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Re: [ietf-dkim] Not exactly not a threat analysis

2005-08-23 13:54:55
Going back a lot of messages, but only a few hours (apologies if I'm beating a dead horse):

Keith Moore wrote:

Part of the idea that DKIM seems to propose is that more than one party can potentially sign a message. For instance, an author might sign a message, or a list might sign the same message. But different parties mean different things when they sign the message. If the author signs a message, it means "I wrote this". If a list signs a message, it means "I sent this".

But DKIM never gives an assertion of authorship (use PGP or S/MIME for that). Even if there is a valid signature that is associated with the origination address, it means "the supposed author's domain authorized this message".

This goes to what we have been very generically calling first-party and third-party signatures. The original submission of a message would normally result in a first-party signature from the supposed author's domain. A mailing list would apply a third-party signature, which can be distinguished by the fact that i= does not match the originator's address. There are other circumstances where third-party signatures would be applied as well, but I can't think of why it would be significant whether the third-party signer is a mailing list, some other resender, or a greeting card or something.

BTW, a good reason for the local-part on i= is that it if the original purported author and the mailing list are in the same domain, it's still possible for the list to apply a signature and not have it look like a first-party signature.

Is there other significance to signature semantics that is needed that is not conveyed by the comparison of i= to the originator address?

-Jim
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