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<p class="MsoNormal">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.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Also the CA software we are using currently always add SKI to all certificates; there is no option to not use it.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Pekka<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> Aaron Gable <aaron@letsencrypt.org> <br>
<b>Sent:</b> torstai 1. joulukuuta 2022 20.42<br>
<b>To:</b> Paul van Brouwershaven <Paul.vanBrouwershaven@entrust.com><br>
<b>Cc:</b> Hubert Chao <hchao@google.com>; Lahtiharju, Pekka <pekka.lahtiharju@teliacompany.com>; CA/Browser Forum Validation SC List <validation@cabforum.org>; Corey Bonnell <Corey.Bonnell@digicert.com><br>
<b>Subject:</b> Re: [EXTERNAL] Re: [cabf_validation] RFC 5280 conflict for SKI in subscriber certificates<o:p></o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal">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 :)<o:p></o:p></p>
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<p class="MsoNormal">Aaron<o:p></o:p></p>
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<p class="MsoNormal">On Thu, Dec 1, 2022 at 7:18 AM Paul van Brouwershaven <<a href="mailto:Paul.vanBrouwershaven@entrust.com">Paul.vanBrouwershaven@entrust.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:12.0pt;color:black">The SKI is useful to quickly search for certificates with the same key.<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:12.0pt;color:black">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?<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:12.0pt;color:black">Are we sure that this would not cause any client incompatibility issues? Almost<span style="background:white"> 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.</span><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:12.0pt;color:black">Paul<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="color:black">From:</span></b><span style="color:black"> Hubert Chao <<a href="mailto:hchao@google.com" target="_blank">hchao@google.com</a>><br>
<b>Sent:</b> Thursday, December 1, 2022 15:59<br>
<b>To:</b> Lahtiharju, Pekka <<a href="mailto:pekka.lahtiharju@teliacompany.com" target="_blank">pekka.lahtiharju@teliacompany.com</a>>; CA/Browser Forum Validation SC List <<a href="mailto:validation@cabforum.org" target="_blank">validation@cabforum.org</a>><br>
<b>Cc:</b> Aaron Gable <<a href="mailto:aaron@letsencrypt.org" target="_blank">aaron@letsencrypt.org</a>>; Paul van Brouwershaven <<a href="mailto:Paul.vanBrouwershaven@entrust.com" target="_blank">Paul.vanBrouwershaven@entrust.com</a>><br>
<b>Subject:</b> [EXTERNAL] Re: [cabf_validation] RFC 5280 conflict for SKI in subscriber certificates</span>
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<p class="MsoNormal">On Thu, Dec 1, 2022 at 5:21 AM Lahtiharju, Pekka via Validation <<a href="mailto:validation@cabforum.org" target="_blank">validation@cabforum.org</a>> wrote:<o:p></o:p></p>
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<p>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.<o:p></o:p></p>
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<p class="MsoNormal">Could you list out the use cases where this extension is useful for a TLS certificate? The discussion that Corey linked to (<a href="https://urldefense.com/v3/__https:/lists.cabforum.org/pipermail/validation/2021-July/001672.html__;!!FJ-Y8qCqXTj2!bhb6QGSEpqEOi6JyHDzixLHA_ziEpOs6UQYkMiffRA4PH_9fFgyIiZRW3epCZqq0_V5K5pDehK6XTaH3PNBz1ibt$" target="_blank">https://lists.cabforum.org/pipermail/validation/2021-July/001672.html</a>)
specifically says "... a TLS certificate [SKI] should not be needed ... ".<o:p></o:p></p>
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<p class="MsoNormal">/hubert <o:p></o:p></p>
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