"SIP Certificates", Certificate Management Service for The SIP, draft-ietf-sip-certs-11

SIP Certificates Draft-ietf-sip-certs-11 snapvoip.blogspot.com
This focuses on a SIP Credential service that allows Session Initiation Protocol (SIP) User Agents (UAs) to use a SIP event package to discover the certificates of other users.  The User agents will be able to reach a given Address-of-Record (AOR) to retrieve that AOR's certificate by subscribing to the Credential Service, which returns an authenticated response containing that certificate.  The Credential Service also allows users to store and retrieve their own certificates and private keys.

IETF