[perpass] Another mail-related proposal

Jim Fenton <fenton@bluepopcorn.net> Sat, 17 August 2013 20:43 UTC

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Date: Sat, 17 Aug 2013 13:43:55 -0700
From: Jim Fenton <fenton@bluepopcorn.net>
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Subject: [perpass] Another mail-related proposal
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When I read the purpose of this mailing list, an idea that had been in
the back of my brain came to mind, and I wonder if it fits with that
purpose. To Dave Crocker's point, I'm not sure whether you'd
characterize this as a privacy enhancement or confidentiality
enhancement, but here goes:

SMTP is biased in favor of delivering the message if at all possible.
While some MTAs use TLS to communicate, many do not, so TLS is
negotiated by mutual consent, but if that doesn't work out the message
is sent in the clear.  So if TLS breaks for some reason, people
frequently don't notice.

There might be times when I'm interested in sending a message that I'd
rather not be in the clear on the wire, and I'd rather that the message
bounce rather than be sent in the clear. How about an SMTP option that
allows a sender to specify whether the message transmission requires (1)
TLS and (2) that the receiving MTA also enforce this option. It could
also specify whether the recipient MTA is required to have a certificate
trusted (e.g., via trust chain) by the sending MTA, or whether any TLS
negotiation (e.g., self-signed cert) is OK.

This of course doesn't address a threat model that includes the MTAs
themselves, since they have the message in the clear.

So,

- Is this proposal the sort of thing this list is for?
- If so, any thoughts on the proposal?

-Jim