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Re: [ietf-smtp] why you shouldn't even try to canonicalize local parts

2016-03-15 17:30:14
On Thu, Mar 10, 2016 at 2:01 PM, John R Levine <johnl(_at_)taugh(_dot_)com> 

Its certainly true that the issuer of a cert using such a regex represented
name matters greatly now.  (I wondered about the same issue in another
thread)  A third party CA is going to have a much harder time
the local practices, and that represents a spoofing risk, though I suspect
it might be possible to communicate local practices to prevent that.  A
workable scenario is that the certificate issuer is the email domain owner
who understands intimately the email local practices.

This is almost certainly a blind alley, and it'd help to back up and think
about what problem you're trying to solve.

It seems to me that even though the same S/MIME certificate is used for
signing and encryption, the two applications are quite different.

For signing, a random recipient having verified that the signature matches
the message body wants to check that an address in the certificate matches
the sender's address, typically the one on the From: line.  In my
experience, even those of us with a zillion inbound addresses don't use all
that many addresses for outbound mail, so it's practical to enumerate them
all in certificates.  Or if a sender does want to make an address on the
fly for which it does not yet have a certificate, if the CA is the sender's
domain owner, it should be straightforward invent to ask the CA at that
time for a certificate that matches the the address, and the CA can use
whatever local rules it wants to decide whether the desired address is one
that belongs to the requester and provide one or not. This is extra work,
but in the context of mail submission, it's not an unreasonable amount of
extra work.

Sorry about belaboring a point.  Popping out a level, what defines an
identity? even when we send mail to someone?  Is it a mailbox or rather an
authenticated user?  Most users probably would say they mean for their mail
to reach a person, and that the recipient would care about the person who
sent it.  How it gets there is almost invisible to them.  This probably
pedantic to many reading this, but doesn't RFC7542 specify a notion of a
Network Access Identity which has a notion of canonicalization for email
addresses?  Granted this is at the edge/client but the point is that is
separate from the authentication etc (AAA) service that consumes the
canonical NAI.  The analogy I'm try to draw is that the signature verifier
at the recipient MUA is an external entity to the SMTP, and is trying to do
a similar canonicalization for the purposes authentication.


For encryption of inbound mail, it doesn't matter what address is in the
certificate; if the recipient can decode the message it's the right one,
otherwise it's not.  So the sender can ask the domain's key oracle for a
certficate for the address, the key oracle applies local rules and provides
a certificate that might have the exact requested address, or might have
another address that goes to the same recipient.

I'd think that something along these lines would not be particularly hard
to implement, and would require semantic changes neither to PKIX nor to


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