On Tue, Nov 10, 2015 at 01:02:19PM +0000, Ian Eiloart wrote:
> > On 3 Nov 2015, at 16:32, Viktor Dukhovni <exim-users@???> wrote:
> >
> >> https://letsencrypt.org/getinvolved/
> >
> > On port 25 CA-issued certificates are pointless. Opportunistic
> > TLS does not check them.
>
> But it could. And Exim could include flexible configuration such that certain domains required more rigorous checks.
Checking is easy, making that check add meaningful security is
not possible.
https://tools.ietf.org/html/rfc7672#section-1.3
> > https://tools.ietf.org/html/rfc7672#section-1.3
> >
> > Self-signed certificates work better on port 25. Their "expiration"
> > need not create regular pre-scheduled outages.
>
> Let�s Encrypt offers 90 day certificates, but with automated renewal: so
> no intervention is required. Of course, it's possible that renewal
> mechanisms will fail, so the renewal system would require an alerting
> mechanism.
That renewal breaks TLSA records (which can actually secure SMTP
TLS) unless it is combined with pre-publication of TLSA record
updates, and an approriate delay in certificate and private key
deployment (to allow all cached copies of previous TLSA records to
time out). As tools for that don't exist yet, deploying LE for
SMTP would do more harm than good.
> > Just make them
> > last 100 years, and do key rotation when you're good and ready,
> > not when some certificate is pre-programmed to "expire".
I stand by this recommendation, combined with DNSSEC and properly
managed TLSA records (that are updated in a planned manner a few
days before any key rotation). With certificates not auto-expiring,
there are no emergency updates.
--
Viktor.
