Certificate Update in TLS 1.3

This document defines a mechanism that enables TLS 1.3 endpoints to update their certificates during the lifetime of a connection using Exported Authenticators. A new extension is introduced to negotiate support for certificate update at handshake time. When negotiated, either endpoint can provide a post-handshake authenticator containing an updated certificate, delivered via a new handshake message. This mechanism allows long-lived TLS connections to remain valid across certificate rotations without requiring session termination.