Re: [Uta] Eric Rescorla's Discuss on draft-ietf-uta-smtp-require-tls-07: (with DISCUSS and COMMENT)

[Jim Fenton <[e-mail]>]

On 2/21/19 7:07 AM, Eric Rescorla wrote:
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
>
> I support Benjamin's DISCUSS.
>
> To elaborate on one point a bit: it seems to me that it's harmful to
> security to allow the sender to unilaterally override the recipient's
> preferences that something be encrypted. To forestall one argument,
> yes, the sender knows the contents of the message, but the recipient
> knows their own circumstances, and they may be at particular risk


The general approach of REQUIRETLS is that the sender is in a good
position to determine the sensitivity of the message(s) they are
sending, because they know what the message is. Ultimately, for the last
hop anyway, the recipient has the ultimate control: they can refuse to
accept a message unless STARTTLS has been negotiated. For intermediate
hops, it's a matter of trying to balance the (perhaps) conflicting
wishes of the sender and recipient. And as Viktor has pointed out, this
provides a workaround for misconfigured mail systems. RequireTLS: NO is
not a very robust mechanism in any case, because it's very possible that
a relay MTA will not support REQUIRETLS and will ignore the header
field. But it's an attempt to get explicitly non-sensitive messages through.

>
>
>        The choices of key lengths and algorithms change over time, so a
>        specific requirement is not presented here.
>
> This is not a verifiable conformance requirement. You
> either need to not have a 8174 SHOULD here, or actually specify what
> "meaningfully secure" means.


This text was removed in the -07 draft, and now has a reference to RFC
7525 for TLS negotiation requirements. Let me know if this addresses
this issue.

>
>
> ----------------------------------------------------------------------
> COMMENT:
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>
>    email by giving the originator of a message an expectation that it
>    will be transmitted in an encrypted form "over the wire".  When used,
>    REQUIRETLS changes the traditional behavior of email transmission,
>
> This does not seem to accurately describe "RequireTLS: NO"
>
This text was also corrected in the -07 draft.