Internet Engineering Task Force (IETF) C. Wendt

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.

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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
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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

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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.

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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.

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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

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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

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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>.

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[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>.

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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: [email protected]

Mary Barnes
iconectiv

Email: [email protected]

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