From: Bill Frantz <frantz@netcom.com>
Date: Mon, 20 Jan 1997 08:26:27 -0800 (PST)
To: cryptography@c2.net
Subject: Server Authentication
Message-ID: <199701201626.IAA12835@toad.com>
MIME-Version: 1.0
Content-Type: text/plain


I recently came aware of an interesting problem in server authentication.
I.e. How does a browser plugin validate the server it is working for.
There are many reasons for a plugin to want to validate its web server
including contractual relations, but the one that most appeals to me is
that the plugin provides access to confidential data which is used in an
application distributed between the client machine and the server.  Since
the data is confidential, the plugin doesn't want to send it to just any
server who can serve up a web page in the correct format, so it needs to
authenticate the server.

Now the obvious way to validate the server would be through the
certificates exchanged when the SSL session was set up.  (I am assuming a
SSL session here because you shouldn't send confidential data over a
non-encrypted link.)  However, I haven't found an API where the plugin can
discover the certificate used by the server, so it appears you have to roll
your own.

Rolling your own seems to come up against the problem mentioned by the
IPSEC people, i.e. if you separate authentication and encryption the places
you can end up are:

(1) Encrypted and authenticated.
(2) Encrypted but not authenticated.
(3) Not encrypted and subject to a man in the middle authentication attack.
(i.e. If an IP router can route your authentication packets, so can one
running Mallory's special code.  In the case above, the hostile server acts
as a router for authentication.)
(4) Unauthenticated and unencrypted.

Does anyone have a solution to this authentication problem?

Are signed applets discriminating enough to differentiate between different
validated hosts and adjust local permissions differently (at the file level
at least) for each?  (Or is it more like, "Oh this applet is from
marketspam.com which is signed by the US Post Office, it can read or write
anything on the machine."  Yea, right :-( )


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