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==Diffie-Hellman in SSL/TLS==

There are three versions of Diffie-Hellman used in SSL/TLS.

* Anonymous Diffie-Hellman
* Fixed Diffie-Hellman
* Ephemeral Diffie-Hellman

'''Anonymous Diffie-Hellman''' uses Diffie-Hellman, but without authentication. Because the keys used in the exchange are not authenticated, the protocol is susceptible to Man-in-the-Middle attacks. Note: if you use this scheme, a call to <tt>SSL_get_peer_certificate</tt> will return <tt>NULL</tt> because you have selected an anonymous protocol. This is the only time <tt>SSL_get_peer_certificate</tt> is allowed to return <tt>NULL</tt> under normal circumstances.

You should not use Anonymous Diffie-Hellman. You can prohibit its use in your code by using <tt>"!ADH"</tt> in your call to <tt>SSL_set_cipher_list</tt>.

'''Fixed Diffie-Hellman''' embeds the server's public parameter in the certificate, and the CA then signs the certificate. That is, the certificate contains the Diffie-Hellman public-key parameters, and those parameters never change.

'''Ephemeral Diffie-Hellman''' uses temporary, public keys. Each instance or run of the protocol uses a different public key. The authenticity of the server's temporary key can be verified by checking the signature on the key. Because the public keys are temporary, a compromise of the server's long term signing key ''does not'' jeopardize the privacy of past sessions. This is known as ''Perfect Forward Secrecy (PFS)''.

You should always use Ephemeral Diffie-Hellman because it provides PFS. You can specify ephemeral methods by providing <tt>"kEECDH:kEDH"</tt> in your call to <tt>SSL_set_cipher_list</tt>.