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SSL Labs: Added test for ephemeral DH parameters

December 23, 2010

Ephemeral Diffie-Hellman key exchange (EDH or DHE, depending on where you look) allows two parties with no prior knowledge of each other to establish a shared secret. In SSL, DHE is used together with some method of authentication (most commonly RSA) in the handshake phase. Ephemeral DH is valued because it provides perfect forward secrecy -- the session keys cannot be recovered if the authentication method is broken (e.g., someone retrieves the server's private key).

DH parameters are typically generated at runtime. Because of that they are not very visible and are often forgotten during the testing. As of 1.0.69, the SSL Labs online test will examine the DH parameters and warn if they are too weak. The scoring system has been modified to take into account the strength of the DH parameters when they are used for key exchange.

Thanks to Adrian Dimcev and Brian Smith for their help with the research of DH parameter evaluation.