[Servercert-wg] Compromised/Weak Keys Ballot Proposal

Fri Mar 29 08:55:14 UTC 2024

Could we limit the Debian Weak keys to key sizes up to RSA 4096 bit? I don’t think that anybody “accidentally” creates an 8192 bit RSA key on a system vulnerable to Debian Weak keys.

Kind regards
Roman

PS: Can somebody explain, why we only test close primes with 100 rounds and not e.g. 1000? 😉

From: Servercert-wg <servercert-wg-bounces at cabforum.org> On Behalf Of Wayne Thayer via Servercert-wg
Sent: Freitag, 29. März 2024 00:14
To: CA/B Forum Server Certificate WG Public Discussion List <servercert-wg at cabforum.org>
Subject: Re: [Servercert-wg] Compromised/Weak Keys Ballot Proposal

There was further discussion of this ballot proposal on today's teleconference. It was suggested that rather than omitting any reference to Debian weak keys, the ballot should retain the current language. Unfortunately, the ballot restructures the language in such a way that this is challenging. My new proposal is that TLS BR Section 6.1.1.3(5) be updated to read as follows (https://github.com/wthayer/servercert/pull/1/files):
=====
5. The Public Key corresponds to an industry-demonstrated weak Private Key. For requests submitted on or after November 15, 2024, at least the following precautions SHALL be implemented:
    1. The CA SHALL reject Debian weak keys (https://wiki.debian.org/SSLkeys).
    2. In the case of ROCA vulnerability, the CA SHALL reject keys identified by the tools available at https://github.com/crocs-muni/roca or equivalent.
    3. In the case of Close Primes vulnerability (https://fermatattack.secvuln.info/), the CA SHALL reject weak keys which can be factored within 100 rounds using Fermat’s factorization method.

    Suggested tools for checking for weak keys can be found here: https://cabforum.org/resources/tools/
=====
I would greatly appreciate feedback from any member that finds this language unacceptable, or that has suggestions to improve it.

Thanks,

Wayne

On Fri, Mar 15, 2024 at 11:19 AM Wayne Thayer via Servercert-wg <servercert-wg at cabforum.org<mailto:servercert-wg at cabforum.org>> wrote:
On yesterday's SCWG teleconference, Mads suggested that a way forward would be to leave the existing requirements in place for Debian weak keys. I've interpreted that to mean that we would just remove references to Debian, resulting in this: https://github.com/wthayer/servercert/pull/1/files

I'm not satisfied by this approach because it doesn't achieve the clarity intended to result from the original weak keys ballot and will seemingly result in CAs continuing to have varying interpretations of the specific requirements for rejecting Debian weak keys, but perhaps this is the best we can all agree to.

What  do others think? Should we proceed with this approach?

Thanks,

Wayne

On Sat, Mar 9, 2024 at 9:15 AM Dimitris Zacharopoulos (HARICA) via Servercert-wg <servercert-wg at cabforum.org<mailto:servercert-wg at cabforum.org>> wrote:
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.

Dimitris.
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<mailto: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 4.9.1.1(4) 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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