Thus the configuration parameters related to Elliptic-Curve forward secrecy are available when Postfix is linked with Open SSL ≥ 1.0.0 (provided EC support has not been disabled by the vendor, as in some versions of Red Hat Linux).
Elliptic curves used in cryptography are typically identified by a "name" that stands for a set of well-known parameter values, and it is these "names" (or associated ASN.1 object identifiers) that are used in the TLS protocol.
On the other hand, with TLS there are no specially designated prime field groups, so each server is free to select its own suitably-strong prime and generator.
The Postfix ≥ 2.2 SMTP server supports forward secrecy in its default configuration.
The server decrypts this with its private key, and uses it together with other data exchanged in the clear to generate the session key.
Later revisions to the TLS protocol introduced forward-secrecy cipher suites in which the client and server implement a key exchange protocol based on ephemeral secrets.
Sessions encrypted with one of these newer cipher suites are not compromised by future disclosure of long-term authentication keys.
The key-exchange algorithms used for forward secrecy require the TLS server to designate appropriate "parameters" consisting of a mathematical "group" and an element of that group called a "generator".
Early implementations of the SSL protocol do not provide forward secrecy (some provide it only with artificially-weakened "export" cipher suites, but we will ignore those here).
The client sends a random "pre-master secret" to the server encrypted with the server's RSA public key.