Re: [Trans] CT for opportunistic STARTTLS in SMTP

On 25 February 2014 22:13, Trevor Freeman
<trevorf@exchange.microsoft.com> wrote:
> The point with opportunistic TLS is its OPPORTUNISTIC, if there is any
> failure it will fall back to send without TLS. In opportunistic TLS the
> sender does no checks whatsoever on the certificate so why would it care
> about CT?

Then perhaps the word "opportunistic" should be removed from the
question. What is the relevance of CT to SMTP+STARTTLS?

>
>
>
> Besides the whole STARTTLS negation is already subject to downgrade attacks.
> Some firewalls actively block the STARTTLS negotiation.
>
>
>
> From: Phillip Hallam-Baker [mailto:hallam@gmail.com]
> Sent: Tuesday, February 25, 2014 1:48 PM
> To: Trevor Freeman
> Cc: Paul Hoffman; Ben Laurie; trans@ietf.org; Daniel Kahn Gillmor
>
>
> Subject: Re: [Trans] CT for opportunistic STARTTLS in SMTP
>
>
>
> The idea seems to be that there is an append only log server that clients
> are gong to be checking. So we can use that to stuff security policy
> information into the system.
>
>
>
> The problem is that the client only checks the CT logs in the normal case
> after it has decided to use TLS.
>
>
>
> So there is certainly a value in using an append only log to publish
> security policy data and in fact I do this in PPE. But it is only a solution
> to the downgrade attack problem with a lot of extra infrastructure. For
> example an Omnibroker scanning the CT log and using that to build the
> connection profile.
>
>
>
>
>
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>
> On Tue, Feb 25, 2014 at 2:47 PM, Trevor Freeman
> <trevorf@exchange.microsoft.com> wrote:
>
> I don't see the relevance of CT to opportunistic STARTTLS.
>
> Opportunistic STARTTLS is a feature of the sender whereby the sender picks
> STARTTLS if offered, but otherwise will send the email.  If the alternative
> was send unprotected over plain TCP, you may as well negotiate TLS if
> offered.  Moreover, if TLS negotiation fails for whatever reason, the send
> remembers the fact and done not attempt to negotiate next time.
>
> The sender does have a list of SMTP domains where it requires TLS
> authentication, but that is mandatory  STARTTLS.
>
>
> -----Original Message-----
> From: Trans [mailto:trans-bounces@ietf.org] On Behalf Of Paul Hoffman
> Sent: Tuesday, February 25, 2014 9:54 AM
> To: Ben Laurie
> Cc: trans@ietf.org; Daniel Kahn Gillmor
> Subject: Re: [Trans] CT for opportunistic STARTTLS in SMTP
>
> On Feb 25, 2014, at 9:36 AM, Ben Laurie <benl@google.com> wrote:
>
>>> 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.
>>
>> That does not seem effective to me.
>
> It is more effective than doing nothing; it may not be effective enough to
> prevent overwhelm by spam. I was just pointing it out as something that was
> proposed, not well-thought-out.
>
> --Paul Hoffman
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