[cabf_validation] New TLS-ALPN Validation Method

[Doug Beattie]

Hey Aaron,

 

If there is a concern with expanding existing methods to support this, then perhaps we should create a new one specifically for this.  That way we could enumerate the permitted ports (and other rules/checks) for this method without opening up existing methods to possible weaknesses.  Maybe that was your intent towards the end of your email, but was not sure.

 

Doug

 

 

From: Validation <validation-bounces at cabforum.org> On Behalf Of Aaron Gable via Validation
Sent: Friday, October 28, 2022 12:25 PM
To: Ben Wilson <bwilson at mozilla.com>; CA/Browser Forum Validation SC List <validation at cabforum.org>
Subject: Re: [cabf_validation] New TLS-ALPN Validation Method

 

A reply on that thread does bring up the very good point that it has (appropriately) become harder to add new validation methods such as variants on ACME's TLS-ALPN-01 because they can no longer fall under "any other method".

 

It seems like support for this at the BRs level would require two changes:

First, we'd need to expand the definition of "Authorized Port" to include any port pointed at by an SVBC, HTTPS, or other future SVBC-compatible Resource Record's "port" SvcParamKey.

Second, we'd need to expand BRs 3.2.2.4.20 (TLS Using ALPN) to not solely reference RFC 8737 (ACME TLS-ALPN-01), but instead operate more like BRs 3.2.2.4.7 (DNS Change), allowing multiple different specific implementations to comply as long as they meet certain criteria. Perhaps in this case the criteria would place limitations on the ALPN protocol negotiated, and the contents of the certificate presented in the TLS handshake.

 

These are both changes that explicitly widen the scope of validation methods acceptable under the BRs, which goes against the general desire to make validation methods stricter in all ways. So they'd need to be carefully considered. But it may be worthwhile to establish a generic "TLS Using ALPN" that multiple implementations can satisfy, similar to how "DNS Change" works today.

 

Aaron

 

On Fri, Oct 28, 2022 at 1:08 AM Ben Wilson via Validation <validation at cabforum.org <mailto:validation at cabforum.org> > wrote:

Thoughts with regard to the following?

 

https://mailarchive.ietf.org/arch/msg/acme/dIfbBLij_SCeXKoE47tpIVkavTs/

Right now, most of ACME’s validation methods can only be used by clients with IP addresses in A/AAAA records corresponding to the identifier, as well as specific open ports. This is perfectly acceptable for most use cases right now, but it becomes problematic when managing certificates for the likes of HTTP alternative services or SVBC/HTTPS targets. Such configurations require a certificate for the original identifier, but (usually) do not share the same IP addresses.
 
dns-01 sidesteps this limitation, but is often less secure since it usually requires credentials for DNS zone modifications to be accessible by clients.
 
I don’t think it is too early to start thinking about more practical solutions, in advance of draft-ietf-dnsop-svcb-httpssvc being finalized. Perhaps a new form of TLS-ALPN method that uses an SVBC/HTTPS record instead of 443/tcp and A/AAAA records? It would need to ignore the normal precedence rules, as they would preclude lower-priority targets from getting certificates.

 

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