(Sending from the right e-mail this time)
Thanks for the responses! I think this is a great thing to bring here,
because there are some interplays with policy and implications that can
affect the design, as I discuss below. I'm trying to be mindful of
proffering solutions outside of the TLS-WG, so mostly, I'm trying to
provide context and framing and create some supporting paper trail. Totally
happy to engage in the TLS-WG if there are substantive technical changes to
address the specific policy concerns.
On Fri, Mar 8, 2019 at 7:16 PM watson--- via dev-security-policy <
> > I ask, because in the context of HTTP Signed Exchanges
> > ,
> > there was an effort to introduce additional validation requirements, most
> > notably, the explicit consent and opt-in by a site (via CAA) and a policy
> > expectation encoded in the spec that CAs SHALL NOT issue unless that CAA
> > constraint is satisfied. While in the case of HTTP Signed Exchanges,
> > represent an extension of capability, and especially the ability to be
> > in the absence of a direct connection to the authoritative origin (as
> > determined by DNS), it would be useful to know what sort of
> > have been made - whether it's ruling out particular concerns or including
> > particular concerns (e.g. the inclusion within Section 3.2 of the draft
> > the delegationUsage extension)
> Are there specific concerns that you think should be addressed? Note that
> any substantive discussions of changes beyond the editorial should take
> place on the TLS-WG list.
I was trying to capture a request to understand what the thinking had been.
The design shape tends to evolve over time, and various considerations may
be raised and ruled out, or result in design changes. Typically in the IETF
drafts, the "Security Considerations" tend to focus on "Here's the concerns
with the document in the current state", but I suppose I was trying to
understand if there is more documentation on "Here were the concerns that
motivated this particular decision", if there were any. It may very well be
that various decisions were made by throwing darts at a dart board ;)
The discussion of validation requirements naturally lead to a desire to
understand more of the reasoning about how the conclusion was reached, not
necessarily a disagreement with the conclusion. HTTP Signed Exchanges took
a similar approach, but reached different conclusions. Those conclusions
may have been (and likely are) due to their very different threat models,
but understanding whether all this had been discussed and bashed out and
reasoned about it just as useful as understanding the conclusion (i.e. that
no additional validation is required), since that's what gives CAs and the
community confidence that it's a sensible conclusion.
I don't think these are things that necessarily need to be addressed in the
draft; in many ways, they're answers to the policy questions that the
technical draft raises.
> Excellent editorial note! As for why not mark critical we want these
> certificates to also work
> for clients that do not support the extension. I can imagine a situation
> where a certificate is on a low-performance HSM, and delegated credentials
> are usually used, but then there is the occasional fallback.
> The expectation is that certificates bearing the delegatedUsage extension
> would not need to be revoked as they would not have been compromised. This
> of course depends on the details: if a webserver was happily serving up the
> entire disk, private keys included that otherwise were living in a
> different process, then the certificates would be compromised and hence
> need to be revoked.
I think these two things may be incompatible. Given that it's not a
critical extension, and thus MAY be used to terminate the connections, from
a policy perspective, CAs would likely need to assume the key "probably" is
compromised, and thus revoke. This means it doesn't really address the
Heartbleed scenario - the certificates still end up revoked.
Making the extension critical provides a greater signal to CAs that it's
unlikely to have been used on the server (since conforming clients would
have rejected it), and thus make it less likely to need to revoke the
extension-bearing certificates vs the 'traditional' certificates.
Not to pick on Cloudflare, but since they ran into a large revocation event
re: Heartbleed, it actually makes a great example. If Cloudflare had been
using delegated credential-enabled certificates for both DC-enabled and
non-DC-enabled clients, then the safest path a CA could take to defend
themselves against an adverse interpretation of the BRs or Mozilla Policy
would be to revoke these certificates. If such certificates were unusable
for non-DC-enabled clients, then there's stronger protocol-level indicators
that such certificates would not have had their private keys exposed (only
the DC keys would have been exposed), and thus it wouldn't be necessary to
revoke the DC-enabled-certificates.
Hopefully that makes sense; there's some interplay here between the policy
implications and expectations of revocation, particularly around
Heartbleed-like events, and what DC is trying to achieve. While DC (and
things like keyless SSL) MIGHT mean the certificates and keys were less
likely to be exposed for a given Subscriber/certificate, the conservative
CA would take the conservative approach of assuming they had been, and
require a key rotation regardless.
> I do not understand what 22.214.171.124(b) is saying as applied to this matter.
> What assertion is being made about what with this OID in place? I think
> what you are saying is this OID should be opt-in on the part of the
> Applicant and the BRs require it to be opt-in, and the security analysis
> might be assuming this.
The thinking here is this: Reading the BRs in a very conservative
perspective, CAs need to be very careful about what additional extensions
they include within a certificate, both as a matter of policy and as it
matters to compliance with the BRs. A CA including an extension not
enumerated in the BRs bears the burden of proof to demonstrate why the
inclusion is permissible by the BRs. 126.96.36.199 places restrictions on the
types of extensions, and thus the CA has to demonstrate that both
conditions have been satisfied, even under a maximally pessimistic
Given this framing, my hope was to get a clear interpretation on the record
for why it would be OK for CAs to include such an extension, and how they
could demonstrate both criteria have been satisfied, even under a
pessimistic reading. By discussing it here on m.d.s.p., that then makes it
easier for CAs to point back to the public discussion and the
interpretation that was discussed and the consensus reached, and thus
allows a CA to respond to anyone that might file an incident report
claiming non-compliance with the BRs (due to including the extension) with
Having that discussion now makes it easier for CAs wanting to adopt these
certificates to have confidence that including it is defensible. :)