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

[Eric Rescorla <[e-mail]>]

On Thu, Feb 21, 2019 at 10:37 AM Viktor Dukhovni <[e-mail]>
wrote:

> > On Feb 21, 2019, at 10:07 AM, Eric Rescorla <[e-mail]> wrote:
> >
> > 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
>
> A recipient has no expectation that the sending MTA supports any of
> DANE, MTA-STS, REQUIRETLS, or even STARTTLS.


Nor do Web servers have any expectation that clients support HSTS, but we
still don't allow it to be overridden by some http-no-really:// link.




> The most the recipient can do is abort the SMTP transaction at any
> pre-STARTTLS "MAIL FROM" (typically sent with the pipelined recipient
> list) when STARTTLS is not used by the sender (or MITM attacker).
>
> More harmful to security than acknowledging that either participant
> has the freedom to choose the policy that works best for them, is
> restricting their choices to the point of making the use of security
> mechanisms too burdensome to deploy.
>

The problem with this mechanism is that it is denying the recipient the
right
to choose what works for them.

-Ekr