<b>Does forward secrecy protect data you already sent before a key compromise?</b>

<b>Does forward secrecy protect data you already sent before a key compromise?</b>

The phrase invites a misreading: "forward secrecy means past traffic is safe even if my key leaks — so I'm covered retroactively." The protection is real but its direction is precise, and getting it backwards leads to false comfort. Forward secrecy (PFS), delivered in TLS 1.3 by mandatory ephemeral Diffie-Hellman (ECDHE), means each session derives its symmetric keys from a temporary key pair that is discarded after the handshake. Compromising the server's <i>long-term private key</i> later does not let an attacker decrypt previously recorded sessions, because that long-term key never encrypted the traffic — it only authenticated the handshake.

The boundary people miss: PFS protects past sessions against a <i>future</i> compromise of the long-term key. It does <i>not</i> protect a session whose ephemeral key is captured in memory <i>during</i> its lifetime, nor does it help if the same ephemeral key is reused across sessions (an implementation bug that voids the property). "Harvest now, decrypt later" against TLS 1.3 is also why ephemerality, not certificate strength, is the relevant defense.

— PFS uses per-session ephemeral keys
— It defends past sessions from future long-term-key leaks
— Reused ephemeral keys silently break it

Further reading: RFC 8446, §2 and §7.1.
Bottom line: Forward secrecy protects already-sent data against a later key compromise — provided ephemeral keys are truly per-session and discarded.

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