From: Hal <hfinney@shell.portal.com>
Date: Thu, 3 Feb 94 23:24:53 PST
To: cypherpunks@toad.com
Subject: Re: contemplating remailer postage
Message-ID: <199402040715.XAA18357@jobe.shell.portal.com>
MIME-Version: 1.0
Content-Type: text/plain


As Jim points out, Matthew's scheme for one-bit-per-stamp has the
problem that it requires non-anonymous stamps.  Jim suggested a variant
on Chaum's digital cash where the stamp numbers would be re-blinded by
the recipient so that the remailer would not recognize them (but could
verify their validity).

Matthew's bitmap idea could still be used, though.  The incoming stamp
numbers could be hashed down to, say, 24 bits.  This could then be an
index into a 2^24-bit file, which would take 2 MB.  Set the bit when the
stamp is used, and reject the mail if the bit is already set.

Granted, this would create false rejections.  But email is already not
perfectly reliable.  You could send 160,000 messages before you had as
many as 1% false rejections (2^24 / 100).  I think this would be better
than trying to save this many digital stamps and check through the list
each time for duplications.

Hal