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

[Stainless Steel Rat <ratinox@peorth.gweep.net>]

* "Patrick J. LoPresti" <patl@curl.com>  on Fri, 28 Jun 2002
| Right, so you have to try them all.  Checking the validity of a coin
| is "fast", so this is OK, in theory.

Not even in theory.  It is a linear problem, and linear problems do not
scale.

[...]

| Then again, it is not disastrous if you miss a message.

And if that lost message is the job offer I am expecting?  -Anything- that
causes loss of legitimate mail is BAD.  Really bad.  Unacceptably bad, in
my opinion and that of the 350 employees in my company who expect mail not
to be lost.

[...]
| Well, you get to decide how many bits you require the sender to match,
| so you can make it one out of however many you like.

| Or am I misunderstanding what you mean?

You are misunderstanding.  Say that you (not necessarilly "you" personally,
but anyone or thing that relies on X-Hashcash headers) want 20 bits
collision (that is a 1:2^20 probability of any two hashes of the same total
length meeting the criteria, or approximately 1 in 1 million, just so you
know).  And say that I use something like Sub7 to distribute my X-Hashcash
DoS system to a mere five thousand machines, which can calculate hashes at
a rate of 1 every 10 seconds, just to pull some numbers out of my behind
(10 seconds is rather slow by today's standards, anyway).  That is 1.8
million hashes per hour.

All those hashes being dumped into your spent coin database.  And five
thousand Sub7 variant infections is a very conservative number.

Do you begin to see the vulnerabilities in X-Hashcash?

| I think X-hashcash is a creative attempt at a technological solution
| to spam.

It is an attempt at blocking or preventing spam from being sent to
mail2news gateways.  It has a -very- narrow focus.

| And for some applications, like a public mailing list, it might even be
| practical.  Who cares whether you can BCC ding@gnus.org, for instance?

Ask the spammers who's mail has recently been sent here. :)

BCC has legitimate uses for legitimate mail.  Today I used it to inform a
number of people who were using excessvie ammounts of disk space on one of
my machines without embarrassing them by revealing their identities.

| It seems unlikely to catch on as a user-to-user filtering scheme;
| digital signatures are probably a better approach for that.

Or a real hashcash system.

-- 
Rat <ratinox@peorth.gweep.net>    \ When not in use, Happy Fun Ball should be
Minion of Nathan - Nathan says Hi! \ returned to its special container and
PGP Key: at a key server near you!  \ kept under refrigeration.
       That and five bucks will get you a small coffee at Starbucks.