Re: Hashcash for IM2000

Ricardo SIGNES (Fri, Sep 05, 2003 at 09:31:05AM -0400):
> * Wes Morgan <[email protected]> [2003-09-05T09:20:02]
> > I was reading an article on K5 about TMDA, and one of the commenters posted a 
> > link to Hashcash (http://www.hashcash.org). This system looks really 
> > promising.
> 
> Does it?
> 
> What keeps a spammer from computing a large number of hashes and then
> sending them in bulk?  The FAQ says that hashes have an expiry.  Is it
> less than a day?  If each hash takes one second (let's assume the
> spammer has a whiz-bang PC) to compute, that's 86400 computed per day.
> Then the spammer sends them all within seconds or minutes...

If you demand a collision of 255 of 256 bits you need longer than the
age of the universe (on avarage) or similiar. I.e. no spammer can afford
it.  For any avarage computer speed there is a collision size which is
suitable for spam prevention.

One problem is the ability to resend identical messages to the same
person. Using an expiry is only of limited use. The disk space may
become short.

Of course, hashcash should be one implementation of a general
challenge/response system. Then the receiver can set the collision
length and the bitpattern. In this case no expiry is needed, since a new
calculation is needed every time.

Sincerly,
-- 
Stefan