Glossary · Encryption
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
Also called
PCS · Future secrecy
What Post-compromise security means
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.
The mechanism is fresh entropy. When a member performs an update, it contributes new random material to the group key, and the attacker who holds only the old state cannot derive the new one. This is why post-compromise security requires the compromised party to eventually be excluded or to update; it is not automatic and it is not instantaneous.
The practical requirement this creates is device management. If a compromise cannot be attributed to a specific device, it cannot be excluded, and the healing property never fires. This is why protocols with post-compromise security treat the device rather than the account as the unit of membership.
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 during phase 2 rather than assumed from the protocol.
Where Post-compromise security is specified
Related terms
Read further
The security page explains how Enclessa protects data and which certifications it does not hold. The trust centre covers the processing agreement, the subprocessors and the residency position. The FAQ answers the questions buyers ask most often.
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