On 2005-06-11 06:15:49 -0000, John Levine wrote: > >This can be addressed by holding all incoming challenges and > >preventing them from reaching the user's inbox for 10 minutes (or > >whatever length of time). The challenge is passed to the user's inbox > >once it is clear that the database in up-to-date. > > Even assuming this synchronization is practical (in a large system > with many MTAs, it probably isn't) this makes no sense whatsoever. > > If the MTA can tell what incoming mail is a challenge, and it knows > what's in the database (it must, if it knows when all of the updates > have been posted), why is it delivering the challenge to the user at > all rather than just answering it? It can't, because the challenge contains a CAPTCHA, so only the user can answer it correctly (at least if the CAPTCHA works as intended). > But the more important question is why bother to create the expensive > giant database and the complex synchronization and the special purpose > challenges when remotely verifiable message signatures a la DK solve > the problem much better, with no database and no challenges. Do they? I think DK, bounce-verification systems and C/R systems solve different problems. DK (and similar systems) answers the question "was this mail sent by somebody authorized to use this domain". The question "was this bounce triggered by by a mail sent by one of our users or by a mail with a faked return-path?" can be answered by DK-like schemes only if the bounce contains the information needed to verify the signature. For DK in particular, this is often not true, because it signs the body (which is a good thing, IMHO) and the body is often omitted or trunkated in bounces (which is also a good thing, IMHO). So to verify if a bounce was triggered by a legitimate mail you need to include something in your mails which is typically included intact in a bounce. About the only thing which is guarantueed to work is the envelope sender. Finally, C/R systems answer the question "was this mail sent by somebody who cares enough about the mail to answer my challenge?" which is a stronger version of question 1. hp -- _ | Peter J. Holzer | Ich sehe nun ein, dass Computer wenig |_|_) | Sysadmin WSR | geeignet sind, um sich was zu merken. | | | hjp at hjp.at | __/ | http://www.hjp.at/ | -- Holger Lembke in dan-am
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