## Background and Problem Statement

The Secure Telephone Identity Revisited (STIR) Working Group specifies Internet-based mechanisms that allow verification of the authorization to use a particular telephone number through cryptographic authentication. These mechanisms are widely deployed in SIP-based telephony networks and are used to mitigate impersonation, spoofing, and other forms of unwanted or fraudulent communications.

Operational experience has revealed that while STIR effectively authenticates the event of a call and the authorization to use a telephone number, it does not provide a means to identify the entity behind that authorization. Telephone numbers are assigned to entities that may use them through multiple services or providers acting on their behalf, and verifiers have no standardized way to determine which entity a delegate certificate represents. Additionally, existing mechanisms do not provide transparency into credential issuance. This rechartering extends the Working Group's scope to address these gaps while preserving compatibility with existing deployments and the STIR trust model.

## Scope of Work

The STIR Working Group will continue to specify and maintain the baseline protocols for cryptographic authentication of telephone calling party information using PASSporT and related mechanisms.

The Working Group will define a means to associate a PASSporT with an identifier representing the entity holding the right to use an assigned telephone number. 

The Working Group will define certificate transparency mechanisms for telephone identity credentials that improve auditability and support detection of mis-issuance within the STIR ecosystem.

The Working Group will extend existing out-of-band mechanisms to support discovery of STIR credentials and services associated with telephone numbers, including the application of connected identity in scenarios where SIP signaling is not end-to-end. The Working Group will consider extensions that support the use of authenticated telephone identity in other communications contexts, including messaging.

The Working Group will continue to leverage existing Internet security mechanisms, including PKI, and will coordinate with other IETF working groups as appropriate.

## Out of Scope

The following items are out of scope for the STIR Working Group:

- Definition of jurisdictional or legal policy topics, including vetting policies, regulatory requirements, governance, or authorization frameworks.
- Definition of governance requirements for numbering authorities, certificate authorities, or transparency services.
- Revisiting the trust model for the core first-party assertion that binds a telephone number to a call, the assurance that the calling number is not spoofed. This assertion relies on the existing certificate-based, administratively-governed model, regardless of the certificate profile used, and alternative trust anchors or non-certificate identifier mechanisms for this core binding are out of scope. Changing this dependence on the certificate-based approach would require a future recharter. This does not constrain separate signers of auxiliary or supplementary information associated with a call, provided such information remains bound to and subordinate to a first-party assertion within the certificate-based trust model.
- Any mechanism requiring changes to circuit-switched telephony technologies.