Forward secrecy — PFS¶
Short definition¶
Keys compromised later should not reveal old messages protected by deleted ephemeral keys.
Concrete example¶
A long-term key is stolen today; that should not be enough to decrypt previously recorded traffic.
What it protects¶
Confidentiality of cryptographic history within the protocol model.
What it does not protect¶
Not already decrypted local databases, recipient screenshots or keys never erased.
Why it matters¶
Guarantees may differ between a phone and a linked desktop.
Example messenger behaviour¶
Threema Desktop 2.0 beta does not support PFS in the report.
Technical details
PFS depends on protocol, implementation, erasure and linked devices.