X.509 Extensions for PQC or Composite Certificate Hosting Continuity

This document specifies a new X.509 certificate extension, Post- Quantum or Composite Hosting Continuity (PQCHC), which enables a certificate subject to signal a intent to continue serving PQC or composite certificates for a defined continuity period after certificate expiration. This extension helps relying parties detect downgrade and man-in-the-middle (MitM) attacks during transition phases, where a cryptographically relevant quantum computer (CRQC) would make traditional certificates insecure.