Smartcard Torture

["Timothy C. May" <tcmay@got.net>]

At 2:41 AM -0700 10/20/96, Bill Stewart wrote:
>At 04:45 PM 10/18/96 -0700, John Gilmore <gnu@toad.com> wrote:
>>Note that this attack requires physical access to the DES chip, to
>>stress it so it will fail.  It works great against "tamper-proof"
>>devices such as smart cards.  It doesn't work against encryption
>>happening at any distance from the attacker (e.g. across the network).
>
>It's probably most useful for defeating attempts to force smart cards
>on the public as the government's solution to Key Recovery
>(e.g. Clipper 4 fails, so after the election they come out with Clipper 5
>or the Anti-Terrorism Airplane Traveller's License Smartcard.)

I think Bill just hit on a VIP (Very Important Point).

In most _legitimate_ uses of a smart card, e.g., where one is using it to
store one's own data (passwords, so one doesn't have to remember long
strings), there is essentially nothing to be gained by thwarting or
subverting the card. After all, _you_ programmed it, so you can program it
again, and again.

However, in the application Bill described, the Anti-Terrorism Airplane
Traveller's License Smartcard, this is a credential issued by some
government, giving one their permission to do something, to be someplace,
etc. There is a very high incentive for some holders of these cards to
thwart or subvert the intent of these credentials. A market will likely
develop wherein people bring in their identity cards to be "twiddled in the
cyclotron."

In other words, for applications where the smartcard is basically a "memory
aid," the holder has no incentive to twiddle the card and every incentive
to stop others from getting ahold of it for twiddling. But for applications
in which the smartcard is _holding someone else's data_ (cash, permissions,
etc.), and the card holder wishes to change the data, he has every
incentive to try to break the encryption and all the time in the world to
do it. He could coax (I think of it as "smartcard torture") the card into
generating the 200 or so pairs Biham and Shamir cite over a period of many
days, even.

There are some defenses, for the card issuers, that I think are reasonable.
(I haven't read the Biham-Shamir paper yet, so I don't know if they
discussed defenses.) One obvious defense against twiddling is to have the
issuing authority digitally sign some of the data in the card, just as
lottery tickets have a digital hash/signature of the actual number printed
on the back.

(Lottery tickets are the canonical example of something that are easy to
forge--even with special ticket paper (and it doesn't look very special to
me, certainly not as special as a currency note), a forger has a lot of
time (months or more) to carefully forge a ticket with the winning number
on it. And the payoff can be enormous. Given that a relatively high
percentage of winning tickets are never redeemed (are lost or forgotten),
this would seem to be a winning approach. However, the number printed on
the face of the ticket is digitally hashed/signed by a secret key and the
resulting number is printed on the _back_ of the ticket (at least in
California, and I presume nearly everywhere). Unless the forger knows the
hashing key he cannot forge a valid ticket. If public key methods are used,
verification of the hash/signature can of course be done locally, even at
the local 7-11, with the secret key safely locked in a vault under several
levels of protection.)

(Local color note: Scientific Games, the leading printer of lottery
tickets, has a major facility--possibly the main facility--a few ridges
away from me, in Gilroy, I hear. I've also heard that John Koza, of genetic
programming fame, made his fortune in this business before going to
Stanford. Don't know if he was connected to Scientific Games.)

--Tim May


"The government announcement is disastrous," said Jim Bidzos,.."We warned IBM
that the National Security Agency would try to twist their technology."
[NYT, 1996-10-02]
We got computers, we're tapping phone lines, I know that that ain't allowed.
---------:---------:---------:---------:---------:---------:---------:----
Timothy C. May              | Crypto Anarchy: encryption, digital money,
tcmay@got.net  408-728-0152 | anonymous networks, digital pseudonyms, zero
W.A.S.T.E.: Corralitos, CA  | knowledge, reputations, information markets,
Higher Power: 2^1,257,787-1 | black markets, collapse of governments.
"National borders aren't even speed bumps on the information superhighway."