README.SSL

SSL (TLS) support for jTDS
==========================

  Author: Rob Worsnop
  Version: $Id: README.SSL,v 1.1 2005-01-13 17:26:50 alin_sinpalean Exp $

The challenge
=============

At this point it should be noted that the solution has been tested against SQL
Server 2000 (SQL2K), and nothing else. Everything in this section applies to SQL
Server 2000. Having said that, the changes made to support SSL will not affect
the behavior of jTDS with other products when using plain sockets.

  Requesting encryption
  ---------------------

  SQL2K allows clients to discover whether or not SSL is supported before
  starting an SSL session. This is done via a special TDS message. SQL2K's
  response depends on whether or not a digital certificate is installed. The
  request for encryption is also a notification of encryption. In other words,
  once a client has requested encryption, and it is supported, SQL2K then
  expects an SSL handshake.

  Header confusion
  ----------------

  To my mind, the most logical approach for SSL in SQL2K would be for the server
  to switch from expecting TDS packets to expecting SSL or TLS records. The
  application data SSL records would then contain TDS packets, whereas the
  handshake records would just be pure SSL. This would make adding SSL trivially
  easy ­ JSSE would do all of this for us.

  Unfortunately this is not how it works. Instead, the handshake records are
  enclosed in TDS packets. Then, after the handshake, things get reversed: the
  TDS packets are delivered inside SSL records (as encrypted application data).

  Handshake batching fussiness
  ----------------------------

  After sending the Client Hello message, and receiving a response from the
  server, the client then sends two or three (depending on whether or not an
  existing SSL session if being resumed) more records. These are Client Key
  Exchange (new sessions only), Change Cipher Spec, and Finished.

  SQL2K will not tolerate these records being sent separately: they must all be
  delivered in the same TDS packet.

  JSSE transmits these records separately; hence the challenge.

The requirements
================

o If SSL is configured (see further down for more details on this), then jTDS
  must send an encryption request, and handle the response appropriately.
o All SSL handshake records written by JSSE must be intercepted and enclosed in
  TDS packets before transmission. Application data records must be passed on
  unmodified.
o Client Key Exchange and Change Cipher Spec messages written by JSSE must be
  intercepted and deferred until a Finished message is written. Then all three
  (or two if resumed session) messages are transmitted in a TDS packet.
o All traffic returned from SQL2K must be inspected. When a TDS header is
  discovered, it must be stripped off before passing on the data up to JSSE.

The approach
============

The JSSE API allows SSL to be tunneled through an existing socket. The existing
socket specified becomes a delegate.

So, if SSL is switched on, we create a plain socket, send the encryption request
on it, and then specify that socket as the delegate for JSSE. Well, almost.
Actually, after the encryption request, the plain socket is wrapped in a socket
implementation of our own, and this wrapper is actually what JSSE sees. By
overriding Socket.getOutputStream and Socket.getInputStream, and providing
custom implementations of those interfaces, we are able to intercept all traffic
being sent by JSSE and SQL2K. It is, therefore, in the custom I/O streams that
we are able to provide the mediation between JSSE and SQL2K.

The input stream is relatively simple. Traffic from SQL2K is inspected and it is
determined whether or not a TDS packet or an SSL record has been sent. If a TDS
packet is detected then its header is stripped off. To ensure that we know where
the packet/record boundaries are, the stream will, using the length attribute,
read in the entire packet/record and store it in a buffer. JSSE is then fed
bytes from this buffer until it is exhausted.

The output stream is more complex. Although it will always receive SSL or TLS
records from JSSE, its required action depends on the contents of those records.
It is therefore necessary to parse the records to a greater extent than is done
by the input stream. This is true particularly of the handshake records, which
have a sub-type descriptor as part of the record body (which has its own
header). To reflect this requirement, there is a fairly elaborate set of
classes, one for each record type. Some of these classes add little value, and
are basically identical to others. But constructing them adds virtually no
overhead and their existence makes the code more readable and debugging a lot
simpler ­ seeing in the debugger, for example, that a TLSChgCipherSpecRecord has
just been written is certainly helpful. They are justifiable also for reasons of
symmetry: it is necessary, for example, to have a class that parses a handshake
record, so why not have classes for other record types, too?

How to use it
=============

An optional property, "ssl", has been added to the URL. Its legal values are:

  o off ­ SSL is not request or used. This is the default.
  o request ­ SSL is requested; if the server does not support it then a plain
    connection is used.
  o require ­ SSL is requested; if the server does not support it then an
    exception is thrown.
  o authenticate ­ Same as require except the server's certificate must be
    signed by a trusted CA.

Additional libraries
====================

If JDK 1.3 or below is used, the optional JSSE 1.0.3 will be required. JSSE has
been integrated into JDK 1.4.

Future work
===========

Some of the classes in the net.sourceforge.jtds.ssl package contain code that
deals with TDS packets. Much of this code duplicates functionality that already
exists in jTDS. But, without refactoring other parts of jTDS, it was not
possible to make use of it. If any refactoring is to be done, it probably ought
to be done as a separate task, by jTDS developers.