@node How to use TLS in application protocols
@section How to use @acronym{TLS} in application protocols

This chapter is intended to provide some hints on how to use
@acronym{TLS} over simple custom made application protocols.  The
discussion below mainly refers to the @acronym{TCP/IP} transport layer
but may be extended to other ones too.

@menu
* Separate ports::
* Upward negotiation::
@end menu

@node Separate ports
@subsection Separate ports

Traditionally @acronym{SSL} was used in application protocols by
assigning a new port number for the secure services. By doing this two
separate ports were assigned, one for the non-secure sessions, and one
for the secure sessions. This method ensures that if a user requests a
secure session then the client will attempt to connect to the secure port
and fail otherwise. The only possible attack with this method is to perform
a denial of service attack. The most famous example of this method is
``HTTP over TLS'' or @acronym{HTTPS} protocol @xcite{RFC2818}.

Despite its wide use, this method has several issues. This
approach starts the @acronym{TLS} Handshake procedure just after the
client connects on the ---so called--- secure port.  That way the
@acronym{TLS} protocol does not know anything about the client, and
popular methods like the host advertising in HTTP do not
work@footnote{See also the Server Name Indication extension on
@ref{serverind}.}.  There is no way for the client to say ``I
connected to YYY server'' before the Handshake starts, so the server
cannot possibly know which certificate to use.

Other than that it requires two separate ports to run a single
service, which is unnecessary complication. Due to the fact that there
is a limitation on the available privileged ports, this approach was
soon deprecated in favor of upward negotiation.

@node Upward negotiation
@subsection Upward negotiation

Other application protocols@footnote{See LDAP, IMAP etc.}  use a
different approach to enable the secure layer.  They use something
often called as the ``TLS upgrade'' method. This method is quite tricky but it
is more flexible. The idea is to extend the application protocol to
have a ``STARTTLS'' request, whose purpose it to start the TLS
protocols just after the client requests it.  This approach
does not require any extra port to be reserved.
There is even an extension to HTTP protocol to support 
this method @xcite{RFC2817}.

The tricky part, in this method, is that the ``STARTTLS'' request is
sent in the clear, thus is vulnerable to modifications.  A typical
attack is to modify the messages in a way that the client is fooled
and thinks that the server does not have the ``STARTTLS'' capability.
See a typical conversation of a hypothetical protocol:

@quotation
(client connects to the server)

CLIENT: HELLO I'M MR. XXX

SERVER: NICE TO MEET YOU XXX

CLIENT: PLEASE START TLS

SERVER: OK

*** TLS STARTS

CLIENT: HERE ARE SOME CONFIDENTIAL DATA
@end quotation

And an example of a conversation where someone is acting
in between:

@quotation
(client connects to the server)

CLIENT: HELLO I'M MR. XXX

SERVER: NICE TO MEET YOU XXX

CLIENT: PLEASE START TLS

(here someone inserts this message)

SERVER: SORRY I DON'T HAVE THIS CAPABILITY

CLIENT: HERE ARE SOME CONFIDENTIAL DATA
@end quotation

As you can see above the client was fooled, and was na@"ive enough to
send the confidential data in the clear, despite the server telling the
client that it does not support ``STARTTLS''.

How do we avoid the above attack? As you may have already noticed this
situation is easy to avoid.  The client has to ask the user before it
connects whether the user requests @acronym{TLS} or not.  If the user
answered that he certainly wants the secure layer the last
conversation should be:

@quotation
(client connects to the server)

CLIENT: HELLO I'M MR. XXX

SERVER: NICE TO MEET YOU XXX

CLIENT: PLEASE START TLS

(here someone inserts this message)

SERVER: SORRY I DON'T HAVE THIS CAPABILITY

CLIENT: BYE

(the client notifies the user that the secure connection was not possible)
@end quotation

This method, if implemented properly, is far better than the
traditional method, and the security properties remain the same, since
only denial of service is possible. The benefit is that the server may
request additional data before the @acronym{TLS} Handshake protocol
starts, in order to send the correct certificate, use the correct
password file, or anything else!