ietf-dkim
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Re: [ietf-dkim] Responsibility vs. Validity

2007-11-28 16:21:45
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I have to strongly disagree with many of the things said here. I am  
one of the original authors and designers, and while I don't speak  
for the other authors and designers, I believe that I can reasonably  
authoritatively say something about DKIM's original intent.

This working group can, of course, *change* DKIM's intent with  
nothing more than rough consensus. However, I have to object when I  
hear the intent misstated. Rather than write a huge note, I'm going  
to break my major objections up.

DKIM is subtly different from OpenPGP [1] and S/MIME. DKIM is  
analogous to a postmark, as opposed to the signature on a letter  
inside the envelope. To use another analogy, it is like the routing  
tag that an airline puts on baggage to make sure it wends its way  
through an airport. There's no corresponding equivalent to the  
OpenPGP-S/MIME signatures in this analogy, but DKIM is a statement  
about the container rather than the contents.

Because of the way that digital signatures work, the mechanics of the  
signature has to cover the body of the message. But the airport  
routing tag also by necessity covers the content of the bag because  
the laws of physics just work that way. That's not the intent.

Each of these metaphors will break down if I go further with them, so  
I won't. I'll move to a use case.

The buck for the "administrative domain" of callas.org lies  
ultimately with me. I run it. I have a number of users, who are all  
members of my family. The way that I run the system, it's possible  
for us to forge messages from each other. Now, the MTA will also put  
in a header line that says who the authenticated sender is, but  
that's in a "Received" line, and isn't going to be signed by DKIM.

My policy is that an authenticated user can "forge" senders. If that  
policy turns out to be unwise, it's my problem. It is the intent of  
DKIM that the administrative domain has the right to be stupid.  
Nonetheless, a DKIM signature means that I accept responsibility for  
a message I (meaning one of my authenticated users) put into the mail  
stream.

        Jon

[1] PGP is software, OpenPGP is an IETF standards-track protocol. The  
PGP software implements a number of standards including OpenPGP, S/ 
MIME. The name PGP is a trademark of PGP Corporation.

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