> On 18 Jun 2016, at 14:35, Aaron Zauner <[email protected]> wrote:
> 
> 
>> On 18 Jun 2016, at 13:59, Viktor Dukhovni <[email protected]> wrote:
>> 
>> On Sat, Jun 18, 2016 at 01:53:20PM +0800, Aaron Zauner wrote:
>> 
>>> RFC6844 defines a method by which domain owners can limit the CA allowed
>>> to issue certificates for their domain.
>> 
>> Critically, this signalling channel is *exclusively* between the
>> domain and any CA that might consider issuing a certificate for
>> the domain.  It MUST NOT be used by relying parties.
>> 
>> Unfortunately, the CA/B forum voted to make support for this
>> optional, so this standard is stillborn.
> 
> Thank you for the information. I'm not very familiar with this standard nor 
> it's history, so that's much appreciated.
> 
>> 
>>> As far as I can tell this isn't widely implemented in DNS Daemons (KnotDNS
>>> and Bind9 [urgh]) do have support though. Is this something that might
>>> make sense including in the MTA-STS document?
>> 
>> See above, CAA does not apply to relying parties, and has no
>> relevance to STS.
> 
> Yea, I *was* thinking it could be mentioned in the security considerations 
> section as additional protection - if that makes sense.

That being said; an option to pin to the public key of a certain intermediate 
CA is far more useful, with the caveat of roll-over and broken/bouncing mail 
transfer.

Aaron

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