what fields to hash with hashcash (Re: A Trial Balloon to Ban Email?)

[Adam Back]

I'm not sure what the comment alludes to as it includes a ;-), but you
can find multiple collisions against the same email address on the
same day, viz:

0:030514:frantz at pwpconsult.com:49916794a98728f2
0:030514:frantz at pwpconsult.com:ffbead9be92472a3

etc.  In fact they are also this way because you want there to be a
relatively low probability of there being an accidental collision in
the tokens created by different users sending you mail.  

The example implementation chooses the random string from a 2^64
space, however on average only 2^44 of those will be valid tokens (if
you use 20 bit collisions), and so if you imagine someone receiving
256 mails in a day, they have a birthday probability of 2^-29 of
having a mail falsely deleted because of an accidental collision.

I guess that is a fairly low probability compared to email
reliability, but anyway the safety margin can be increased simply by
increasing the random string search space.

Adam

On Wed, May 14, 2003 at 11:14:56AM -0700, Bill Frantz wrote:
> This approach seems like a good direction.  However, it does limit me to
> email per address per day.  :-)
> 
> At 7:56 AM -0700 5/14/03, Adam Back wrote:
> >The day is matched against the day in the token, as Bill said the
> >tokens contain the date and the email address, in fact they look like
> >this:
> >
> >0:030514:foo at bar.com:482d3c37d5b5c112
> >
> >where the first field is a version number, 2nd field is date
> >(year,month,day), 3rd field is resource name (for email the
> >recipient's email address) and last field is random junk to make it
> >hash to trailing zeros.