Hashcash in mail (was: New Whitepaper: Anti Brute Force Resource Metering)

From: Peter J. Holzer (hjp_at_wsr.ac.at)
Date: 03/24/05

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    Date: Thu, 24 Mar 2005 10:58:39 +0100
To: bugtraq@securityfocus.com

    On 2005-03-23 21:25:03 -0000, Gunter Ollmann (NGS) wrote:
    > > You claim that hashcash "has already proven to positively reduce the
    > > success" of spammers. Is there any example of hashcash being
    > > deployed in e-mail systems? I don't know any and I can't even
    > > offhand think of any feasible method of how it could be deployed.
    >
    > Checkout the following:
    > SpamAssassin - http://wiki.apache.org/spamassassin/
    > TMDA - http://wiki.tmda.net/TmdaHashCashHowto
    > CANRAM - http://www.camram.org/

    Yes, but are they actually used?

    The X-hashcash header has to be sent "blind". There is no way for the
    recipient to tell the sender that it uses hashcash and how many bits it
    requires. At best, this information could be included in the reject
    message, but 1) that requires the sender to read the bounce message
    (which the average user doesn't) and 2) especially Spamassassin is often
    used not to reject messages but only to mark them so they can be sorted
    into a Junk folder.

    This makes it very likely that the sender must be told to use hashcash
    in some other way (probably by telephone: "did you get my mail?" - "no,
    you have to use hashcash" - "what's that?"), which makes it IMHO highly
    unlikely that it is used anywhere without prior agreement - but if you
    agree in advance to use hashcash, you can agree on simpler measures, too
    (e.g. whitelisting each others outgoing mail servers).

            hp 

    -- 
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