Double spending, i.e. X in S == not X not in S

[Patrick Chkoreff]

On Thursday, April 24, 2003, at 04:17 PM, R. A. Hettinga wrote:

> At 3:52 PM -0400 4/24/03, Patrick Chkoreff wrote:
>> Alternatively, I think a copy of the non-spent coins will do the trick
>> also.
>
> Patrick, no offense, but have you actually *read* this stuff?
>
> You *delete* the spent coins after some pre-arranged period. They're 
> useless.
>
> You don't *care* about the unspent coins. You're going to *have* to 
> keep the spent coins to prevent double spending.
>
> Get it?


No, which indicates there is one huge unshared premise at work here.  I 
assert that I can prevent double spending without keeping the spent 
coins, even for a limited time period.

It's simple.  Alice gives Bob a coin X.  Bob immediately swaps coin X 
for a brand new coin Y.  The server deletes coin X completely, 
forgetting the bits with extreme prejudice.

Now Alice tries to give Charles the same coin X.  Charles immediately 
attempts to swap coin X for a new one.  The server tries to look up X 
in the set of valid coins and does not find it.  The server says 
"Sorry, Charlie, that is not a valid coin."

The whole thing depends on the recipient doing an immediate swap.  But 
that's no big requirement, because the recipient must contact the 
server to see if it's a valid coin anyway.  So you just go ahead and do 
a swap at that point.

-- Patrick
http://fexl.com