Forward Secrecy Explained: How Past Sessions Stay Safe
What perfect forward secrecy is, how ephemeral Diffie-Hellman key exchange enables it, and why it's enabled by default in TLS 1.3.
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Tags: security, cryptography, tls
Perfect Forward Secrecy: Protecting Past Sessions Perfect forward secrecy (PFS) is a property of cryptographic key exchange protocols that ensures compromise of a server's long-term private key does not allow decryption of past recorded sessions. Without it, a single key compromise could expose years of archived traffic. With it, the damage is limited to the current session at most. TLS 1.3 mandates perfect forward secrecy for this reason. The Problem Without Forward Secrecy In classic RSA key exchange (TLS 1.2 without ECDHE): An adversary who records this session has: The encrypted (encrypted with the server's public RSA key) All the encrypted traffic Years later, if the server's private RSA key is exposed (from a breach, a subpoena, a legal order, or an insider), the adversary can:…
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