[Servercert-wg] Compromised/Weak Keys Ballot Proposal

Dimitris Zacharopoulos (HARICA) dzacharo at harica.gr
Sat Mar 9 16:15:24 UTC 2024

FWIW, I think in the recent years, it was mostly security researchers 
that attempted to request certificates with Debian weak keys to test if 
a CA was properly blocking them.

If an Applicant uses an outdated OS that generates weak keys, imagine 
the actual web server or other software that runs on that server, 
putting Relying Parties at risk. CAs don't have control over that but 
they could have control over a common set of weak keys using common 
parameters/algorithms which could be enforced by all CAs.


On 9/3/2024 12:05 π.μ., Wayne Thayer via Servercert-wg wrote:
> Hi Clint,
> Thank you for your response. Unfortunately, it leads me to the 
> conclusion that there is not a path forward and we're stuck with the 
> status quo. Having said that, I'll reply to a few of your points below 
> and encourage others to do the same if there is a desire to move 
> forward with an update to our weak keys requirements.
> On Thu, Mar 7, 2024 at 12:59 AM Clint Wilson <clintw at apple.com> wrote:
>     Hi Wayne,
>     Thank you for carrying this work item forward. I have a few
>     concerns regarding the proposed removal of Debian weak key
>     checking, outlined below.
>     I don’t believe there has been sufficient explanation or data
>     presented to justify the removal of the requirement to check for
>     Debian weak keys. It seems to me there are feasible methods for
>     CAs to continue performing this check. Similar to what Martijn has
>     pointed out, the reasoning provided is lacking (hasty
>     generalization, fallacy of composition, etc.).
> The argument that I find compelling is that any system capable of 
> generating a Debian weak key in 2024 is also riddled with a decade of 
> vulnerabilities, so preventing the use of said weak key in a 
> certificate is security theater. In what scenario do you envision the 
> rejection of a Debian weak key having a meaningful impact on the 
> security of a service that relies on it? It's just not obvious to me 
> that these checks continue to provide any practical value at this 
> point in time.
>     I don’t believe a compromise where Debian weak keys only need be
>     checked for specific key sizes addresses the core issue, unless
>     tied together with a restriction from accepting key sizes which
>     are not included in such a list (which I do see as reasonable and
>     something I’m under the impression is already being done by some CAs).
> My understanding is that the objections some CAs had to the original 
> ballot was the requirement to maintain a sizable database of Debian 
> weak keys for every key size they support. Limiting the requirement to 
> the most popular key sizes would resolve that issue and be more inline 
> with my understanding of some current practices. This approach would 
> also greatly reduce the threat of the accidental use of a Debian weak key.
>     The removal of this check seems to shift a burden currently placed
>     on CAs to a risk (long assumed resolved) for Relying Parties and
>     Subscribers. From my reading of the ballot, the requirement that a
>     CA revoke Certificates with Debian weak keys remains in effect,
>     serving only to remove the pre-issuance “blocking” requirement,
>     but retaining an expectation that certificates are checked
>     post-issuance based on the CA’s awareness of this method of
>     compromising a Private Key; this generally seems a significantly
>     worse experience for Subscribers. Have I missed something
>     regarding the intent of the changes in this regard?
> This is a great point. It makes no sense to allow a CA to issue a cert 
> that is then immediately subject to a revocation requirement. I am 
> open to either exempting Debian weak keys from BR or for 
> CAs to be required to revoke a certificate containing a Debian weak 
> key only if they are "made aware" using the mechanism specified in 4.9.3.
> Thanks,
> Wayne
>     There have been incidents involving certificates issued to Debian
>     weak keys in recent years; some CAs have indicated that they don’t
>     receive Debian weak keys in requests, but evidence exists to the
>     contrary for the ecosystem as a whole.
>     Thank you!
>     -Clint
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> Servercert-wg at cabforum.org
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