Digital Currency

[Daniel C.]

I just finished reading Cryptonomicon for the third or fourth time,
and it's got me wondering (again) about how digital currency would be
possible.  He talks a lot about how "strong crypto" can enable you to
have digital currency that is unforgeable, but I've thought and
thought about it and I just don't see how it's possible to issue a
digital, transferable document that you couldn't just make a copy of,
thereby doubling the amount of this currency that you own.  Is digital
currency really possible, or is it purely a product of Neal
Stephenson's imagination?

Thanks,
Dan

-- 
What is the largest power of two that has three zeros at the end of
its base 7 representation? (before the radix point, aka "decimal
point")