Temporary, non-certified, private keys, used with a
server_key_exchange message. The rules are as follows:
If the negotiated cipher_suite has the "exportable" property, and
short_rsa is not zero, send a server_key_exchange message with the
(public part of) the short_rsa key.
If the negotiated cipher_suite does not have the exportable
property, and long_rsa is not zero, send a server_key_exchange
message with the (public part of) the long_rsa key.
Otherwise, dont send any server_key_exchange message.