Ephemeral Diffie Hellman parameters, offer perfect forward secrecy.
\\
\hline
-DHE\_DSS & The DSS\footnote{DSS stands for Digital Signature Standard} algorithm is used to sign Ephemeral Diffie Hellman
-parameters which are sent to the peer.
+DHE\_DSS & The DSS algorithm is used to sign Ephemeral Diffie Hellman
+parameters which are sent to the peer. DSS stands for Digital Signature
+Standard.
\\
\hline
\end{tabular}
authentication\index{Certificate authentication!Client} of the client is
optional in \tls{}. A server may request a certificate from the client -- using the
\printfunc{gnutls_certificate_server_set_request}{gnutls\_certificate\_server\_set\_request}
-function. If a certificate is to be requested by the client, at the handshake
-procedure, the server will send an extra packet,
-than contains a list of acceptable certificate signers, and indicates the
-request of a certificate. The client may then send a certificate, signed
+function. If a certificate is to be requested from the client during the handshake,
+the server will send a certificate request message that contains
+a list of acceptable certificate signers. The client may then send a certificate, signed
by one of the server's acceptable signers. In \gnutls{} the server's acceptable
signers list is constructed using the trusted CA certificates in the
credentials structure.
The discussion below mainly refers to the \emph{TCP/IP} transport layer
but may be extended to other ones too.
-\section{The traditional way}
+\section{Separate ports}
Traditionally \ssl{} was used in application protocols by assigning
-a new port number for the secure services. That way two ports were assigned, one for the
+a new port number for the secure services. That way two separate
+ports were assigned, one for the
non secure sessions, and one for the secured ones. This has the benefit
that if a user requests a secure session then the client will try to
connect to the secure port and fail otherwise. The only possible attack
the available privileged ports, this approach was soon obsoleted.
-\section{A different approach}
+\section{Upward negotiation}
Other application protocols\footnote{See LDAP, IMAP etc.}
use a different approach to enable the secure layer.
They use something called the ``TLS upgrade'' method. This method