[cabf_validation] [EXTERNAL] Re: RFC 5280 conflict for SKI in subscriber certificates

[Lahtiharju, Pekka]

CA can use SKI for internal purposes to help finding their own certificates with the same public key. For example, the new Key Compromise revocation reason code specification requires CA to revoke all certificates using the same key. Our own certificates use always the same algorithm for SKI so we can be sure that search result is correct. We haven’t used it to search external certificates.

Also the CA software we are using currently always add SKI to all certificates; there is no option to not use it.

Pekka

From: Aaron Gable <aaron at letsencrypt.org>
Sent: torstai 1. joulukuuta 2022 20.42
To: Paul van Brouwershaven <Paul.vanBrouwershaven at entrust.com>
Cc: Hubert Chao <hchao at google.com>; Lahtiharju, Pekka <pekka.lahtiharju at teliacompany.com>; CA/Browser Forum Validation SC List <validation at cabforum.org>; Corey Bonnell <Corey.Bonnell at digicert.com>
Subject: Re: [EXTERNAL] Re: [cabf_validation] RFC 5280 conflict for SKI in subscriber certificates

If you're searching for certificates with the same key, the SKID can easily lead you astray -- there's no requirement that two different CAs use the same derivation function to compute the SKID from the Public Key. The SKID is useful in CA certs because it is required to byte-for-byte match the AKID in issued certs. I don't believe the SKID in end-entity certs serves any purpose in the modern webpki.

I'd love to hear more from Corey and/or Ryan Sleevi on the original motivation for this from July 2021, in case I'm missing something, but obviously I'm convinced already :)

Aaron

On Thu, Dec 1, 2022 at 7:18 AM Paul van Brouwershaven <Paul.vanBrouwershaven at entrust.com<mailto:Paul.vanBrouwershaven at entrust.com>> wrote:
The SKI is useful to quickly search for certificates with the same key.

Is saving a few bytes a sufficient reason to 'deviate' from RFC 5280, where we try to get everyone to focus on RFC 5280 adherence at the same time?

Are we sure that this would not cause any client incompatibility issues? Almost all certificates include the SKI today and while this might be fine for the major browsers, we also know that there are other clients/libraries that interact with web websites.

Paul

________________________________
From: Hubert Chao <hchao at google.com<mailto:hchao at google.com>>
Sent: Thursday, December 1, 2022 15:59
To: Lahtiharju, Pekka <pekka.lahtiharju at teliacompany.com<mailto:pekka.lahtiharju at teliacompany.com>>; CA/Browser Forum Validation SC List <validation at cabforum.org<mailto:validation at cabforum.org>>
Cc: Aaron Gable <aaron at letsencrypt.org<mailto:aaron at letsencrypt.org>>; Paul van Brouwershaven <Paul.vanBrouwershaven at entrust.com<mailto:Paul.vanBrouwershaven at entrust.com>>
Subject: [EXTERNAL] Re: [cabf_validation] RFC 5280 conflict for SKI in subscriber certificates

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On Thu, Dec 1, 2022 at 5:21 AM Lahtiharju, Pekka via Validation <validation at cabforum.org<mailto:validation at cabforum.org>> wrote:

I support Paul’s idea to change this to SHOULD. Why should we create new recommendations against RFC when this extension is useful in several use cases and almost everybody is using it now.

Could you list out the use cases where this extension is useful for a TLS certificate? The discussion that Corey linked to (https://lists.cabforum.org/pipermail/validation/2021-July/001672.html<https://urldefense.com/v3/__https:/lists.cabforum.org/pipermail/validation/2021-July/001672.html__;!!FJ-Y8qCqXTj2!bhb6QGSEpqEOi6JyHDzixLHA_ziEpOs6UQYkMiffRA4PH_9fFgyIiZRW3epCZqq0_V5K5pDehK6XTaH3PNBz1ibt$>) specifically says "... a TLS certificate [SKI] should not be needed ... ".

/hubert
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