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Re: [Asrg] 6. Proposals - DNS + PKI - Yahoo's "Domain Keys"

2003-12-09 13:26:25
Yakov Shafranovich <research(_at_)solidmatrix(_dot_)com> wrote:
Another difference here is that LMAP addresses MAIL FROM forgery in the 
SMTP transaction, while DK addresses forgery in the mail headers and 
email message itself.

  Which makes me wonder what STARTTLS was for.

  That's not entirely true...  STARTTLS encrypts a message hop by
hop, but you've got no guarantee that the hop you're talking to isn't
lying about the existence of previous hops.

  But for the vast majority of email, which flows directly from MTA to
MTA, DK is equivalent to using STARTTLS, with public keys looked up in
DNS.  Why sign the message, and then distribute it in the clear, when
you can instead distribute the message via an encrypted tunnel?

  But why use an encrypted tunnel or sign a message when it's being
delivered from your user, using your MX, to my MX, for my user?  The
audit trail is pretty straightforward there.

  The drawback is you can't delegate authority like you can in LMAP.

What about if you use subkeys?

  Then the DK headers will have to include not just the domain
information, but which sub-key to use.  The extreme is that each user
will have their own sub-key, in which case spammers will have them,
and key maintenance become a nightmare.

  LMAP allows you to say that a machine is permitted to send mail,
without giving that machine permission to make those statements
itself.  DK gives a machine the ability to "hand-off" it's delegated
authority to third parties, without the knowledge, or consent, of the
originating authority.

  If you're going to have roaming users, you MUST use sub-keys, which
will be impossible to keep secure.  LMAP doesn't have this problem.

  Alan DeKok.

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