[Servercert-wg] Transitive Trust and DCV (was Re: Ballot SC-080 V1)

[Dimitris Zacharopoulos (HARICA)]

Hi Mike,

On 19/9/2024 5:22 μ.μ., Mike Shaver via Servercert-wg wrote:
> Hi Dimitris,
>
> I've been thinking about your email all night, and I want to figure 
> out where our reasoning about the integrity of DCV via WHOIS-1932 or 
> HTTP-nonce (3.2.2.5.1) diverge. I was quite surprised by your 
> statements about the properties of internet security, the 
> responsibility of TLS and SCWG, and the limitations that should be 
> accepted in performing DCV. I'm hoping that your new responsibilities 
> as Chair (congratulations!) will still leave you a bit of time to 
> point out where my reasoning doesn't connect. :)

Thank you for that :) New officers will start their duties on 2024-12-01 
but I believe Member's contributions to the Forum are a lot more 
important when we're having these discussions, working on ballots and 
continuously improving the standards! Officers mainly have to do the 
"administrative" work and make sure the WG Charter and Bylaws are followed.

>
> Because I'm not sure where we diverge, I'm going to walk through my 
> understanding of the principles underlying the DCV methods used 
> historically by CA/BF (and CAs in the dark days of private deals with 
> individual browsers), and the direction towards which the SCWG and 
> root programs seem to be headed in terms of those principles. Please, 
> *please* do not take this as me implying that you don't know these 
> things: I'm not trying to lecture, but to be as explicit as reasonable 
> in explaining my thinking, so that it's easiest for yourself or others 
> to help me correct error in it. I would be quite grateful for such 
> generosity, honestly! I also apologize in advance for the length of 
> this message. I lack the wisdom to make it smaller, perhaps.
>
> (Some of this is entangled with "what is this even for?" sorts of 
> discussion about who the web PKI should ultimately benefit, but I've 
> tried to keep that separate.)
>
> On Wed, Sep 18, 2024 at 12:18 PM Dimitris Zacharopoulos (HARICA) 
> <dzacharo at harica.gr> wrote:
>
>     We don't need Domain Name Registrars to go through WebTrust or
>     ETSI audits suitable for Trust Service Providers. These Registrars
>     are the source of truth for the DNS on which all Internet
>     connections, and the WebPKI relies on. It's so fundamental to the
>     ecosystem that IMO it doesn't make sense to ask ourselves how this
>     Forum can make them better. Other authorities should be working on
>     that.
>
>
> A recurring theme throughout web PKI operations has been that a 
> participant in the ecosystem thinks that they are doing a good job, 
> and perhaps indeed are doing a good job of some kind, but are not 
> providing the guarantees or protections that others are assuming are 
> in place. I don't mean "good job" in the sense of competence or 
> ethics, but rather "meeting the requirements through actions". The 
> farther that the participant is from the core of the web PKI 
> ecosystem, which is to say the less likely they are to evaluate all 
> their actions by the effects of those actions on the trustworthiness 
> of web certificates and server authentication. This means, IMO, that 
> this group and other stewards of the PKI should be very conservative, 
> and very explicit, when depending on another participant of the 
> ecosystem to maintain certain properties. Otherwise, those 
> dependency-bearing organizations may not even know what is being 
> expected of them.
>
> I am not suggesting that CA/BF in any way place requirements on what 
> an organization must do in order to be a domain registrar, and 
> reporter of that registry. As you say, that is beyond both the scope 
> and the expertise of this group, and this group has plenty to keep it 
> busy! I am, rather, suggesting that SCWG should establish criteria 
> against which it will determine if a registrar's publication of domain 
> information should be considered to be reliable enough to accomplish 
> DCV. Similarly, the group does not put any requirements forcing public 
> DNS servers to strictly check DNSSEC, but if a CA is going to use a 
> DNS server for checking CAA records or doing 3.2.2.4.7 "DNS Change" 
> validation, then that's only acceptable if the DNS server enforces 
> DNSSEC in the presence of a validation chain. (I think that might only 
> be in Bugzilla and not yet captured in the BRs, but it's well-enough 
> understood that failure to adhere to it has required reissuance.)

I see your point. In general, as you say, it is difficult for a group 
like the CA/B Forum to enforce rules and expectations placed upon 
another industry group (Domain Name Registrars/TLD Operators) if those 
other industry groups do not participate or don't even have knowledge 
about those expectations. I would be very concerned if the SCWG or the 
Forum at large, would create a set of "expectations from a Registrar/TLD 
Operator to be considered good-enough for the WebPKI to rely upon". I 
think this group would be heavily criticized for doing that.

If we believe this area is so security-critical for the WebPKI, the best 
think we could do is to have members of this Forum engage with IANA or 
other ccTLD-coordinating venues to promote ideas for improved security, 
external monitoring and transparency. It feels similar to what Members 
of this Forum have done in the past when we bring up ideas for improving 
the security of our ecosystem, and bring those ideas to IETF (LAMPS WG 
or other similar venues) to standardize.

I'm not opposed to adding requirements that prohibit the use of external 
information in egregious failure cases, but it's not easy to find the 
right language in a standard for this.

>
>     If a ".cookoo" TLD operator is not functioning properly, then the
>     entire TLD is in jeopardy and every Domain owner under that TLD is
>     at risk. Certificates are the least of people's problems when
>     relying parties connect to websites operated under that unsafe TLD
>     operator.
>
>
> As above, this depends very much on exactly what form "not functioning 
> properly" takes. If their systems are broken such that it's impossible 
> to make changes to registrations, or to look up registrations, I think 
> we would all agree that the TLD operator is not functioning properly, 
> but that would not have as much risk to the integrity of the web PKI 
> as those systems being broken such that *anyone* can change any TLD.
>
> For CAs to rely on email address information from registrars for 
> issuance of certificates, I think it's only appropriate that they 
> ensure that the information is "fit for purpose" and that it's managed 
> (and accessed) in a way consistent with the level of trust placed in it.
>
> This means making sure that the integrity of the response is 
> maintained, which is a role that WHOIS-3912 simply cannot perform.
>
>     No, the SCWG and TLS is not here to solve the unencrypted nature
>     of the DNS protocol. IETF and DNSSEC is. There is a great number
>     of Domain Names in the DNS without DNSSEC, and there is still
>     heavy reliance on the unencrypted DNS protocol in almost EVERY
>     Domain Validation under 3.2.2.4.
>
>
> DNS being unencrypted is not a problem, and indeed even DNSSEC doesn't 
> encrypt traffic. DNS results being *unauthenticated* is a problem for 
> clients who wish to be certain that they have been given an authorized 
> address in response to their lookup, but even with DNSSEC that leaves 
> the issue of ensuring that the unauthenticated IP layer reached the 
> "real" home of that address (thus DANE).
>
>     Even the Agreed-upon change to website method, 3.2.2.4.18, relies
>     on "Authorized Ports that are offered via non-encrypted channels
>     (ports 80, 25, 22).
>
>
> Again, encryption during validation is not necessary for there to be a 
> reliable chain of trust from the browser to the site certificate, via 
> CA-operated root certificates. We need authentication, but only 
> authentication, of every step of the delegation of trust.
>
>     I would go as far as to say that even the ACME methods connecting
>     to https URLs are untrusted, because the endpoints are not
>     protected by publicly trusted certificates and anyone could launch
>     a MiTM attack.
>
>
> This is a part that really got stuck in my head. Are you saying that 
> ALPN-1 is vulnerable to a MITM attack during validation? That would be 
> a pretty shocking situation, in my opinion!
>
> How would the attacker get access to the key material needed to 
> complete the challenge? It never leaves the subscriber machine from 
> what I can tell. Similarly, the ACME account key is used in HTTP-01 to 
> render MITM attacks ineffective.
>
> This is how the trust in that connection is bootstrapped based on the 
> trust in the connection between subscriber and CA when the certificate 
> is requested. Presumably publicly-trusted certificates are used when 
> the subscriber connects to the CA's server to make that request and 
> obtain the account key!
>
> This transitivity of trust goes to the issue with WHOIS. Even if the 
> registrar is maximally diligent in their key ceremonies and internal 
> processes and generating certificates correctly, WHOIS-3912 puts a 
> *maximum* trustworthiness on the entire operation, which is that of 
> unauthenticated TCP. In my opinion, that is not appropriate for web 
> PKI, and is a relic of a time before this community took technical 
> registration-time attacks as seriously as we do now.

I consider Andrew's response 
<https://lists.cabforum.org/pipermail/servercert-wg/2024-September/004883.html> 
a lot clearer than mine. I believe it answers your questions and how to 
MiTM the ACME TLS-ALPN-01 challenge.


>
>     In order for the SCWG measures to be proportionate, we should not
>     blame the entire WHOIS protocol but work on additional controls to
>     minimize the risk of CAs using those problematic WHOIS libraries.
>
> Could you describe how a non-problematic WHOIS-3912 library could 
> provide assurance of the validity of the data returned, equivalent to 
> the integrity of the results from HTTP-01/DNS-01/ALPN-01?

Assuming no MiTM, it's no different than any of those methods. The CA 
"trusts" that the information is coming from an authoritative source or 
from a source that demonstrated control of the Authorization Domain 
Name. In the HTTP-01 case, it retrieves a random value or request token 
from a designated URL that contains the FQDN to-be-included in the 
Certificate. In the WHOIS case, it retrieves the email address of a tech 
or admin contact associated with the registration of that Base Domain Name.

The .mobi issue, in my understanding, is that some CAs were (still are?) 
using WHOIS libraries that were not looking for the currently 
authoritative Registrar of that TLD. If the SCWG could identify those 
insecure libraries, or if criteria were added to use libraries that 
check for the most recent authoritative source for each Base Domain Name 
Registrar, it would be a good emergency mitigation.

We had a similar discussion when discussing the linting tools. At some 
point we need to add rules for continuous updates, taking into account 
proper testing and change management procedures.

>>     Instead, we could focus on requiring immediate/emergency measures
>>     for CAs to use the WHOIS protocol securely
>>
>>
> This is definitely one place that our reasoning diverges: I don't see 
> a way to use WHOIS-3912 securely, where by "securely" I mean "in such 
> a way as to not weaken the DCV guarantees that the web PKI wishes to 
> make".
>> I disagree. We are trying to move from "http" to a "trustworthy 
>> https" (note: "trustworthy https" in the sense that it doesn't use 
>> untrusted certificates). At some point, CAs need to rely on 
>> unencrypted communication to achieve that.
>>
> Why do CAs need to rely on unauthenticated communication to achieve 
> that? If we were establishing the very first root, someone would have 
> to carry the key physically to the browser developer as a form of IRL 
> authentication, but we have sufficient systems in place now to 
> inductively create a completely authenticated chain of validation if 
> we should want to.
>
> Where is that chain impossible to authenticate, in your opinion?

Andrew's answer covers this very clearly IMO. This is just how it works. 
There is no single source of truth or "one key to rule them all" that 
everyone trusts. The closest we have is the DNS.

>
>     I'm confused by this statement. Is this a plea to the CAs to stop
>     using what you think is an insecure method?
>
>
> Yes, it is exactly that. I don't know how anyone can seriously claim 
> that WHOIS-3912 is a secure method, regardless of how high quality the 
> data on the other side is.

Then the second sentence of my snipped quoted statement applies:

"Everyone is entitled to an opinion but that's why we are having these 
discussions publicly so that the SCWG members can find "substantial 
consensus" as mandated in the Bylaws. I'm sure some CAs are already 
working, or have already stopped using WHOIS, proactively, until this 
discussion comes to an end."

>>     Domain Registries for validation of domain contacts: domain
>>     registry information should, IMO, only be used at *all*,
>>     independent of protocol, if the SCWG can be confident that IANA
>>     or another trusted body will be able to ensure that all those
>>     registries, for all domains present and future, will meet the
>>     SCWG's requirements for reliability.
>>
>     This is like saying that the Registrars, the main stewards
>     assigned to run the DNS which is fundamental for how the Internet
>     works and practically the World Wide Web, need to meet the SCWG
>     requirements for reliability.
>
>
> As above, domain registrars do not need to meet any SCWG requirements. 
> CAs need to meet the SCWG requirements, and I think that those 
> requirements should include minimum properties that must hold of a 
> registrar's data management practices, and the means of accessing it, 
> if such data is to be used as authoritative proof of domain control.
>
> An example: do registrars inform registrants that anyone who can 
> receive email at the listed contact address can be issued a 
> certificate for their domain? I think that the significance of that 
> address as used for DCV is not well-understood by most registrants, 
> and that many will send that email to ticket/CS lists composed of 
> people who are *not* authorized to make changes to DNS, and are not 
> assumed to have the same authority for requesting certificates. 
> Similarly, I don't think that registrants understand that someone 
> popping a domain-privacy provider not only gets their email and 
> contact information, but could plausibly also silently have 
> certificates issued against their domain.
>
> If these participants in the ecosystem (registrar and registrants) 
> don't regard the email contact field as having such immense security 
> significance, then it is unlikely that they will manage it with 
> appropriate care. Given that, as you correctly point out, the SCWG is 
> in no position to put requirements on registrars or registrants, all 
> that we can do is say that CAs must not use registrar data unless it 
> is managed to an appropriate standard, and accessed in an appropriate 
> way. That is *well* within the mandate of the SCWG, and consistent 
> with all of its other activities in defining validation standards.

This is a very tough sell because there is no stop to this line of 
arguments. Should CAs also monitor the behavior of Internet Service 
Providers responsible of the Internet connectivity? Should they monitor 
Telecommunication Providers for how they run the SMS GSM networks used 
by CAs to contact Applicants? Should they monitor the Postal Services 
when sending letters to Applicants?

The publicly-trusted ecosystem (WebPKI in the SCWG) needs to rely on 
some fundamental services offered by entities outside the scope of the 
CA/Browser Forum, that are governed by their own policies and practices. 
The CABF cannot police or directly intervene with these fundamental 
services at a global level.

>> I believe the WHOIS deprecation could follow a similar pattern but 
>> for sure the SCWG should urgently focus on requiring CAs to use WHOIS 
>> libraries that query the proper Registrar endpoints, IF they are 
>> using the WHOIS method to query Domain Contact information
>>
> If CAs are going to continue to use WHOIS-3912, I think that the SCWG 
> should require that the traffic be carried over an authenticated TLS 
> channel, or that the response be signed. Anything less doesn't address 
> the fundamental insecurity of the *access protocol*, whatever the 
> truth of the data returned in the request. Do you feel that 
> unauthenticated requests over the public internet really have a place 
> in DCV?

Based on Andrew's and my answers, I hope you can see that this point 
just doesn't apply.

>
> Saying that the internet is fundamentally insecure is like saying that 
> electricity is fundamentally insecure, to me. The base protocols of 
> the internet, like IP, don't provide sufficient security properties 
> given the importance of the modern web. But just as TCP provides 
> in-order delivery over un-ordered IP, and TLS provides privacy and 
> integrity over TCP (which features neither of those, save a trivial 
> attempt at integrity aimed at signalling errors and not malicious 
> interception), the web PKI can provide authentication of domain 
> ownership and connection validity by layering appropriate protocols 
> (computer and human) atop the less-capable lower layers. That to me is 
> the essence of the mandate of the SCWG. We are to TLS what ICANN is to 
> DNS: ICANN doesn't say that you can't return arbitrary nonsense in a 
> DNS response from your server on a private network, but it *does* say 
> that if you want to operate a DNS server on behalf of a gTLD, you need 
> to meet certain requirements or ICANN simply won't point traffic at 
> you. We should "stop pointing traffic" at certificates that were 
> validated using WHOIS-3912 DCV, and I wish I'd pushed to do so 
> multiple decades ago.

Apologies for oversimplifying, but I just want to clarify what I meant. 
The Internet was built on some principles. These principles included 
anonymity and clear-text communication. Encryption and authentication 
came later, but they were built on top of the anonymous/clear-text layers.

What does "secure the Internet" mean? There are various answers to that 
statement but one answer is "the assurance that communication between 
two points (point-to-point) is authenticated and encrypted". For this 
you need cryptography but you need to solve the key distribution 
problem. One solution is the WebPKI. There are other solutions (DANE, 
VPN, etc).

In order to validate a Domain Name and verify a binding between a key 
and a name, a CA must use some parts of the unauthenticated/unencrypted 
Internet in order to get assurance that it is 
interacting/communicating/contacting the proper and authorized 
recipient. I don't think there is a way around that.

Dimitris.
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