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Re: mental cryptography
Scott Brickner writes:
> Nathan Loofbourrow writes:
> >It may be that the approach is off anyway. Credit cards have only
> >signature verification -- if the salesperson bothers -- because stolen
> >cards are reported. You don't need a strong authentication technique
> >if a stolen card is easy to cancel.
> The card's easy to cancel, but the cash ain't. Credit cards are
> cleared with the issuer. Digital cash with smart cards acting as
> transaction observers don't need this. The thief need only transfer
> the cash from the stolen card to his own, just like he does with
> regular cash.
Er, um, right.
Well, then, perhaps on-line systems need to consider cash
revocation in case of theft. Actually, this is a no-brainer: just
exchange the cash for some new coin, and the old stuff goes
invalid. Admittedly, this means a footrace for the mugger and the
victim, so I guess the mugger is encouraged to knock you out cold.
Maybe you just shouldn't carry too much cash with you. Gee, that
sounds like good advice even without digital protocols.
> >Even with a weak PIN system for authentication, you can always provide
> >a "duress" PIN, right?
> Sounds like a better choice.
Duress PINs liberally sprinkled through the keyspace also drop the
efficacy of brute-force PIN search for the thief.