Subject: WG Review: Secure Telephone Identity Revisited (stir)

From: The IESG <[email-masked]>
Date: Fri, 17 Dec 2021 10:41:44 -0800

The Secure Telephone Identity Revisited (stir) WG in the Applications and
Real-Time Area of the IETF is undergoing rechartering. The IESG has not made
any determination yet. The following draft charter was submitted, and is
provided for informational purposes only. Please send your comments to the
IESG mailing list ([email-masked]) by 2021-12-27.

Secure Telephone Identity Revisited (stir)
-----------------------------------------------------------------------
Current status: Active WG

Chairs:
  Ben Campbell <[email-masked]>
  Russ Housley <[email-masked]>
  Robert Sparks <[email-masked]>

Assigned Area Director:
  Murray Kucherawy <[email-masked]>

Applications and Real-Time Area Directors:
  Murray Kucherawy <[email-masked]>
  Francesca Palombini <[email-masked]>

Mailing list:
  Address: [email-masked]
  To subscribe: https://www.ietf.org/mailman/listinfo/stir
  Archive: https://mailarchive.ietf.org/arch/browse/stir/

Group page: https://datatracker.ietf.org/group/stir/

Charter: https://datatracker.ietf.org/doc/charter-ietf-stir/

The STIR working group will specify Internet-based mechanisms that allow
verification of the calling party's authorization to use a particular
telephone number for an incoming call.  Since it has  become fairly easy to
present an incorrect source telephone number, a growing set of problems have
emerged over the last decade.  As with email, the claimed source identity of
a SIP request is not verified, permitting unauthorized use of the source
identity as part of deceptive and coercive activities, such as robocalling
(bulk unsolicited commercial communications), vishing (voicemail hacking, and
impersonating banks) and swatting (impersonating callers to emergency
services to stimulate unwarranted large scale law enforcement deployments). 
In addition, use of an incorrect source telephone number facilitates wire
fraud or can lead to a return call at premium rates.

SIP is one of the main VoIP technologies used by parties that want to present
an incorrect origin, in this context an origin telephone number. Several
previous efforts have tried to secure the origins of SIP communications,
including RFC 3325, RFC 4474, and the VIPR working group.  To date, however,
true validation of the source of SIP calls has not seen any appreciable
deployment.  Several factors contributed to this lack of success, including:
failure of the problem to be seen as critical at the time; lack of any
technical means of producing a proof of authorization to use telephone
numbers; misalignment of the mechanisms proposed by RFC 4474 with the complex
deployment environment that has emerged for SIP; lack of end-to-end SIP
session establishment; and inherent operational problems with a transitive
trust model.  To make deployment of this solution more likely, consideration
must be given to latency, real-time performance, computational overhead, and
administrative overhead for the legitimate call source and all verifiers.

As its priority mechanism work item, the working group will specify and
maintain a SIP header-based mechanism for verification that the originator of
a SIP session is authorized to use the claimed source telephone number, where
the session is established with SIP end to end.  This is called an in-band
mechanism. The mechanism will use a canonical telephone number representation
specified by the working group, including any mappings that  might be needed
between the SIP header fields and the canonical telephone  number
representation.  The working group will consider choices for protecting
identity information and credentials used.  This protection will likely be
based on a digital signature mechanism that covers a set of information in
the SIP header fields, and verification will employ a credential that
contains the public key that is associated with the one or more telephone
numbers.  Credentials used with this mechanism will be derived from existing
telephone number assignment and delegation models.  That is, when a telephone
number or range of telephone numbers is delegated to an entity, relevant
credentials will be generated (or modified) to reflect such delegation.  The
mechanism must allow a telephone number holder to further delegate and revoke
use of a telephone number without compromising the global delegation scheme.

In addition to its priority mechanism work item, the working group will work
on mechanisms for verification of the originator during session establishment
in an environment with one or more non-SIP hops, most likely requiring an
out-of-band authorization mechanism. It is important to note that while the
main focus of this working group is telephone numbers, the STIR working group
will not develop any mechanisms that require changes to circuit-switched
technologies. Moreover, the work of this group is limited to developing a
solution for telephone numbers. Expansion of the authorization mechanism to
identities using the user@domain or other name forms is out of scope.

The group will also consider extensions that leverage STIR to solve related
identity problems around telephone calls and other telephone-number based
communication, including call diversion and forwarding, rich identity
presentation for delivery to a called party, messaging that uses telephone
numbers, connected identity (mechanisms that identify the called party
reached to the calling party), and similar use cases related to fraud and
security.

The working group will coordinate with the Security Area on credential
management and signature mechanics.

The working group will coordinate with other working groups in the ART Area
regarding signaling through existing deployments.

The working group welcomes input from potential implementors or operators of
technologies developed by this working group.  For example, national
numbering authorities might consider acting as credential authorities for
telephone numbers within their purview.

Authentication and authorization of identity is closely linked to privacy,
and these security features sometimes come at the cost of privacy.  Anonymous
calls are already defined in SIP standards, and this working group will not
propose changes to these standards.  In order to support anonymity, the
working group will provide a solution in which the called party receives an
indication that the source telephone number is unavailable.  This working
group, to the extent feasible, will specify privacy-friendly mechanisms that
do not reveal any more information to user agents or third parties than a
call that does not make use of secure telephone identification mechanisms.

Milestones:

   - Submit PASSPorT Extension for rich call data for publication as
   Proposed Standard

   - Submit Assertion Values for a Resource Priority Header Claim in Support
   of Emergency Services Networks as Proposed Standard

   - Submit STIR Certificate Delegation as Proposed Standard

   - Submit Privacy analysis for Informational