From: Matt Blaze <mab@crypto.com>
Date: Wed, 1 Feb 95 07:27:03 PST
To: "James A. Donald" <jamesd@netcom.com>
Subject: Re: The security characteristics of crypto modules with secrets
In-Reply-To: <Pine.3.89.9501312316.A19818-0100000@netcom4>
Message-ID: <199502011528.KAA23229@crypto.com>
MIME-Version: 1.0
Content-Type: text/plain


>> The prevalent use of modules further reduces the likelihood of initial
>> attacks based on spoofing.  Since active IP attacks require the
>> subversion of routers, and since router software is much more
>> difficult to subvert than general purpose servers, adding crypto
>> modules to routers would be a big win.
>
>This does not make sense:  The advantage of a tamper resistant module
>is that if somebody physically gets to the system, he still cannot
>get the key.  But if he physically gets to the router, he can
>make it do his will, even if he does not get the key.  So one
>might as well have the key in software in the router.
>
>If the router is hard to subvert, and the attacker cannot 
>physically get to it, then there is little need for a separate
>tamper resistant module.  Software will do fine.
>
>If the router can be got at, you are stuffed regardless, tamper
>resistant module or not.

The advantage of a secure crypto module on an insecure server (or
router or whatever) is in limiting the scope of successful attack. 
As Eric pointed out, if you can subvert a general purpose machine that
does all its crypto through a secure module that you can't subvert,
you can still add a covert "service" to the machine that lets
a future spoofer use the module remotely.  The main important
difference between this attack and just learning the server's secret
is that it only remains useful as long as the attack is undiscovered.
In the case of software keys, it is sufficient for the attacker to subvert
the machine that knows the secret ONCE.  He or she can put things back
to normal on the original machine and still know the secret forever, with
little chance of future detection.  With a secure module, the attacker has
to either steal (physically) the hardware (which will be discovered when
the real server stops working) or set up the kind of future access that
Eric mentioned (which, once discovered, will likely be turned off or
investigated).

If you have secure crypto hardware, you only have to worry about and
detect whether the server is being compromised continuously.  Otherwise,
without special hardware, you have to worry about and detect whether the
server was ever compromised since it was last rekeyed.  Personally,
the former seems like a realistic thing to try to do while the latter
doesn't, at least in the environments in which I live.

If the server hardware or software is insecure, cryptographic techniques
can't provide any absolute guarantees, period.   In the real world, though,
you're not interested in absolute guarantees, you just want to
reduce risks.   How effective the mechanisms to do this are depends on
how accurately they reflect the real world threats.

-matt