DRM for batteries

[Leichter, Jerry]

| Date: Fri, 04 Jan 2008 16:38:07 +1300
| From: Peter Gutmann <pgut001 at cs.auckland.ac.nz>
| To: cryptography at metzdowd.com
| Subject: DRM for batteries
| 
| http://www.intersil.com/cda/deviceinfo/0,1477,ISL6296,0.html
| 
| At $1.40 each (at least in sub-1K quantities) you wonder whether it's
| costing them more to add the DRM (spread over all battery sales) than
| any marginal gain in preventing use of third-party batteries by a
| small subset of users.
For laptop batteries - which can cost $100 each - some might see it as
a win.  Of course, if you can eliminate the competition, you can also
raise prices.

The spec sheets have links to PDF description of the algorithm, but
distribution of that is restricted - talk to your salesman.

Can anyone make any sense of the following claim:

	Non-unique mapping of the secret key to an 8-Bit
	authentication code maximizes hacking difficulty due
	to need for exhaustive key search (superior to
	SHA-1).

The only thing I can come up with is the old idea that you compute (say)
a 32-bit keyed MAC but then only use the bottom 16 bits.  This makes it
more difficult for an attacker to use the MAC on some data to determine
the key - on average, you'd probably need about 2^16 samples to give
a unique key.  This was use in some old X.something-or-other bank
hashing algorithm, which predates functions that we believe to be
one-way.

Over all, I find it hard to see how such a product can really make
sense, however.  If there's enough money to make it worth trying to keep
the clone makers out, there's enough money in it for the clone makers to
be willing to invest in determining the secret information.  Given the
nature of the proposed solution, all batteries (or other protected
objects) have to have the same secret - break into one, and you can make
as many as you like, so any cost for breaking in is amortized over
however many of the things you can sell.  There's no effective way to
change the secret - even if you could somehow make a patch to the
devices involved, you couldn't change it in such a way that it would
refuse to use the batteries already in it (with the old secret).

Meanwhile, at $1.40 a unit, you can't make anything really tamper
protected.  (Given some of the reverse engineering expertise already out
there, it's not clear how "tamper protected" you can really make
something these days at *any* cost.  But you certainly can't do it on
the cheap.)

Still, I'm sure people will try - and life will become even more
annoying.
							-- Jerry

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