The security characteristics of crypto modules with secrets

[eric@remailer.net (Eric Hughes)]

   From: Matt Blaze <mab@research.att.com>

   I think better than expecting the world to switch over to
   cumbersome, multilevel secure OSs is to equip such servers with
   inexpensive tamper-resistant cryptographic modules that never reveal
   their secrets.

This is certainly the first step to take, even if it's not a complete
answer.

   At least then you're guaranteed that there can be only
   one instance of a machine's identity out there at a time, and have some
   hope of detecting the theft of the key material.  

Unfortunately, this isn't really true.

Let's take as our model general purpose computers which can't store
secrets connected directly to crypto modules which can.  Furthermore,
let us assume that these general purpose computer are subject to
intrusion.  In other words, it's today's servers with attached crypto.

Now, the crypto module can't authenticate the machine it's plugged
into, because, by definition, that machine can't keep a secret.  One
ends up in an infinite regress here in one tries to assume a secret in
the place we have assumed otherwise.  Because the crypto module can't
authenticate the machine, it will reply to service requests from both
the local and approved machine and any remote and unapproved machines
that can gain access to the module.  The software on the server can be
subverted in order to allow the local crypto module to service remote
requests.

The attack works like this.  First, subvert the system software on
some server, probably through existing implementation defects.  Now,
install new software on that server that allows other machines to make
remote procedure calls to the module.  Set up a client on an
impersonating machine that make remote calls to the subverted server
whenever it needs to spoof.  The remote calls will most likely be an
encrypted protocol, to boot.  The easiest way to detect this
externally will be an additional delay in response, that is, doable
but not particularly reliable.

This is not to say that crypto modules are useless.  They have a great
use in recovery.  Because the secret doesn't leave the module, you
have an assurance after recovery (reboot from CDROM, for example) that
nobody else has the secret.  There won't be a need for an immediate
key change, at least.  What you don't get is a complete loss of
assurance of identity.

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.

Eric