VESPER Out-of-Band OOB

This document describes a mechanism for delivering authenticated telephone call identity information using the VESPER framework in environments where SIP signaling is unavailable or unsuitable. By supporting an out-of-band (OOB) transport model, this approach enables entities to publish and retrieve signed PASSporT assertions independent of end-to-end delivery within SIP-based VoIP networks. These PASSporTs are signed with delegate certificates that were authorized for issuance by corresponding authority tokens, which represent the trust and validation of telephone number control and related claim information. Transparency features ensure that these authorizations are publicly auditable and cryptographically provable, supporting a higher standard of trust. This document also introduces support for Connected Identity to the STIR OOB model, enabling the called party to respond with a signed PASSporT asserting its identity, thereby binding the identities of both parties to the transaction and enhancing end-to-end accountability. The OOB mechanism serves as an alternative delivery path for PASSporTs in cases where end-to-end in-band SIP delivery is not possible, enabling verifiers to confirm the association between the originating telephone number and the identity asserting authority as part of the broader VESPER trust framework.