[Trans] CT for opportunistic STARTTLS in SMTP

[Paul Hoffman <[e-mail]>]

Sorry for using a valid subject line. :-)

On Feb 24, 2014, at 11:28 PM, Ben Laurie <[e-mail]> wrote:

> On 24 February 2014 21:43, Daniel Kahn Gillmor <[e-mail]> wrote:
>> On 02/24/2014 04:25 PM, Melinda Shore wrote:
>> 
>>> As for relevance, right now therightkey is the best place
>>> for discussion of other approaches to fixing PKI, while trans
>>> is specifically for discussion of certificate transparency.
>>> The only thing that's in our charter at the moment is 6962bis.
>>> That doesn't mean that other applications of CT are out-of-
>>> scope, but that we'd need to recharter to take them on
>>> as work items.
>> 
>> I think you're saying you want the slot in London to focus on getting
>> the mechanism right, and not trying to propose policy, which is
>> completely reasonable.  I'm happy to stay focused.
>> 
>> There's nothing in RFC 6962 (and i hope there won't be in 6962bis) that
>> is HTTPS-specific, though; it's defined as a mechanism for logging X.509
>> certificates for use in TLS, regardless of the application layer traffic
>> within the TLS session.
>> 
>> So i hope that the use of CT in SMTP+STARTTLS isn't seen as an "other
>> application" -- it's still TLS.  If we suspect that CT is somehow valid
>> only for X.509 certs used by HTTPS servers, we should make that more
>> explicit in the draft (but i hope we don't!)
> 
> I agree. One observation: CT as applied to HTTPS uses the CA signature
> as a spam limitation mechanism.
> 
> I believe most SMTP certs are not CA issued, so the question arises:
> how would you propose to limit spam?

At the earlier CT meeting, I think someone proposed that there could be a check that the cert was in actual use at the place it said it was.

> I am open, by the way, to running CT logs at Google with alternate
> spam limitation mechanisms to allow this kind of usage. I think that
> not having a spam limitation mechanism is dangerous.

Spam limitation is needed for both attacks on CT to weaken it and accidental misconfiguration that could look a lot like an attack.

--Paul Hoffman