-Asserted-Identity header field value for
environments where that is applicable.
In some networks, the P-Asserted-Identity header field value is used 7.1. Authority for Telephone Numbers
in lieu of the From header field to convey the telephone number of
the sender of a request; while it is not envisioned that most of
those networks would or should make use of the Identity mechanism
described in this specification, where they do, local policy might
therefore dictate that the canonical string derive from the P-
Asserted-Identity header field rather than the From. In any case
where local policy canonicalizes the number into a form different
from how it appears in the From header field, the use of the "canon"
parameter by authentication services is RECOMMENDED, but because
"canon" itself could then divulge information about users or
networks, implementers should be mindful of the guidelines in
Section 10.
First, implementations must assess if the user-portion of the URI In order for telephone numbers to be used with the mechanism
constitutes a telephone number. In some environments, numbers described in this document, authentication services must enroll with
will be explicitly labeled by the use of TEL URIs or the an authority that issues credentials authoritative for telephone
'user=phone' parameter, or implicitly by the presence of the '+' numbers or telephone number ranges, and verification services must
indicator at the start of the user-portion. Absent these trust the authority employed by the authentication service that signs
indications, if there are numbers present in the user-portion, a request. Per Section 6.4, enrollment procedures and credential
implementations may also detect that the user-portion of the URI management are outside the scope of this document; approaches to
contains a telephone number by determining whether or not those credential management for telephone numbers are discussed in
numbers would be dialable or routable in the local environment -- [I-D.ietf-stir-certificates].
bearing in mind that the telephone number may be a valid E.164
number, a nationally-specific number, or even a private branch
exchange number.
Once an implementation has identified a telephone number, it must 7.2. Telephone Number Canonicalization Procedures
construct a number string. Implementations MUST drop any leading
+'s, any internal dashes, parentheses or other non-numeric Once an implementation has identified a telephone number in the URI,
characters, excepting only the leading "#" or "*" keys used in it must construct a number string. That requires performing the
some special service numbers (typically, these will appear only in following steps:
the To header field value). This MUST result in an ASCII string
limited to "#", "*" and digits without whitespace or visual Implementations MUST drop any leading +'s, any internal dashes,
separators. parentheses or other non-numeric characters, excepting only the
leading "#" or "*" keys used in some special service numbers
(typically, these will appear only in the To header field value).
This MUST result in an ASCII string limited to "#", "*" and digits
without whitespace or visual separators.
Next, an implementation must assess if the number string is a Next, an implementation must assess if the number string is a
valid, globally-routable number with a leading country code. If valid, globally-routable number with a leading country code. If
not, implementations SHOULD convert the number into E.164 format, not, implementations SHOULD convert the number into E.164 format,
adding a country code if necessary; this may involve transforming adding a country code if necessary; this may involve transforming
the number from a dial string (see [RFC3966]), removing any the number from a dial string (see [RFC3966]), removing any
national or international dialing prefixes or performing similar national or international dialing prefixes or performing similar
procedures. It is only in the case that an implementation cannot procedures. It is only in the case that an implementation cannot
determine how to convert the number to a globally-routable format determine how to convert the number to a globally-routable format
that this step may be skipped. This will be the case, for that this step may be skipped. This will be the case, for
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If the result of this procedure forms a complete telephone number, If the result of this procedure forms a complete telephone number,
that number is used for the purpose of creating and signing the that number is used for the purpose of creating and signing the
signed-identity-string by both the authentication service and signed-identity-string by both the authentication service and
verification service. Practically, entities that perform the verification service. Practically, entities that perform the
authentication service role will sometimes alter the telephone authentication service role will sometimes alter the telephone
numbers that appear in the To and From header field values, numbers that appear in the To and From header field values,
converting them to this format (though note this is not a function converting them to this format (though note this is not a function
that [RFC3261] permits proxy servers to perform). The result of the that [RFC3261] permits proxy servers to perform). The result of the
canonicalization process of the From header field value may also be canonicalization process of the From header field value may also be
recorded through the use of the "canon" parameter of the Identity(see recorded through the use of the "canon" parameter of the Identity(see
Section 8). If the result of the canonicalization of the From header Section 8).
field value does not form a complete telephone number, the
authentication service and verification service should treat the
entire URI as a SIP URI, and apply a domain signature per the
procedures in Section 7.2.
7.2. Domain Names If the result of the canonicalization of the From header field value
does not form a complete and valid telephone number, the
authentication service and/or verification service SHOULD treat the
entire URI as a SIP URI, and apply the procedures in Section 7.4.
7.3. Authority for Domain Names
When a verifier processes a request containing an Identity-Info When a verifier processes a request containing an Identity-Info
header with a domain signature, it must compare the domain portion of header with a domain signature, it must compare the domain portion of
the URI in the From header field of the request with the domain name the URI in the From header field of the request with the domain name
that is the subject of the credential acquired from the "info" that is the subject of the credential acquired from the "info"
parameter. While it might seem that this should be a straightforward parameter. While it might seem that this should be a straightforward
process, it is complicated by two deployment realities. In the first process, it is complicated by two deployment realities. In the first
place, credentials have varying ways of describing their subjects, place, credentials have varying ways of describing their subjects,
and may indeed have multiple subjects, especially in 'virtual and may indeed have multiple subjects, especially in 'virtual
hosting' cases where multiple domains are managed by a single hosting' cases where multiple domains are managed by a single
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A verifier MUST evaluate the correspondence between the user's A verifier MUST evaluate the correspondence between the user's
identity and the signing credential by following the procedures identity and the signing credential by following the procedures
defined in RFC 2818 [RFC2818], Section 3.1. While RFC 2818 [RFC2818] defined in RFC 2818 [RFC2818], Section 3.1. While RFC 2818 [RFC2818]
deals with the use of HTTP in TLS and is specific to certificates, deals with the use of HTTP in TLS and is specific to certificates,
the procedures described are applicable to verifying identity if one the procedures described are applicable to verifying identity if one
substitutes the "hostname of the server" in HTTP for the domain substitutes the "hostname of the server" in HTTP for the domain
portion of the user's identity in the From header field of a SIP portion of the user's identity in the From header field of a SIP
request with an Identity header. request with an Identity header.
7.4. URI Normalization
Just as telephone numbers may undergo a number of syntactic
transformation during transit, the same can happen to SIP and SIPS
URIs without telephone numbers as they traverse certain
intermediaries. Therefore, when generating a PASSporT object based
on a SIP request, any SIP and SIPS URIs must be transformed into a
canonical form which captures the address-of-record represented by
the URI before they are provisioned in PASSporT claims such as "uri".
The URI normalization procedures required are as follows.
Following the ABNF of RFC3261, the SIP or SIPS URI in question MUST
discard all elements after the "hostport" of the URI, including all
uri-parameters and headers, from its ayntax. Of the userinfo
component of the SIP URI, only the user element will be retained: any
password (and any leading ":" before the password) MUST be removed,
and since this userinfo necessarily does not contain a telephone-
subscriber component, no further parameters can appear in the user
portion.
The hostport portion of the SIP or SIPS URI MUST similarly be
stripped of any trailing port along with the ":" that proceeds the
port, leaving only the host.
The ABNF of this canonical URI form (following the syntax defined in
RFC3261) is:
canon-uri = ( "sip" / "sips" ) ":" user "@" host
Finally, the URI will be subject to syntax-based URI normalization
procedures of [RFC3986] Section 6.2.2, especially to perform case
normalization and percent-encoding normalization. However, note that
normalization procedures face known challenges in some
internationalized environments (see [I-D.ietf-iri-comparison]) and
that perfect normalization of URIs may not be possible in those
environments.
For future PASSporT applications, it may be desirable to provide an
identifier without an attached protocol scheme. Future
specifications that define PASSporT claims for SIP as a using
protocol could use these basic procedures, but eliminate the scheme
component. A more exact definition is left to future specifications.
8. Header Syntax 8. Header Syntax
The Identity and Identity-Info headers that were previously defined The Identity and Identity-Info headers that were previously defined
in RFC4474 are deprecated. This revised specification collapses the in RFC4474 are deprecated. This revised specification collapses the
grammar of Identity-Info into the Identity header via the "info" grammar of Identity-Info into the Identity header via the "info"
parameter. Note that unlike the prior specification in RFC4474, the parameter. Note that unlike the prior specification in RFC4474, the
Identity header is now allowed to appear more than one time in a SIP Identity header is now allowed to appear more than one time in a SIP
request. The revised grammar for the Identity header is (following request. The revised grammar for the Identity header is (following
the ABNF [RFC4234] in RFC 3261 [RFC3261]): the ABNF [RFC4234] in RFC 3261 [RFC3261]):
skipping to change at page 22, line 21 skipping to change at page 23, line 9
For example: For example:
{ "typ":"passport", { "typ":"passport",
"alg":"ES256", "alg":"ES256",
"x5u":"https://www.example.com/cert.pkx" } "x5u":"https://www.example.com/cert.pkx" }
To populate the PASSporT claims JSON object from a SIP request, the To populate the PASSporT claims JSON object from a SIP request, the
following elements MUST be placed as values corresponding to the following elements MUST be placed as values corresponding to the
designated JSON keys: designated JSON keys:
First, if the originating identity is a telephone number, the JSON First, the JSON "orig" array MUST be populated. If the
key "otn" MUST be used, set to the value of the quoted originating originating identity is a telephone number, then the array MUST be
identity, a canonicalized telephone number (see Section 7.1.1). populated with a "tn" claim with a value set to the value of the
Otherwise, the JSON key "ouri" MUST be used, set to the value of quoted originating identity, a canonicalized telephone number (see
the AoR of the UA sending the message as taken from addr-spec of Section 7.2). Otherwise, the array MUST be populated with a "uri"
the From header field. claim, set to the value of the AoR of the UA sending the message
as taken from addr-spec of the From header field, per the
procedures in Section 7.4.
Second, if the destination identity is a telephone number, the Second, the JSON "dest" array MUST be populated. If the
JSON key "dtn" MUST be used, set to the value of the quoted destination identity is a telephone number, then the array MUST be
destination identity, a canonicalized telephone number (see populated with a "tn" claim with a value set to the value of the
Section 7.1.1). Otherwise, the JSON key "duri" MUST be used, set quoted destination identity, a canonicalized telephone number (see
to the value of the addr-spec component of the To header field, Section 7.2). Otherwise, the array MUST be populated with a "uri"
which is the AoR to which the request is being sent. claim, set to the value of the addr-spec component of the To
header field, which is the AoR to which the request is being sent,
per the procedures in Section 7.4.
Third, the JSON key "iat" MUST appear, set to the value of a Third, the JSON key "iat" MUST appear, set to the value of a
quoted encoding of the value of the SIP Date header field as a quoted encoding of the value of the SIP Date header field as a
JSON NumericDate (as UNIX time, per [RFC7519] Section 2). JSON NumericDate (as UNIX time, per [RFC7519] Section 2).
Fourth, if the request contains an SDP message body, and if that Fourth, if the request contains an SDP message body, and if that
SDP contains one or more "a=fingerprint" attributes, then the JSON SDP contains one or more "a=fingerprint" attributes, then the JSON
key "mky" MUST appear with the algorithm(s) and value(s) of the key "mky" MUST appear with the algorithm(s) and value(s) of the
fingerprint attributes (if they differ), following the format fingerprint attributes (if they differ), following the format
given in [I-D.ietf-stir-passport] Section 3.2.2.2. given in [I-D.ietf-stir-passport] Section 3.2.2.2.
For example: For example:
{ "otn":"12155551212", { "orig":{"tn":"12155551212"},
"dtn":"12155551213", "dest":{"tn":"12155551213"},
"iat":"1443208345" } "iat":"1443208345" }
For more information on the security properties of these SIP message For more information on the security properties of these SIP message
elements, and why their inclusion mitigates replay attacks, see elements, and why their inclusion mitigates replay attacks, see
Section 11 and [RFC3893]. Note that future extensions to the Section 12 and [RFC3893]. Note that future extensions to the
PASSporT object could introduce new claims, and that further SIP PASSporT object could introduce new claims, and that further SIP
procedures could be required to extract further information from the procedures could be required to extract further information from the
SIP request to populate the values of those claims; see Section 9. SIP request to populate the values of those claims; see Section 9.
The "orig" and "dest" arrays may contain identifiers of heterogeneous
type; for example, the "orig" array might contain a "tn" claim, while
the "dest" contains a "uri" claim. Also note that in some cases, the
"orig" and "dest" arrays might be populated with more than one value.
This could for example occur when multiple "dest" identities are
specified in a meshed conference. Defining how a SIP implementation
would provision multiple originating or destination identities is
left as a subject for future specification.
After these two JSON objects, the header and the claims, have been After these two JSON objects, the header and the claims, have been
constructed, they must each be hashed per [I-D.ietf-stir-passport] constructed, they must each be hashed per [I-D.ietf-stir-passport]
Section 3.3. The signed value of those concatenated hashes then Section 3.3. The signed value of those concatenated hashes then
becomes the signed-identity-string of the Identity header. The becomes the signed-identity-string of the Identity header. The
hashing and signing algorithm is specified by the 'alg' parameter of hashing and signing algorithm is specified by the 'alg' parameter of
the Identity header and the mirrored "alg" parameter of PASSporT. the Identity header and the mirrored "alg" parameter of PASSporT.
This specification inherits from the PASSporT specification one value This specification inherits from the PASSporT specification one value
for the 'alg' parameter: 'ES256', as defined in [RFC7519], which for the 'alg' parameter: 'ES256', as defined in [RFC7519], which
connotes an ECDSA P-256 digital signature. All implementations of connotes an ECDSA P-256 digital signature. All implementations of
this specification MUST support the required signing algorithms of this specification MUST support the required signing algorithms of
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extension may then trigger appropriate application-level behavior in extension may then trigger appropriate application-level behavior in
the presence of an extension; authors of extensions should provide the presence of an extension; authors of extensions should provide
appropriate extension-specific guidance to application developers on appropriate extension-specific guidance to application developers on
this point. this point.
If any claim in an extension contains a JSON value that does not If any claim in an extension contains a JSON value that does not
correspond to any field of the SIP request, but then the optional correspond to any field of the SIP request, but then the optional
"canon" parameter MUST be used for the Identity header containing "canon" parameter MUST be used for the Identity header containing
that extension. that extension.
10. Privacy Considerations 10. Backwards Compatibililty with RFC4474
This specification introduces several significant changes from the
RFC4474 version of the Identity header. However, due to the problems
enumerated in [I-D.rosenberg-sip-rfc4474-concerns], it is not
believed that the original Identity header has seen any deployment,
or even implementation in deployed products.
As such, this mechanism contains no provisions for signatures
generated with this specification to work with RFC4474-compliant
implementations, nor any related backwards-compatibility provisions.
Hypothetically, were an RFC4474-compliant implementation to receive
messages containing this revised version of the Identity header, it
would likely fail the request due to the absence of an Identity-Info
header with a 436 response code. Implementations of this
specification, for debugging purposes, might interpret a 436 with a
reason phrase of "Bad Identity-Info" as an indication that the
request has failed because it reached a (hypothetical)
RFC4474-compliant verification service.
11. Privacy Considerations
The purpose of this mechanism is to provide a strong identification The purpose of this mechanism is to provide a strong identification
of the originator of a SIP request, specifically a cryptographic of the originator of a SIP request, specifically a cryptographic
assurance that an authority asserts the originator can claim the URI assurance that an authority asserts the originator can claim the URI
given in the From header field. This URI may contain a variety of given in the From header field. This URI may contain a variety of
personally identifying information, including the name of a human personally identifying information, including the name of a human
being, their place of work or service provider, and possibly further being, their place of work or service provider, and possibly further
details. The intrinsic privacy risks associated with that URI are, details. The intrinsic privacy risks associated with that URI are,
however, no different from those of baseline SIP. Per the guidance however, no different from those of baseline SIP. Per the guidance
in [RFC6973], implementers should make users aware of the privacy in [RFC6973], implementers should make users aware of the privacy
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Identity with the Identity "canon" parameter in this fashion is NOT Identity with the Identity "canon" parameter in this fashion is NOT
RECOMMENDED outside of environments where SIP requests will never RECOMMENDED outside of environments where SIP requests will never
leave the trust domain. As a side note, history shows that closed leave the trust domain. As a side note, history shows that closed
networks never stay closed and one should design their implementation networks never stay closed and one should design their implementation
assuming connectivity to the broader Internet. assuming connectivity to the broader Internet.
Finally, note that unlike [RFC3325], the mechanism described in this Finally, note that unlike [RFC3325], the mechanism described in this
specification adds no information to SIP requests that has privacy specification adds no information to SIP requests that has privacy
implications. implications.
11. Security Considerations 12. Security Considerations
This document describes a mechanism that provides a signature over This document describes a mechanism that provides a signature over
the Date header field of SIP requests, parts of the To and From the Date header field of SIP requests, parts of the To and From
header fields, and when present any media keying material in the header fields, and when present any media keying material in the
message body. In general, the considerations related to the security message body. In general, the considerations related to the security
of these headers are the same as those given in [RFC3261] for of these headers are the same as those given in [RFC3261] for
including headers in tunneled 'message/sip' MIME bodies (see including headers in tunneled 'message/sip' MIME bodies (see
Section 23 of RFC3261 in particular). The following section details Section 23 of RFC3261 in particular). The following section details
the individual security properties obtained by including each of the individual security properties obtained by including each of
these header fields within the signature; collectively, this set of these header fields within the signature; collectively, this set of
header fields provides the necessary properties to prevent header fields provides the necessary properties to prevent
impersonation. It addresses the solution-specific attacks against impersonation. It addresses the solution-specific attacks against
in-band solutions enumerated in [RFC7375] Section 4.1. in-band solutions enumerated in [RFC7375] Section 4.1.
11.1. Protected Request Fields 12.1. Protected Request Fields
The From header field value (in ordinary operations) indicates the The From header field value (in ordinary operations) indicates the
identity of the sender of the message. The SIP address-of-record identity of the sender of the message. The SIP address-of-record
URI, or an embedded telephone number, in the From header field is the URI, or an embedded telephone number, in the From header field is the
identity of a SIP user, for the purposes of this document. Note that identity of a SIP user, for the purposes of this document. Note that
in some deployments the identity of the sender may reside in P- in some deployments the identity of the sender may reside in P-
Asserted-Id instead. The sender's identity is the key piece of Asserted-Id instead. The sender's identity is the key piece of
information that this mechanism secures; the remainder of the signed information that this mechanism secures; the remainder of the signed
parts of a SIP request are present to provide reference integrity and parts of a SIP request are present to provide reference integrity and
to prevent certain types of cut-and-paste attacks. to prevent certain types of cut-and-paste attacks.
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legitimate request for one user is cut-and-pasted into a request for legitimate request for one user is cut-and-pasted into a request for
a different user. Second, it preserves the starting URI scheme of a different user. Second, it preserves the starting URI scheme of
the request, which helps prevent downgrade attacks against the use of the request, which helps prevent downgrade attacks against the use of
SIPS. The To identity offers additional protection against cut-and- SIPS. The To identity offers additional protection against cut-and-
paste attacks beyond the Date header field. For example, without a paste attacks beyond the Date header field. For example, without a
signature over the To identity, an attacker who receives a call from signature over the To identity, an attacker who receives a call from
a target could immediately forward the INVITE to the target's a target could immediately forward the INVITE to the target's
voicemail service within the Date interval, and the voicemail service voicemail service within the Date interval, and the voicemail service
would have no way knowing that the Identity header it received had would have no way knowing that the Identity header it received had
been originally signed for a call intended for a different number. been originally signed for a call intended for a different number.
However, note the caveats below in Section 11.1.1. However, note the caveats below in Section 12.1.1.
When signing a request that contains a fingerprint of keying material When signing a request that contains a fingerprint of keying material
in SDP for DTLS-SRTP [RFC5763], this mechanism always provides a in SDP for DTLS-SRTP [RFC5763], this mechanism always provides a
signature over that fingerprint. This signature prevents certain signature over that fingerprint. This signature prevents certain
classes of impersonation attacks in which an attacker forwards or classes of impersonation attacks in which an attacker forwards or
cut-and-pastes a legitimate request. Although the target of the cut-and-pastes a legitimate request. Although the target of the
attack may accept the request, the attacker will be unable to attack may accept the request, the attacker will be unable to
exchange media with the target as they will not possess a key exchange media with the target as they will not possess a key
corresponding to the fingerprint. For example, there are some corresponding to the fingerprint. For example, there are some
baiting attacks, launched with the REFER method or through social baiting attacks, launched with the REFER method or through social
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prevented by securing a fingerprint for DTLS-SRTP. While this is a prevented by securing a fingerprint for DTLS-SRTP. While this is a
different form of impersonation than is commonly used for different form of impersonation than is commonly used for
robocalling, ultimately there is little purpose in establishing the robocalling, ultimately there is little purpose in establishing the
identity of the user that originated a SIP request if this assurance identity of the user that originated a SIP request if this assurance
is not coupled with a comparable assurance over the contents of the is not coupled with a comparable assurance over the contents of the
subsequent media communication. This signature also, per [RFC7258], subsequent media communication. This signature also, per [RFC7258],
reduces the potential for passive monitoring attacks against the SIP reduces the potential for passive monitoring attacks against the SIP
media. In environments where DTLS-SRTP is unsupported, however, no media. In environments where DTLS-SRTP is unsupported, however, no
field is signed and no protections are provided. field is signed and no protections are provided.
11.1.1. Protection of the To Header and Retargeting 12.1.1. Protection of the To Header and Retargeting
The mechanism in this document provides a signature over the identity The mechanism in this document provides a signature over the identity
information in the To header field value of requests. This provides information in the To header field value of requests. This provides
a means for verifiers to detect replay attacks where a signed request a means for verifiers to detect replay attacks where a signed request
originally sent to one target is modified and then forwarded by an originally sent to one target is modified and then forwarded by an
attacker to another, unrelated target. Armed with the original value attacker to another, unrelated target. Armed with the original value
of the To header field, the recipient of a request may compare it to of the To header field, the recipient of a request may compare it to
their own identity in order to determine whether or not the identity their own identity in order to determine whether or not the identity
information in this call might have been replayed. However, any information in this call might have been replayed. However, any
request may be legitimately retargeted as well, and as a result request may be legitimately retargeted as well, and as a result
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guidance is given for implementers here regarding the 'connected guidance is given for implementers here regarding the 'connected
party' problem (see [RFC4916]); authentication service behavior is party' problem (see [RFC4916]); authentication service behavior is
unchanged if retargeting has occurred for a dialog-forming request. unchanged if retargeting has occurred for a dialog-forming request.
Ultimately, the authentication service provides an Identity header Ultimately, the authentication service provides an Identity header
for requests in the backwards dialog when the user is authorized to for requests in the backwards dialog when the user is authorized to
assert the identity given in the From header field, and if they are assert the identity given in the From header field, and if they are
not, an Identity header is not provided. And per the threat model of not, an Identity header is not provided. And per the threat model of
[RFC7375], resolving problems with 'connected' identity has little [RFC7375], resolving problems with 'connected' identity has little
bearing on detecting robocalling or related impersonation attacks. bearing on detecting robocalling or related impersonation attacks.
11.2. Unprotected Request Fields 12.2. Unprotected Request Fields
RFC4474 originally had protections for the Contact, Call-ID and CSeq. RFC4474 originally had protections for the Contact, Call-ID and CSeq.
These are removed from RFC4474bis. The absence of these header These are removed from RFC4474bis. The absence of these header
values creates some opportunities for determined attackers to values creates some opportunities for determined attackers to
impersonate based on cut-and-paste attacks; however, the absence of impersonate based on cut-and-paste attacks; however, the absence of
these headers does not seem impactful to preventing the simple these headers does not seem impactful to preventing the simple
unauthorized claiming of an identity for the purposes of robocalling, unauthorized claiming of an identity for the purposes of robocalling,
voicemail hacking, or swatting, which is the primary scope of the voicemail hacking, or swatting, which is the primary scope of the
current document. current document.
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guarantee that no Via hops are inserted between the sending user guarantee that no Via hops are inserted between the sending user
agent and the authentication service, it could not prevent an agent and the authentication service, it could not prevent an
attacker from adding a Via hop after the authentication service, and attacker from adding a Via hop after the authentication service, and
thereby preempting responses. It is necessary for the proper thereby preempting responses. It is necessary for the proper
operation of SIP for subsequent intermediaries to be capable of operation of SIP for subsequent intermediaries to be capable of
inserting such Via header fields, and thus it cannot be prevented. inserting such Via header fields, and thus it cannot be prevented.
As such, though it is desirable, securing Via is not possible through As such, though it is desirable, securing Via is not possible through
the sort of identity mechanism described in this document; the best the sort of identity mechanism described in this document; the best
known practice for securing Via is the use of SIPS. known practice for securing Via is the use of SIPS.
11.3. Malicious Removal of Identity Headers 12.3. Malicious Removal of Identity Headers
In the end analysis, the Identity header cannot protect itself. Any In the end analysis, the Identity header cannot protect itself. Any
attacker could remove the header from a SIP request, and modify the attacker could remove the header from a SIP request, and modify the
request arbitrarily afterwards. However, this mechanism is not request arbitrarily afterwards. However, this mechanism is not
intended to protect requests from men-in-the-middle who interfere intended to protect requests from men-in-the-middle who interfere
with SIP messages; it is intended only to provide a way that the with SIP messages; it is intended only to provide a way that the
originators of SIP requests can prove that they are who they claim to originators of SIP requests can prove that they are who they claim to
be. At best, by stripping identity information from a request, a be. At best, by stripping identity information from a request, a
man-in-the-middle could make it impossible to distinguish any man-in-the-middle could make it impossible to distinguish any
illegitimate messages he would like to send from those messages sent illegitimate messages he would like to send from those messages sent
by an authorized user. However, it requires a considerably greater by an authorized user. However, it requires a considerably greater
amount of energy to mount such an attack than it does to mount amount of energy to mount such an attack than it does to mount
trivial impersonations by just copying someone else's From header trivial impersonations by just copying someone else's From header
field. This mechanism provides a way that an authorized user can field. This mechanism provides a way that an authorized user can
provide a definitive assurance of his identity that an unauthorized provide a definitive assurance of his identity that an unauthorized
user, an impersonator, cannot. user, an impersonator, cannot.
11.4. Securing the Connection to the Authentication Service 12.4. Securing the Connection to the Authentication Service
In the absence of user agent-based authentication services, the In the absence of user agent-based authentication services, the
assurance provided by this mechanism is strongest when a user agent assurance provided by this mechanism is strongest when a user agent
forms a direct connection, preferably one secured by TLS, to an forms a direct connection, preferably one secured by TLS, to an
intermediary-based authentication service. The reasons for this are intermediary-based authentication service. The reasons for this are
twofold: twofold:
If a user does not receive a certificate from the authentication If a user does not receive a certificate from the authentication
service over the TLS connection that corresponds to the expected service over the TLS connection that corresponds to the expected
domain (especially when the user receives a challenge via a domain (especially when the user receives a challenge via a
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constrain UAC behavior, and moreover there will be some deployment constrain UAC behavior, and moreover there will be some deployment
architectures where a direct connection is simply infeasible and the architectures where a direct connection is simply infeasible and the
UAC cannot act as an authentication service itself. Accordingly, UAC cannot act as an authentication service itself. Accordingly,
when a direct connection and TLS are not possible, a UAC should use when a direct connection and TLS are not possible, a UAC should use
the SIPS mechanism, Digest 'auth-int' for body integrity, or both the SIPS mechanism, Digest 'auth-int' for body integrity, or both
when it can. The ultimate decision to add an Identity header to a when it can. The ultimate decision to add an Identity header to a
request lies with the authentication service, of course; domain request lies with the authentication service, of course; domain
policy must identify those cases where the UAC's security association policy must identify those cases where the UAC's security association
with the authentication service is too weak. with the authentication service is too weak.
11.5. Authorization and Transitional Strategies 12.5. Authorization and Transitional Strategies
Ultimately, the worth of an assurance provided by an Identity header Ultimately, the worth of an assurance provided by an Identity header
is limited by the security practices of the authentication service is limited by the security practices of the authentication service
that issues the assurance. Relying on an Identity header generated that issues the assurance. Relying on an Identity header generated
by a remote administrative domain assumes that the issuing domain by a remote administrative domain assumes that the issuing domain
uses recommended administrative practices to authenticate its users. uses recommended administrative practices to authenticate its users.
However, it is possible that some authentication services will However, it is possible that some authentication services will
implement policies that effectively make users unaccountable (e.g., implement policies that effectively make users unaccountable (e.g.,
ones that accept unauthenticated registrations from arbitrary users). ones that accept unauthenticated registrations from arbitrary users).
The value of an Identity header from such authentication services is The value of an Identity header from such authentication services is
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Finally, it is worth noting that the presence or absence of the Finally, it is worth noting that the presence or absence of the
Identity headers cannot be the sole factor in making an authorization Identity headers cannot be the sole factor in making an authorization
decision. Permissions might be granted to a message on the basis of decision. Permissions might be granted to a message on the basis of
the specific verified Identity or really on any other aspect of a SIP the specific verified Identity or really on any other aspect of a SIP
request. Authorization policies are outside the scope of this request. Authorization policies are outside the scope of this
specification, but this specification advises any future specification, but this specification advises any future
authorization work not to assume that messages with valid Identity authorization work not to assume that messages with valid Identity
headers are always good. headers are always good.
11.6. Display-Names and Identity 12.6. Display-Names and Identity
As a matter of interface design, SIP user agents might render the As a matter of interface design, SIP user agents might render the
display-name portion of the From header field of a caller as the display-name portion of the From header field of a caller as the
identity of the caller; there is a significant precedent in email identity of the caller; there is a significant precedent in email
user interfaces for this practice. Securing the display-name user interfaces for this practice. Securing the display-name
component of the From header field value is outside the scope of this component of the From header field value is outside the scope of this
document, but may be the subject of future work, such as through the document, but may be the subject of future work, such as through the
"ppt" name mechanism. "ppt" name mechanism.
In the absence of signing the display-name, authentication services In the absence of signing the display-name, authentication services
might check and validate it, and compare it to a list of acceptable might check and validate it, and compare it to a list of acceptable
display-names that may be used by the sender; if the display-name display-names that may be used by the sender; if the display-name
does not meet policy constraints, the authentication service could does not meet policy constraints, the authentication service could
return a 403 response code. In this case, the reason phrase should return a 403 response code. In this case, the reason phrase should
indicate the nature of the problem; for example, "Inappropriate indicate the nature of the problem; for example, "Inappropriate
Display Name". However, the display-name is not always present, and Display Name". However, the display-name is not always present, and
in many environments the requisite operational procedures for in many environments the requisite operational procedures for
display-name validation may not exist, so no normative guidance is display-name validation may not exist, so no normative guidance is
given here. given here.
12. IANA Considerations 13. IANA Considerations
This document relies on the headers and response codes defined in RFC This document relies on the headers and response codes defined in RFC
4474. It also retains the requirements for the specification of new 4474. It also retains the requirements for the specification of new
algorithms or headers related to the mechanisms described in that algorithms or headers related to the mechanisms described in that
document. document.
12.1. Identity-Info Parameters 13.1. Identity-Info Parameters
The IANA has already created a registry for Identity-Info parameters. The IANA has already created a registry for Identity-Info parameters.
This specification defines a new value called "canon" as defined in This specification defines a new value called "canon" as defined in
Section 6.3. Note however that unlike in RFC4474, Identity-Info Section 6.3. Note however that unlike in RFC4474, Identity-Info
parameters now appear in the Identity header. parameters now appear in the Identity header.
12.2. Identity-Info Algorithm Parameter Values 13.2. Identity-Info Algorithm Parameter Values
The IANA has already created a registry for Identity-Info "alg" The IANA has already created a registry for Identity-Info "alg"
parameter values. Note that now, the "alg" parameter appears in the parameter values. Note that now, the "alg" parameter appears in the
Identity header rather than the deprecated Identity-Info header. Identity header rather than the deprecated Identity-Info header.
Since the algorithms for signing PASSporT objects are defined in Since the algorithms for signing PASSporT objects are defined in
PASSporT rather than in this specification, there is no longer a need PASSporT rather than in this specification, there is no longer a need
for an algorithm parameter registry for the Identity header. This for an algorithm parameter registry for the Identity header. This
registry is therefore deprecated. registry is therefore deprecated.
12.3. Response Codes defined in RFC4474 13.3. Response Codes defined in RFC4474
RFC4474 defined four response codes for failure conditions specific RFC4474 defined four response codes for failure conditions specific
to the Identity header and its original mechanism. These status to the Identity header and its original mechanism. These status
codes are retained in this specification, with some modifications. codes are retained in this specification, with some modifications.
The semantics of the 428 'Use Identity Header' response code are The semantics of the 428 'Use Identity Header' response code are
slightly altered by the potential presence of the "ppt" parameter. slightly altered by the potential presence of the "ppt" parameter.
Now, a 428 response MUST be sent when an Identity header is required, Now, a 428 response MUST be sent when an Identity header is required,
but no Identity header without a "ppt" parameter, or with a support but no Identity header without a "ppt" parameter, or with a supported
"ppt" value, has been received. In the case where one or more "ppt" value, has been received. In the case where one or more
Identity headers with unsupported "ppt" values have been received, Identity headers with unsupported "ppt" values have been received,
then a verification service SHOULD send a 428 with the reason phrase then a verification service SHOULD send a 428 with the reason phrase
"Use Supported PASSporT Format". Note however that this "Use Supported PASSporT Format". Note however that this
specification gives no guidance on how a verification service might specification gives no guidance on how a verification service might
decide to require an Identity header for a particular SIP request. decide to require an Identity header for a particular SIP request.
Such authorization policies are outside the scope of this Such authorization policies are outside the scope of this
specification. specification.
For 436 'Bad Identity-Info' response, the default reason phrase is For 436 'Bad Identity-Info' response, the default reason phrase is
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policy dictates that a broken signature in an Identity header is policy dictates that a broken signature in an Identity header is
grounds for rejecting a request. Note that in some cases, an grounds for rejecting a request. Note that in some cases, an
Identity header may be broken for other reasons than that an Identity header may be broken for other reasons than that an
originator is attempting to spoof an identity: for example, when a originator is attempting to spoof an identity: for example, when a
transit network alters the Date header of the request. Relying on transit network alters the Date header of the request. Relying on
the full PASSporT object presented through the "canon" parameter can the full PASSporT object presented through the "canon" parameter can
repair some of these conditions (see Section 5.2.1), so the repair some of these conditions (see Section 5.2.1), so the
recommended way to attempt to repair this failure is to retry the recommended way to attempt to repair this failure is to retry the
request with "canon". request with "canon".
13. Acknowledgments 14. Acknowledgments
The authors would like to thank Stephen Kent, Brian Rosen, Alex The authors would like to thank Stephen Kent, Brian Rosen, Alex
Bobotek, Paul Kyzviat, Jonathan Lennox, Richard Shockey, Martin Bobotek, Paul Kyzviat, Jonathan Lennox, Richard Shockey, Martin
Dolly, Andrew Allen, Hadriel Kaplan, Sanjay Mishra, Anton Baskov, Dolly, Andrew Allen, Hadriel Kaplan, Sanjay Mishra, Anton Baskov,
Pierce Gorman, David Schwartz, Eric Burger, Alan Ford, Philippe Pierce Gorman, David Schwartz, Eric Burger, Alan Ford, Philippe
Fouquart, Michael Hamer, Henning Schulzrinne, and Richard Barnes for Fouquart, Michael Hamer, Henning Schulzrinne, and Richard Barnes for
their comments. their comments.
14. Changes from RFC4474 15. Changes from RFC4474
The following are salient changes from the original RFC 4474: The following are salient changes from the original RFC 4474:
Generalized the credential mechanism; credential enrollment, Generalized the credential mechanism; credential enrollment,
acquisition and trust is now outside the scope of this document acquisition and trust is now outside the scope of this document
Reduced the scope of the Identity signature to remove CSeq, Call- Reduced the scope of the Identity signature to remove CSeq, Call-
ID, Contact, and the message body ID, Contact, and the message body
Removed the Identity-Info header and relocated its components into Removed the Identity-Info header and relocated its components into
parameters of the Identity header parameters of the Identity header
The Identity header can now appear multiple times in one request The Identity header can now appear multiple times in one request
Replaced previous signed-identity-digest format with PASSporT Replaced previous signed-identity-digest format with PASSporT
(signing algorithms now defined there) (signing algorithms now defined there)
Revised status code descriptions Revised status code descriptions
15. References 16. References
16.1. Normative References
15.1. Normative References
[I-D.ietf-stir-passport] [I-D.ietf-stir-passport]
Wendt, C. and J. Peterson, "Persona Assertion Token", Wendt, C. and J. Peterson, "Persona Assertion Token",
draft-ietf-stir-passport-02 (work in progress), May 2016. draft-ietf-stir-passport-03 (work in progress), June 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000, DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>. <http://www.rfc-editor.org/info/rfc2818>.
skipping to change at page 35, line 19 skipping to change at page 36, line 44
<http://www.rfc-editor.org/info/rfc3280>. <http://www.rfc-editor.org/info/rfc3280>.
[RFC3370] Housley, R., "Cryptographic Message Syntax (CMS) [RFC3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, DOI 10.17487/RFC3370, August 2002, Algorithms", RFC 3370, DOI 10.17487/RFC3370, August 2002,
<http://www.rfc-editor.org/info/rfc3370>. <http://www.rfc-editor.org/info/rfc3370>.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", [RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
RFC 3966, DOI 10.17487/RFC3966, December 2004, RFC 3966, DOI 10.17487/RFC3966, December 2004,
<http://www.rfc-editor.org/info/rfc3966>. <http://www.rfc-editor.org/info/rfc3966>.
[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,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>. <http://www.rfc-editor.org/info/rfc5280>.
[RFC6919] Barnes, R., Kent, S., and E. Rescorla, "Further Key Words [RFC6919] Barnes, R., Kent, S., and E. Rescorla, "Further Key Words
for Use in RFCs to Indicate Requirement Levels", RFC 6919, for Use in RFCs to Indicate Requirement Levels", RFC 6919,
DOI 10.17487/RFC6919, April 2013, DOI 10.17487/RFC6919, April 2013,
<http://www.rfc-editor.org/info/rfc6919>. <http://www.rfc-editor.org/info/rfc6919>.
15.2. Informative References 16.2. Informative References
[I-D.ietf-iri-comparison]
Masinter, L. and M. D&#258;&#378;rst, "Comparison,
Equivalence and Canonicalization of Internationalized
Resource Identifiers", draft-ietf-iri-comparison-02 (work
in progress), October 2012.
[I-D.ietf-stir-certificates] [I-D.ietf-stir-certificates]
Peterson, J., "Secure Telephone Identity Credentials: Peterson, J. and S. Turner, "Secure Telephone Identity
Certificates", draft-ietf-stir-certificates-03 (work in Credentials: Certificates", draft-ietf-stir-
progress), March 2016. certificates-06 (work in progress), July 2016.
[I-D.kaplan-stir-cider] [I-D.kaplan-stir-cider]
Kaplan, H., "A proposal for Caller Identity in a DNS-based Kaplan, H., "A proposal for Caller Identity in a DNS-based
Entrusted Registry (CIDER)", draft-kaplan-stir-cider-00 Entrusted Registry (CIDER)", draft-kaplan-stir-cider-00
(work in progress), July 2013. (work in progress), July 2013.
[I-D.peterson-sipping-retarget] [I-D.peterson-sipping-retarget]
Peterson, J., "Retargeting and Security in SIP: A Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements", draft-peterson-sipping- Framework and Requirements", draft-peterson-sipping-
retarget-00 (work in progress), February 2005. retarget-00 (work in progress), February 2005.
 End of changes. 53 change blocks. 
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