[ietf-dkim] What DKIM provides, again
Murray S. Kucherawy
msk at cloudmark.com
Wed Oct 13 11:37:54 PDT 2010
> -----Original Message-----
> From: ietf-dkim-bounces at mipassoc.org [mailto:ietf-dkim-bounces at mipassoc.org] On Behalf Of Jeff Macdonald
> Sent: Wednesday, October 13, 2010 11:27 AM
> To: DKIM
> Subject: Re: [ietf-dkim] detecting header mutations after signing
> Count me as one of those who was confused early on about what DKIM
> provides. DKIM seems to make assurances to message integrity. But it
> doesn't. I think the reason why many think it does is because of the
> body hash. It is trying to do to much. It should just provide an
> identifier that can be verified. Instead of using the body for
> hashing, use the Message-ID header along with the Date header and just
> hash that. That way most folks would understand DKIM is just providing
> an Identifier.
Then you send me a piece of signed mail, I change everything except the Message-ID and Date, and send it to someone else. And the verifier will green-light it, meaning you've taken responsibility for it. Are you OK with that?
My way of thinking about this is that verification of a message is equivalent to collecting all the pieces (header, body, signature) and coming to you and saying "Do you take responsibility for this?" If I get your public key from DNS and everything lines up, you're implicitly saying "yes".
Now, if I remove the whole body and most of the header from what I'm presenting to you for that question, you're now possibly saying "yes" to content you didn't create. Are you OK with that?
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