Re: [Acme] kinds of proof

Paul Hoffman <paul.hoffman@vpnc.org> Fri, 28 November 2014 21:57 UTC

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From: Paul Hoffman <paul.hoffman@vpnc.org>
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Subject: Re: [Acme] kinds of proof
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On Nov 28, 2014, at 1:37 PM, Viktor Dukhovni <ietf-dane@dukhovni.org> wrote:
> 
> On Fri, Nov 28, 2014 at 10:37:01AM -0800, Paul Hoffman wrote:
> 
>>> * Does the web server have to be on port 443 with TLS enabled
>>>   (perhaps with a temporary self-signed certificate)?  Or is it
>>>   sufficient to control port 80 or some other port with or without
>>>   TLS?
>>> 
>>> * If on port 443, is an existing certificate from any of the
>>>   usual public CAs (other than LE) (if also usable as a TLS client
>>>   cert to authenticate to the provisioning system) accepted as
>>>   evidence of authenticity for bootstrapping trust with LE?
>> 
>> Why should ability to control port 80 or 443 be a determinant
>> for proving control of "web server" at "domain name"? Not all web
>> service is run on port 80, and not all secure web service is run
>> on port 443. A CA can validate that the requestor can control some
>> HTTP service on some port at the domain name. If a CA has a policy
>> that is more restrictive than that, ACME can trivially support that
>> policy with more error codes.
> 
> Because unfortunately, Web PKI certificates are host-wide, they don't
> specify a port.  Anyone who can run some program on a machine can
> bind to some random port and start a web service.  Possibly port-forwarded
> somewhere else via SSH!
> 
> It is far from clear to me that every "shell" user of a machine
> should be authorized to obtain certificates for the whole machine.

And it is clear to me that they should be, if we want to see more encryption of traffic. I have no problem with some CAs saying "we'll issue you a cert only if you control port X", but I absolutely want that to be a policy of the CA, not of the enrollment protocol.

>>> * If the domain is DNSSEC signed, is it reasonable (I hope)
>>>   to raise the bar, and require proof of DNS control?  Since
>>>   bootstrapping with just web server control and no existing
>>>   credentials is vulnerable to MiTM, and signed domains are
>>>   making an effort to harden against such attacks?
>> 
>> I disagree that ACME-the-protocol should raise the bar past what
>> we use for domain verification today. It's fine if a CA who uses
>> ACME does.
> 
> The protocol is just a syntax, and indeed I asking a question about
> appropriate CA policy.  Perhaps some guidance to CAs implementing
> the protocol would not be amiss.

If you want to write such a thing as a separate document whose target audience is CAs, that's grand. It does not belong in the protocol.

--Paul Hoffman