Post-compromise security

Post-compromise security (PCS) is the property of a cryptographic system that enables it to recover security guarantees after an active compromise has ended. Where forward secrecy protects past communications from future key compromise, post-compromise security protects future communications from past endpoint compromise. It is the temporal mirror image of forward secrecy: instead of preventing the future from reaching backward, it prevents the past from reaching forward.\n\nPCS is achieved through continuous key evolution mechanisms — most notably the Double Ratchet Algorithm in the Signal Protocol — that replace compromised key material with fresh material derived through one-way functions. Once both parties have completed a key update that the attacker did not observe, the attacker is locked out. The system has healed.\n\nThe concept challenges the traditional threat model of cryptography, which treats compromise as a binary state: either secure or broken. Post-compromise security treats compromise as a recoverable condition, like an infection that the immune system eventually clears. This reframing has profound implications: it means that perfect endpoint security is not a prerequisite for continued communication security, provided the protocol can out-evolve the compromise faster than the attacker can exploit it.\n\n\n