[Servercert-wg] Compromised/Weak Keys Ballot Proposal

[Tomas Gustavsson]

Parts feel a bit redundant and confusing to me. As 6.1.5 specifies key types and sizes. An EC key pair with 184 bits should never make it to this check since only NIST P-256, NIST P-384 or NIST P-521 are allowed. No other key types than RSA and EC are allowed so what are "all other key types"? Is it that if ML-DSA is added as allowed in 6.1.5 in the future a CA is expected to find a way to generate ML-DSA keys on an old Debian system? That sounds a bit hard, and these keys should be added to the repo in that case, if desired, shouldn't it?

Since adding ML-DSA seems like potentially the most likely future addition of keys, what changes are expected then?

Regards,
Tomas


________________________________
From: Servercert-wg <servercert-wg-bounces at cabforum.org> on behalf of Wayne Thayer via Servercert-wg <servercert-wg at cabforum.org>
Sent: Friday, April 12, 2024 11:35:42 PM
To: Clint Wilson <clintw at apple.com>; ServerCert CA/BF <servercert-wg at cabforum.org>
Subject: Re: [Servercert-wg] Compromised/Weak Keys Ballot Proposal

I've updated https: //github. com/wthayer/servercert/pull/1/files as follows to exclude large key sizes: In the case of Debian weak keys vulnerability (https: //wiki. debian. org/SSLkeys)), the CA SHALL reject all keys found at https: //github. com/cabforum/debian-weak-keys/

I've updated https://github.com/wthayer/servercert/pull/1/files as follows to exclude large key sizes:

In the case of Debian weak keys vulnerability (https://wiki.debian.org/SSLkeys<https://urldefense.com/v3/__https://wiki.debian.org/SSLkeys__;!!BjbSd3t9V7AnTp3tuV-82YaK!yX4W6PVddaeXFMyavnUXrc_MlL3jDlGVklxumGHAbVoCU_3aE3OKUa69ss71NlchtXX0myESs2HNlSlDLGZM5UyI8geLuHbWd_I$>)), the CA SHALL reject all keys found at https://github.com/cabforum/debian-weak-keys/ for each key type (e.g. RSA, ECDSA) and size listed in the repository. For all other key types and sizes, with the exception of RSA key sizes greater than 8192 bits and ECC key sizes greater than 521 bits, the CA SHALL reject Debian weak keys.

- Wayne

On Fri, Apr 12, 2024 at 12:44 PM Clint Wilson <clintw at apple.com<mailto:clintw at apple.com>> wrote:
Hi Wayne,

That was indeed my intent, but I’m happy with the proposal either way.

Thank you,
-Clint

On Apr 12, 2024, at 12:33 PM, Wayne Thayer <wthayer at gmail.com<mailto:wthayer at gmail.com>> wrote:

Thank you Clint and Aaron, this is helpful. Here is what I propose:

In the case of Debian weak keys vulnerability ([https://wiki.debian.org/SSLkeys)]<https://urldefense.com/v3/__https://wiki.debian.org/SSLkeys)*5D__;JQ!!BjbSd3t9V7AnTp3tuV-82YaK!yX4W6PVddaeXFMyavnUXrc_MlL3jDlGVklxumGHAbVoCU_3aE3OKUa69ss71NlchtXX0myESs2HNlSlDLGZM5UyI8geLef72tUc$>), the CA SHALL reject all keys found at [https://github.com/cabforum/debian-weak-keys/] for each key type (e.g. RSA, ECDSA) and size listed in the repository. For all other key types and sizes, the CA SHALL reject Debian weak keys.

This change can be viewed in context https://github.com/wthayer/servercert/pull/1/files

This language allows us to add key sizes in the future without updating the TLS BRs.

Clint Wilson: I did not exclude key sizes larger than 8192 RSA/521 ECDSA bits from the requirements but would be happy to do so if you will confirm that this was your intent?

Rob Stradling: I would like to import your repo to github.com/cabforum/Debian-weak-keys<http://github.com/cabforum/Debian-weak-keys>. May I have your permission to do so?

Thanks,

Wayne

On Thu, Apr 11, 2024 at 10:11 AM Clint Wilson <clintw at apple.com<mailto:clintw at apple.com>> wrote:
Hi Aaron,

Your proposed phrasing sounds good to me and matches what I had in mind as the end result of the changes represented in Set 1, just structured slightly differently.

Cheers,
-Clint

On Apr 11, 2024, at 9:47 AM, Aaron Gable <aaron at letsencrypt.org<mailto:aaron at letsencrypt.org>> wrote:

On Thu, Apr 11, 2024 at 9:12 AM Clint Wilson via Servercert-wg <servercert-wg at cabforum.org<mailto:servercert-wg at cabforum.org>> wrote:
In other words, I believe it satisfactory to establish a constrained set of Debian weak keys which CAs must block (rather than leaving the requirement fully open-ended), but I don’t believe that should obviate the need for a CA to check uncommon key sizes — which are otherwise in the key size ranges of that constrained set’s key sizes — should a CA allow those uncommon key sizes.

I completely concur.

I don't think that either of your Set 1 / Set 2 proposals quite hits the mark for me, for one reason: they both contain the phrase "CAs must not issue certificates containing Debian weak keys". As long as that statement exists, the requirement is "evaluate everything yourself, and if new sets of weak keys come to light, you're already behind" -- the existence of the github repo is just a nicety.

Instead, I would phrase the requirement as "In the case of [list of common RSA and ECDSA key sizes] Debian Weak Keys, the CA SHALL reject keys identified by [link to CABF repository]. For other key sizes, the CA SHALL reject Debian Weak Keys."

In other words -- for these common key sizes, the repository is the source of truth. Every key in it is considered compromised and must be blocked, but you don't need to waste time replicating the work of generating all of these keys to prove to yourself that it has been done correctly. If you want to issue for other key sizes, then the onus is on you to do the relevant due diligence.

Aaron


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