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

[Adam Back]

On Wed, May 14, 2003 at 10:02:42AM -0400, Sunder wrote:
> And what happens when there's a network outage and a message gets stuck in
> the queue for a day on another server?  You know a backup MX server when
> yours is hosed?
> 
> Do you not accept the mail because the current day doesn't match what's in
> the message?  Or do you accept mails from a day ago?  a week ago? a year
> ago? 1922?

I was suggesting 30 days.  You could up that if you want -- the
database won't be that big at say 32 bytes per recieved mail.  

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.

if you hash that with sha1:

% echo -n 0:030514:foo at bar.com:482d3c37d5b5c112 | sha1
00000bea531c1edbcee4fbb69e094026cd83ed75

You can see that this one has 20 leadings 0s (in binary -- 5x4bit hex
digits).

> 2nd, why wouldn't the spammer just adjust and send an email to each
> recipient with a random, but properly hashed token to match the target
> address + today's date?  More work for sure, but if enough targets start
> adopting it, the spammer will adapt.  The token doesn't have to contain an
> actual valid coin, and you'll only find out when you try to cash it.

If the token is random (ie the spammer put no computational work into
it), it won't have the required number of bits of collision and the
recipient will reject it.

eg I'll just type this one in myself:
0:030514:sunder at sunder.net:0123456789abcdef

Then my MTA or my mail-client (or any MTA in the path that does
checking) will check:

% echo -n 0:030514:sunder at sunder.net:0123456789abcdef
aeeaf7971f7e30e2485062b17b43189e5361383f

and see that there are insufficient leading 0 bits (none in this
case).

The tokens are only valid to a given recipient, if a spammer sends you
a token address to me, you'll reject it because it doesn't have an
address you receive mail for in it.

If a spammers sends you the same valid token twice, you'll reject it
because you keep a little database of received tokens.

There is software here (windows GUI, windows cmd line, unix cmd line):

http://www.cypherspace.org/hashcash/

Adam