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Security Fundamentals & Crypto · Intermediate

Perfect Forward Secrecy (PFS)

Property ensuring that compromise of long-term keys does not compromise past session keys.

Definition

PFS (forward secrecy) is achieved by deriving each session key from ephemeral key exchange values that are discarded after use. Even if an attacker records traffic and later steals the server's private key, earlier sessions remain protected. TLS 1.3 mandates forward secrecy by allowing only (EC)DHE key exchange.

Example

Configuring a web server to prefer ECDHE cipher suites enables forward secrecy for TLS 1.2 clients.

Exam tip

RSA key transport does not provide PFS; DHE and ECDHE do.

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