RFC 8588 PROPOSED STANDARD SIP · CALLER ID · SECURITY
PASSporT SHAKEN: Personal Assertion Token (PaSSporT) Extension for Signature-based Handling of Asserted information using toKENs (SHAKEN)
Two extra claims that let a phone carrier say how sure it is about a caller and where the call began.
RFC EDITOR
Wendt, C. and M. Barnes, "Personal Assertion Token (PaSSporT) Extension for Signature-based Handling of Asserted information using toKENs (SHAKEN)", RFC 8588, DOI 10.17487/RFC8588, May 2019, <https://www.rfc-editor.org/info/rfc8588>.
BIBTEX
@misc{rfc8588,
series = {Request for Comments},
number = 8588,
howpublished = {RFC 8588},
publisher = {RFC Editor},
doi = {10.17487/RFC8588},
url = {https://www.rfc-editor.org/info/rfc8588},
author = {C. Wendt and M. Barnes},
title = {{Personal Assertion Token (PaSSporT) Extension for Signature-based Handling of Asserted information using toKENs (SHAKEN)}},
pagetotal = 9,
year = 2019,
month = may,
} RFC text
Contents
Internet Engineering Task Force (IETF) C. Wendt
Request for Comments: 8588 Comcast
Category: Standards Track M. Barnes
ISSN: 2070-1721 iconectiv
May 2019
Personal Assertion Token (PaSSporT) Extension for Signature-based
Handling of Asserted information using toKENs (SHAKEN)
Abstract
This document extends the Personal Assertion Token (PASSporT), which
is a token object that conveys cryptographically signed information
about the participants involved in communications. The extension is
defined based on the "Signature-based Handling of Asserted
information using toKENs (SHAKEN)" specification by the ATIS/SIP
Forum IP-NNI Task Group. It provides both (1) a specific set of
levels of confidence in the correctness of the originating identity
of a call originated in a SIP-based telephone network as well as (2)
an identifier that allows the Service Provider (SP) to uniquely
identify the origin of the call within its network.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8588.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview of "shaken" PASSporT Extension . . . . . . . . . . 4
4. PASSporT "attest" Claim . . . . . . . . . . . . . . . . . . . 4
5. PASSporT "origid" Claim . . . . . . . . . . . . . . . . . . . 4
6. Example "shaken" PASSporT . . . . . . . . . . . . . . . . . . 5
7. Using "shaken" in SIP . . . . . . . . . . . . . . . . . . . . 5
8. Order of Claim Keys . . . . . . . . . . . . . . . . . . . . . 5
9. Security Considerations . . . . . . . . . . . . . . . . . . . 6
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
11.1. JSON Web Token claims . . . . . . . . . . . . . . . . . 7
11.2. PASSporT Types . . . . . . . . . . . . . . . . . . . . . 7
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
12.1. Normative References . . . . . . . . . . . . . . . . . . 7
12.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The Signature-based Handling of Asserted information using toKENs
(SHAKEN) [ATIS-1000074] specification defines a framework for using
Secure Telephone Identity Revisited (STIR) protocols including the
Personal Assertion Token (PASSporT) [RFC8225], SIP Authenticated
Identity Management [RFC8224], and the STIR certificate framework
[RFC8226] for implementing the cryptographic validation of an
authorized originator of telephone calls using SIP. Because the
current telephone network contains traffic originated from both VoIP
and TDM/SS7 (Time Division Multiplexing / Signaling System 7), there
are many scenarios that need to be accounted for where PASSporT
signatures may represent either direct or indirect call origination
scenarios. The SHAKEN [ATIS-1000074] specification defines levels of
attestation of the origination of the call as well as an origination
identifier that can help create a unique association between the
origin of a particular call to the point in the VoIP or TDM telephone
network the call came from to identify, for example, either a
customer or class of service that call represents. This document
specifies these values as claims to extend the base set of PASSporT
claims.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
In addition, the following terms are used in this document:
o Verified association: Typically defined as an authenticated
relationship between a customer and a device that initiated a call
on behalf of that customer, for example, a subscriber account with
a specific SIM card or set of SIP credentials.
o PASSporT: Defined in [RFC8225] is a JSON Web Token [RFC7519]
defined specifically for securing the identity of an initiator of
personal communication. This document defines a specific
extension to PASSporT.
3. Overview of "shaken" PASSporT Extension
The SHAKEN framework is designed to use PASSporT [RFC8225] as a
method of asserting the caller's telephone identity. In addition to
the PASSporT base claims, there are two additional claims that have
been defined for the needs of a service provider to signal
information beyond just the telephone identity. First, in order to
help bridge the transition of the state of the current telephone
network (which has calls with no authentication and non-SIP [RFC3261]
signaling not compatible with the use of PASSporT and Secure
Telephone Identity (STI) in general), there is an attestation claim.
This provides three levels of attestation: a full attestation when
the service provider can fully attest to the calling identity, a
partial attestation when the service provider originated a telephone
call but cannot fully attest to the calling identity, and a gateway
attestation, which is the lowest level of attestation and represents
the service provider receiving a call from a telephone gateway that
does not support PASSporT or STI.
The second claim is a unique origination identifier that should be
used by the service provider to identify different sources of
telephone calls to support a traceback mechanism that can be used for
enforcement and identification of a source of illegitimate calls.
The use of the compact form of PASSporT is not specified in this
document and is not specified for use in SHAKEN [ATIS-1000074].
The next two sections define these new claims.
4. PASSporT "attest" Claim
This indicator allows for both identifying the service provider that
is vouching for the call as well as clearly indicating what
information the service provider is attesting to. The "attest" claim
can be one of the following three values: 'A', 'B', or 'C'. These
values correspond to 'Full Attestation', 'Partial Attestation', and
'Gateway Attestation', respectively. See [ATIS-1000074] for the
definitions of these three levels of attestation.
5. PASSporT "origid" Claim
The purpose of the "origid" claim is described in [ATIS-1000074].
The value of "origid" claim is a Universally Unique Identifier (UUID)
as defined in [RFC4122]. Please refer to Section 10 for a discussion
of the privacy considerations around the use of this value.
6. Example "shaken" PASSporT
Protected Header
{
"alg":"ES256",
"typ":"passport",
"ppt":"shaken",
"x5u":"https://cert.example.org/passport.cer"
}
Payload
{
"attest":"A"
"dest":{"tn":["12155550131"]}
"iat":"1443208345",
"orig":{"tn":"12155550121"},
"origid":"123e4567-e89b-12d3-a456-426655440000"
}
7. Using "shaken" in SIP
The use of the "shaken" PASSporT type and the "attest" and "origid"
claims for SIP is formally defined in [ATIS-1000074] using the SIP
[RFC3261] Identity header field defined in [RFC8224].
8. Order of Claim Keys
The order of the claim keys MUST follow the rules of Section 9 of
[RFC8225]; the claim keys MUST appear in lexicographic order.
Therefore, the claim keys discussed in this document appear in the
PASSporT Payload in the following order:
o attest
o dest
o iat
o orig
o origid
9. Security Considerations
This document defines a new PASSporT [RFC8225] extension. The
considerations related to the security of the PASSporT object itself
are the same as those described in [RFC8225].
[RFC8224] defines how to compare the values of the "dest", "orig",
and "iat" claims against fields in a SIP message containing a
PASSporT as part of validating that request. The values of the new
"attest" and "origid" claims added by this extension are not used in
such a validation step. They are not compared to fields in the SIP
message. Instead, they simply carry additional information from the
signer to the consumer of the PASSporT. This new information shares
the same integrity protection and non-repudiation properties as the
base claims in the PASSporT.
10. Privacy Considerations
As detailed in Section 26 of [RFC3261], SIP messages inherently carry
identifying information of the caller and callee. The addition of
STIR cryptographically attests that the signing party vouches for the
information given about the callee, as is discussed in the Privacy
Considerations of [RFC8224].
SHAKEN [ATIS-1000074] furthermore adds an "origid" value to the STIR
PASSporT, which is an opaque unique identifier representing an
element on the path of a given SIP request. This identifier is
generated by an originating telephone service provider to identify
where within their network (e.g. a gateway or particular service
element) a call was initiated; "origid" can facilitate forensic
analysis of call origins when identifying and stopping bad actors
trying to spoof identities or make fraudulent calls.
The opacity of the "origid" claim value is intended to minimize
exposure of information about the origination of calls labeled with
an "origid" value. It is therefore RECOMMENDED that implementations
generate a unique "origid" value per call in such a way that only the
generator of the "origid" can determine when two "origid" values
represent the same or different elements. If deployed systems
instead use a common or related "origid" for service elements in
their network, the potential for discovering patterns through
correlation of those calls exists. This could allow a recipient of
calls to, for instance, learn that a set of callers are using a
particular service or coming through a common gateway. It is
expected that SHAKEN PASSporTs are shared only within an [RFC3324]
trust domain and will be stripped before calls exit that trust
domain, but this information still could be used by analytics on
intermediary and terminating systems to reveal information that could
include geographic location and even device-level information,
depending on how the "origid" is generated.
11. IANA Considerations
11.1. JSON Web Token claims
IANA has added two new claims to the "JSON Web Token Claims" registry
as defined in [RFC7519].
Claim Name: attest
Claim Description: Attestation level as defined in SHAKEN framework
Change Controller: IESG
Specification Document(s): RFC 8588
Claim Name: origid
Claim Description: Originating Identifier as defined in SHAKEN
framework
Change Controller: IESG
Specification Document(s): RFC 8588
11.2. PASSporT Types
IANA has added a new entry in the "Personal Assertion Token
(PASSporT) Extensions" registry for the type "shaken", which is
specified in this document.
12. References
12.1. Normative References
[ATIS-1000074]
ATIS/SIP Forum IP-NNI Task Group, "Signature-based
Handling of Asserted information using toKENs (SHAKEN)",
January 2017, <https://access.atis.org/apps/group_public/
download.php/32237/ATIS-1000074.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/info/rfc4122>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8224] Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 8224,
DOI 10.17487/RFC8224, February 2018,
<https://www.rfc-editor.org/info/rfc8224>.
[RFC8225] Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
<https://www.rfc-editor.org/info/rfc8225>.
[RFC8226] Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", RFC 8226,
DOI 10.17487/RFC8226, February 2018,
<https://www.rfc-editor.org/info/rfc8226>.
12.2. Informative References
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC3324] Watson, M., "Short Term Requirements for Network Asserted
Identity", RFC 3324, DOI 10.17487/RFC3324, November 2002,
<https://www.rfc-editor.org/info/rfc3324>.
Acknowledgements
The authors would like to thank those that helped review and
contribute to this document including specific contributions from Jon
Peterson, Russ Housley, Robert Sparks, and Andrew Jurczak. The
authors would like to acknowledge the work of the ATIS/SIP Forum
IP-NNI Task Force to develop the concepts behind this document.
Authors' Addresses
Chris Wendt
Comcast
One Comcast Center
Philadelphia, PA 19103
United States of America
Email: chris-ietf@chriswendt.net
Mary Barnes
iconectiv
Email: mary.ietf.barnes@gmail.com
AI ELI5
On this page
PROBLEMTelephone calls today come from both VoIP and older TDM/SS7 networks, so a signed caller identity needs to say how well the provider actually knows the caller, and where in its network the call started.
MECHANISMThe RFC defines a PASSporT extension named shaken with two claims: attest, with levels A, B or C, and origid, a UUID that identifies the call's origin point.
WHY IT MATTERSReceivers get signed information about attestation level and call origin, supporting traceback of illegitimate calls. It is a Proposed Standard.
§1SHAKEN and PASSporT
The SHAKEN framework from ATIS and the SIP Forum uses the STIR protocols to cryptographically validate the authorized originator of a call made with SIP. It builds on PASSporT (RFC 8225), SIP Authenticated Identity Management (RFC 8224) and the STIR certificate framework (RFC 8226).
Today's telephone network carries traffic from both VoIP and TDM/SS7, so a PASSporT signature may represent direct or indirect call origination. This document specifies SHAKEN's attestation levels and origination identifier as claims that extend the base PASSporT claims.
A PASSporT is a JSON Web Token (RFC 7519) made for securing the identity of the initiator of personal communication.
Phone calls come from old and new kinds of networks. This note adds two extra facts to the signed caller ID so everyone knows how far to trust it.
§3The shaken Extension
The extension adds two claims for a service provider to signal more than the telephone identity. The first, attest, helps bridge the move from today's network, which has calls with no authentication and non-SIP signaling that cannot use PASSporT.
The second is a unique origination identifier the provider should use to identify different sources of calls, supporting a traceback mechanism for enforcement and for finding the source of illegitimate calls.
The compact form of PASSporT is not specified here and is not specified for use in SHAKEN.
The first fact tells how sure the carrier is. The second helps investigators follow a bad call back to where it started.
§4The attest and origid Claims
The attest claim identifies the provider vouching for the call and what it is attesting to. Its value is one of three letters: A for Full Attestation, B for Partial Attestation and C for Gateway Attestation. The levels are defined in the SHAKEN specification.
In outline, full means the provider can fully attest to the calling identity. Partial means the provider originated the call but cannot fully attest to the identity. Gateway, the lowest level, means the provider received the call from a telephone gateway that does not support PASSporT or STI.
The origid claim is a Universally Unique Identifier (UUID) as defined in RFC 4122.
A means the carrier is very sure, B means it knows the call started with it but not who exactly, and C means it only passed the call along from an old phone link.
- AFull Attestation — The provider can fully attest to the calling identity.
- BPartial Attestation — The provider originated the call but cannot fully attest to the identity.
- CGateway Attestation — The lowest level: the call came from a gateway that does not support PASSporT or STI.
§6Example PASSporT and Use in SIP
The RFC gives an example with protected header fields alg of ES256, typ of passport, ppt of shaken and an x5u certificate URL. The payload carries attest, dest, iat, orig and origid.
Use in SIP is formally defined in the SHAKEN specification, using the SIP Identity header field from RFC 8224. The claim keys MUST appear in lexicographic order, following Section 9 of RFC 8225, so the payload order is attest, dest, iat, orig, origid.
The signed note has to list its facts in alphabetical order, like names on a class roll, so every computer reads it the same way.
Example shaken PASSporT
Protected Header
{
"alg":"ES256",
"typ":"passport",
"ppt":"shaken",
"x5u":"https://cert.example.org/passport.cer"
}
Payload
{
"attest":"A"
"dest":{"tn":["12155550131"]}
"iat":"1443208345",
"orig":{"tn":"12155550121"},
"origid":"123e4567-e89b-12d3-a456-426655440000"
}§9Security and Privacy
Security of the PASSporT object itself is as described in RFC 8225. RFC 8224 compares the dest, orig and iat claims against the SIP message. The new attest and origid claims are not part of that check. They carry extra information from signer to consumer, with the same integrity protection and non-repudiation as the base claims.
On privacy, origid is meant to be opaque, to limit what it reveals about where a call originated. Implementations are RECOMMENDED to generate a unique value per call so only the generator can tell whether two values name the same element. A shared or related origid could let recipients correlate calls and learn, for instance, that callers use a common gateway. SHAKEN PASSporTs are expected to be shared only within an RFC 3324 trust domain and stripped on exit.
The secret origin code should be different every time. If it were always the same, someone could notice the pattern and learn things about the caller.
§6References
[RFC8225]RFC 8225Wendt, C. and J. Peterson, "PASSporT: Personal Assertion Token", RFC 8225, February 2018.February 2018[RFC8224]RFC 8224Peterson, J., Jennings, C., Rescorla, E., and C. Wendt, "Authenticated Identity Management in the Session Initiation Protocol (SIP)", RFC 8224, February 2018.February 2018[RFC8226]RFC 8226Peterson, J. and S. Turner, "Secure Telephone Identity Credentials: Certificates", RFC 8226, February 2018.February 2018[RFC7519]RFC 7519Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, May 2015.May 2015[RFC4122]RFC 4122Leach, P., Mealling, M., and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, July 2005.July 2005
[ATIS-1000074]ATIS/SIP Forum IP-NNI Task Group, "Signature-based Handling of Asserted information using toKENs (SHAKEN)", January 2017.