Re: [dane] Delivery of email if MX is not signed

["Patrik Fältström " <[e-mail]>]

On 23 Aug 2015, at 20:50, Peter Koch wrote:

> If we'd only be worried about a passive attacker, we'd not
> address the spoofed MX RR, but we'd want the SMTP connection
> encrypted.  In that case (passive only), opportunistic (pre DANE)
> STARTTLS would be sufficient.
>
> If we'd worry enough about active attacks, we'd want the endpoint
> identity verified (thus DANE[1]) as well as its capabilities
> (thus DANE[2]), and at the same time we'd probably prefer (or
> even demand) signed/validated MX RRs.
>
> Apparently there are multiple potential attackers of different background,
> motivation, and means. If your enemy is the helpful corporate
> firewall that intercepts SMTP to 'optimize away' STARTTLS, maybe
> its next version will 'enhance' MX responses?

Agree.

What I think I see in the draft is that "DANE and SMTP" is either "on" or "off", and I want more shades of gray.

1. Unsigned MX, Unsigned A/AAAA, not using TLS at all

2. Unsigned MX, Unsigned A/AAAA, TLS used with self-signed cert

3. Unsigned MX, Unsigned A/AAAA, TLS used with cert signed by CA (i.e. trusted cert)

4. Unsigned MX, Signed A/AAAA, TLS used with cert signed by CA (i.e. trusted cert)

5. Unsigned MX, Signed A/AAAA, TLS used with cert validated with signed TLSA (i.e. trusted cert)

6. Signed MX, Signed A/AAAA, TLS used with cert validated with signed TLSA


I think 4 and 5 are equivalent, both better than 3 (which is better than 1, 2 and 3).

And of course there are more permutations, specifically with signed MX, but the for me interesting alternatives are the ones above, where MX is unsigned.

   Patrik