DalSemi: Add-Only Memory for Storage of Digital Cash

[Nathan Loofbourrow <loofbour@cis.ohio-state.edu>]

A press release breezed through the list a while back about some
"decoder rings" -- basically, a nonvolatile RAM chip embedded in a
Jostens ring.

On further inspection, it looks to be a superior product to the
DataKey products I remember seeing about ten years ago, targeted at
this same sort of niche. The part in question is much smaller -- it
fits in a ring, after all, or on the usual sorts of key fobs and
employee badges -- and establishing electrical contact to transceive
data is trivial.

Cypherpunks relevance? Twofold. One, the Touch MultiKey
(ftp://ftp.dalsemi.com/pub/datasheets/1991.ps), which promises to hold
three 384 bit blocks under 64-bit passwords. The device will transmit
the stored data under the correct password, and "random bits" under
all other passwords. No crypto here, though, just a simple on-chip
comparison with a stored password. So I assume the determined opponent
with physical access can extract the info; but better that than store
your private key on a publically accessible machine, no?

Secondly, an app note: "Use of Add-Only Memory for Secure Storage of
Monetary Equivalent Data" (ftp://ftp.dalsemi.com/pub/datasheets/app84.ps).
A creative idea based on the peculiar nature of the EPROMs* that are
also available in this form factor. In particular, one bits may be
burned to zeros, but not vice versa; so why not burn particular bits
to indicate credits and debits?

Knowing that this scheme depends on keeping an attacker from guessing
which bits to burn, they use the unique serial number to uniquely
permute the bits, so that an attacker is as likely to burn a "debit"
bit as a "credit", and far likelier to burn out-of-sequence so that
the monkey business is apparent to any vendor.

And there's another item of note: each chip has a unique, etched,
machine-readable serial number. What are the bets that Dallas
Semiconductor can tell you who purchased that chip? Well, so much for
an anonymous payment scheme based on *this* product.

Still, if I had to choose a place to keep a secret key, I'd choose my
knuckle over my key ring, let alone a floppy disk, PDA, or portable
computer.

nathan
* Gee, back in my day EPROMs were Eraseable; these folks mean instead
  to indicate an Electrically Programmable chip, which sounds like a
  good old PROM to me. Ahhh, acronyms...