From: Steve Bellovin <smb@research.att.com>
Date: Thu, 29 Oct 1998 02:10:58 +0800
To: Hal Lockhart <Harold.Lockhart@platinum.com>
Subject: Re: log files (was: Re: dbts: Cryptographic Dog Stocks, The Dirigible Biplane, and Sending the Wizards Back to Menlo Park )
Message-ID: <199810281713.MAA29423@postal.research.att.com>
MIME-Version: 1.0
Content-Type: text/plain



In message <3.0.1.32.19981028093825.00a3f2d0@pop.bos.platinum.com>, Hal Lockhar
> 
> I hear you saying, "but what if the guy is trying to break into my system."
>  I would argue, that if you are using strong cryptographic authentication
> (whether his identity can be mapped to a human being or not) then either 1)
> he will fail, in which case you don't care or 2) he will succeed, in which
> case he must have used some technique like stealing a key which cannot be
> detected by monitoring.

Why do you assume that the flaw that lets the bad guy in is cryptographic?
I recently analyzed every CERT advisory ever issued.  85% of them
described problems that crypto couldn't fix.  Buffer overflow is the
culprit du jour, but it's hardly the only one.  (This year, the prize
for second place -- admittedly a distant second -- went to crypto modules.)
> 
> I raise this point, because I see it as part of a current trend to resist
> stong security measures in preference to weak ones.  The current industry
> facination with weak measures, like firewalls and intrusion detection
> systems has caused some people to resist the introduction of strong
> measures, such as cryptographic authentication and data protection.  The
> most popular example of this is the foolish practice of putting an SSL
> server, with its vital server private key, outside the firewall in a DMZ.  

That's only a flaw if you assume that there are other weak points on
the Web server that a firewall can protect.  If you lock down the host
so that only port 80 is exposed (and maybe a cryptographically protected
administrative port), what good does the firewall do?  The weakest point
is probably the CGI scripts, and those have to be exposed in any event.
> 
> But I have also heard system administrators protest the introduction of
> cryptographic solutions because some current half measure will no longer
> work.  There are even misguided individuals who have proposed putting
> master decryption keys at vulnerable locations like border routers, so that
> messages can be inspected in the fly.

There are no panaceas, there is no single technology (and that emphatically
includes crypto) that will solve this problem.  Defense in depth is our
best hope, which means firewalls plus crypto plus IDS plus better operating
systems and programming languages.  Most of all, it means engineering a
solution -- making the right set of tradeoffs, even if unobvious.

Let me give an analogy.  On most electrical equipment, the ground pin
is useless -- *unless* there has been another insulation or air gap
failure within the device, a failure that would render the frame "hot".
But because of the ground pin, such a failure will instead short the
hot lead to ground, tripping the breaker.  It thus takes two failures
to electrocute someone.  *But* -- toasters and other devices with
exposed heating elements don't use this technique.  Why not?  Well,
with an exposed electrode, the probability of direct contact with a live
wire is much greater.  And if you ground the frame of the toaster,
there's now a direct, high-quality path to ground that in turn is
in contact with the other hand of the poor fool who is poking at a
piece of bread with a fork.  In other words, what's a safety mechanism
in your refrigerator is a danger in your toaster -- and sometimes,
that can be true of crypto as well.

For another analogy, think of insecurity as entropy.  You can't destroy
it, but you can move it around.  Using crypto moves the problem from
the (in)security of the links to the (in)security of the keys.  If
you can't protect your keys -- and that generally takes much more than
crypto -- using cryptography hasn't bought you anything.