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In Ephemeral Diffie-Hellman, the key pair is generated every time.

In practice, does this mean I never need a certificate? Or that means I need a new cert every single time?

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You do need a certificate, because the Diffie-Hellman parameters have to be signed. Diffie-Hellman only makes sense if the client can rely on the authenticity of the parameters (meaning: they actually come from the server). Otherwise a man-in-the-middle attack would be trivial.

You just need a standard RSA (or DSA) certificate. In fact, the only difference between DHE-based cipher suites and “classical” RSA suites is how the certificate is used: With DHE, the certificate is only used for signatures, not for encryption.

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DH only takes care of the key-exchange. In practice you will need some form of authentication, eg. RSA or DSA for TLS. IPSec also allows pre-shared keys. Without authentication the communication will always be susceptible to man-in-the-middle attacks.

  • TLS also has an option for DHE authenticated by PSK, if both parties implement it, but that's usually harder to manage. Technically TLS also has an option for DHE with NO authentication, easily missed because it is called "anonymous" instead of "ephemeral". This is subject to active attack as you say, and frequently left unimplemented or prohibited by policy for that reason. – dave_thompson_085 Jan 26 '15 at 8:46

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