[hashcash] Re: stamp collisions

>>>>> "Jonathan" == Jonathan Morton <chromi@xxxxxxxxxxxxxxxxxxxxx> writes:

[...]

Jonathan> To eliminate collisions entirely, a "sender=..." extension
Jonathan> field would be advisable.  This reduces the chance of
Jonathan> collisions to 1 in 2^48 stamps produced by any particular
Jonathan> sender, even if they only have a medium-quality RNG.

Of course, your probabilities get reset at the beginning of the day,
since the date is part of the stamp.  (Although if you live to be over
100 years, you might run into problems, since it only has 2-digit years.
;-) (of course, by then, you would undoubtedly need to generate a larger
bit-value stamp anyways)) If the stamp contained the time down to the
second, along with the sender= extension, collisions would be pretty
much impossible, although this is probably not needed.

Another way to guarantee no collisions (in addition to the sender=
extension) is to have a few bits in the "random" field be a sequential
counter that gets incremented for every stamp generated.

I think a sender= extension is a good idea anyways, to prevent someone
from ripping off my stamp from a mailing list archive and using it to
spam someone.  (If a spammer rips off stamps from an obscure mailing
list, they can be pretty sure that it won't be in most people's
double-spend database.  It's still a limited number of stamps that they
have access to, but it's an easy way to get some free stamps.)

-- 
Hubert Chan <hubert@xxxxxxxxx> - http://www.uhoreg.ca/
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