[hashcash] Re: PR Problem?

On Sun, Nov 12, 2006 at 06:03:45PM -0500, Eric S. Johansson wrote:

> Fair enough.  That makes a lot of sense.  SPF is not a useful antispam
> technique, it's a useful anti-forgery technique which merely limits
> one of the types of spam disguises.  But I think your point of giving

SPF only protects against joe-jobs. That's not a piffling thing, but it
was definitely overhyped as the "one true answer to spam." Domain-keys
(and similar) will have similar problems--and so will hashcash, if
people aren't clear on *what* it is providing or authenticating. All a
hashcash header really proves is that:

    1. Someone was technically proficient enough to set it up, and

    2. The system is originated from was not spewing mail at such high
       volumes that minting a stamp for each message was prohibitive.

I think #2 would be a show-stopper for server-side stamping at a busy
ISP, even if the mail was (generally) legitimate. Consider the
consequences of having a botnet start spewing mail from your customers'
systems, and your mail infrastructure happily minting stamps for them.
Either it will effectively create a DoS situation (bad for business and
sysadmin sanity), or your gi-normous mail server cluster filled with
8-way Xeon systems won't even notice (defeating the value of the
stamping process).

*That* is the main argument I have against server-side stamps: it shifts
the problem from low-end hardware where the minting makes the proof of
work expensive enough to be useful, and onto higher-end hardware where
the existence of minted stamps no longer really proves anything about
the originator.

I'm not saying server-side minting is always bad. But if you do that,
you need to implement some sort of per-user token bucket to ensure that
you're saying something useful about the original sender.

> Okay, who knows Java and is willing to make a fast (only three X
> slower) stamp generator?  I've often thought the same thing about a

Um, squirrelmail is written in PHP, not Java. Even if true, so what? By
definition, expensive stamps are more valuable. If your earlier point is
that server hardware should suck it up for end-users, and that service
providers can just throw more money or hardware at a centralization
problem, then what difference does it make if it takes .5 seconds or 3
seconds to mint a stamp? It's a scaling problem, no doubt about it, but
I think uptake problems have more to do with a lack of tools, and not
with the implementation details or efficiency of the back-end code.

> generic "contact us" mailto.  can a mail-to include random headers in
> a message?

Of course. See here:

    http://xml.resource.org/public/rfc/html/rfc2368.html#anchor2

so you could insert a URL into your mail forms, if you want:

    
mailto:foo@xxxxxxx?X-Hashcash:%201:20:061113:foo@xxxxxxxxxxx::tCXVpAsAD42tT+Gh:004bC6

> I'm still not getting this however.  It may be the migraine and I'm
> being dim.  It seems like what you're saying is that the filter should
> attach some sort of a mime trailer indicating success or failure of
> stamp analysis.

I'm saying that hashcash itself should when it checks the stamp. Failing
that, the filtering tool should be doing that, because you can't assume
that it's the last filter in the toolchain. For example,
postfix->procmail->tmda->bogofilter->procmail might be one example.
Where does hashcash get inserted? And who inserts the resulting header
into the mail? If hashcash itself adds the verification header after
checking a stamp, that would make the most sense to me.

> sorry about the speech recognition error.  If you had a mime
> attachment for holding stamps, and you also signed the document with
> gpg, would gpg be able to cope with a document that had the stamps
> attached post-signing?

AFAIK, GPG will only verify whatever's in a relevent MIME part:

    application/pgp-encrypted
    application/pgp-keys
    application/pgp-signature

If, however, you're using inline PGP, then it will verify whatever's
between its start and end delimiter within the message body. So, either
way, I don't think it would have any effect since neither GPG nor
hashcash is actually hashing the other.

-- 
Unabashedly littering the information superhighway with detritus like
this for over 15 years now.

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