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What is Post-compromise security?

Post-compromise security is the property that a conversation heals after a device is compromised, so that once the compromised device is removed and the group re-keys, the attacker who held its keys can no longer read new messages.

Updated

  • PCS
  • Future secrecy
After the next key update, a stolen key stops opening new messages.

Encryption

Post-compromise security, explained

Forward secrecy protects the past; post-compromise security protects the future. They are separate properties and a protocol can have one without the other. Post-compromise security matters because real compromises are usually discovered late — a laptop is found to have been stolen, a session token turns up somewhere it should not — and the question that follows is whether the conversation can be made safe again without abandoning it.

How Enclessa uses it

Enclessa gets post-compromise security from MLS, and makes it usable by treating each device as a separate group member with its own key. Revoking a device removes its leaf and re-keys the conversation within one catch-up pass, so it can decrypt nothing sent afterwards. That behaviour was verified in testing rather than assumed from the protocol.