Re: [ANNOUNCE] contrib/hashcash.el spam fighter

["Patrick J. LoPresti" <patl@curl.com>]

Stainless Steel Rat <ratinox@peorth.gweep.net> writes:

> | A spammer could compute these, but he could only use it once.
> 
> No, he could only use it once every 28 days by default.

No.  I think you still do not fully understand the hashcash scheme...

The hashed value includes both the recipient's address and the date.
The hash is valid only if the recipient address is correct *and* the
date is recent.  "Recent" can be 28 days, 1 week, 1 day, or whatever
you like; it depends on how long you want to allow the coin to be
valid (for the convenience of the sender) versus how long you want to
keep old coins listed in your local database.

> Still, there may be a way of exploiting the Bcc mechanism or RCPT TO
> at the SMTP level, both common spam tactics.  If a spammer were to
> generate a hash against a bogus To header (like most of the spam I
> get), he could blindly send his spam to millions of recipients with
> only one hash.  Yes, you could check the X-Hashcash coin against
> your known address, but that leads you directly into a false
> negative if a legitimate sender Bccs you something.

The sender must include a separate hash for every recipient, whether
CC'd or BCC'd.  Depending on the structure of the hash, this may or
may not directly reveal to whom you BCC'd.  (It certainly makes it
possible for someone else to *guess* to whom you BCC'd, which is a
downside.  I doubt you can correct this deficiency without public-key
crypto, though.)

> The system is also vunlerable to false positives.  The likelyhood of
> an accidentally duplicated coin increases with the number of users.
> And it is potentially vulnerable to denial of service by the same
> mechanism.

It does not matter if two duplicated coins appear somewhere in the
world; what matters is whether somebody can exploit them.  With any
reasonable number of bits, they cannot.  One false positive out of a
million messages, or a billion messages, is certainly a ratio I could
tolerate.

I do not think hashcash will catch on because it is too complex.  But
it is technically sound, your half-dozen misinformed messages on the
topic notwithstanding :-).

 - Pat