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<p><font face="Calibri">I would agree to consider out-of-scope (of
the BRs) a leaf certificate with EKU=clientAuth issued by a
SubCA that has EKU={server,client}, provided of course the leaf
certificate does not assert a BR TLS policy OID because this
would be contradictory. </font></p>
<p><font face="Calibri">Besides, at least one widely used linter
considers a certificate in-scope if it contains EKU=serverAuth
and/or it contains a BR policy OID, which seems quite logical to
me.</font></p>
<p><font face="Calibri">Adriano</font></p>
<p><font face="Calibri"><br>
</font></p>
<div class="moz-cite-prefix">Il 14/05/2024 11:33, Dimitris
Zacharopoulos (HARICA) via Servercert-wg ha scritto:<br>
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<blockquote type="cite"
cite="mid:0100018f76738e97-739d5cad-6797-4977-b997-150e338d5740-000000@email.amazonses.com">
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<td valign="top" bgcolor="#ffff00"> <span
style="color: red;">NOTICE:</span> Pay attention -
external email - Sender is
<a class="moz-txt-link-abbreviated" href="mailto:0100018f76738e97-739d5cad-6797-4977-b997-150e338d5740-000000@amazonses.com">0100018f76738e97-739d5cad-6797-4977-b997-150e338d5740-000000@amazonses.com</a>
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<br>
Dear Members,
<br>
<br>
Following-up on an interesting <a
href="https://bugzilla.mozilla.org/show_bug.cgi?id=1886467#c11"
moz-do-not-send="true">public
incident</a>
, I would like to have a discussion about the scope of the TLS BRs
as specified in the SCWG Charter and in the actual TLS BRs,
especially when it comes to single-purpose "client authentication"
certificates (i.e. leaf certificates that include the <i>id-kp-clientAuth</i>
and DO NOT include the <i>id-kp-serverAuth</i> KeyPurposeId in
their extKeyUsage extension).
<br>
<p>The TLS BRs describe the profiles of Subordinate CA
Certificates
issued from a Root that is in-scope for server TLS
authentication,
even for the case of Technically-Constrained non-TLS CA
Certificates. There was a lot of discussion about whether this
is
permitted based on the SCWG Charter and there was consensus that
Browsers want to make sure that there are some minimum
expectations
about the structure of such non-TLS CA certificates, especially
the
adherence to RFC 5280. I recall that there was also consensus
that
whatever is issued off of these TC non-TLS CAs is not in scope
of
the TLS BRs.</p>
<p><u>Does this seem like a fair statement about intent of the
group on the expectations of TC non-TLS CAs and their leaf
certificates?</u><br>
</p>
<p>Technically Constrained non-TLS Issuing CAs have a defined
profile in the TLS BRs but IMO it cannot, and should not mandate
the profile of non-TLS leaf certificates based on the <a
href="https://cabforum.org/working-groups/server/charter/"
rel="nofollow" moz-do-not-send="true">CA/Browser Forum Server
Certificate Working Group
Charter</a> which states (emphasis mine):</p>
<blockquote type="cite"><i>(a) To specify Baseline Requirements,
Extended Validation Guidelines, and other acceptable practices
for
the issuance and management of <b>TLS server certificates
used for
authenticating servers accessible through the
Internet</b></i></blockquote>
<p>It gets more interesting when an Issuing CA that is technically
capable of issuing server authentication TLS Certificates (by
including the <i>id-kp-serverAuth</i> KeyPurposeId in its
extKeyUsage extension), also includes the <i>id-kp-clientAuth</i>
KeyPurposeId, thus being technically capable of issuing client
authentication TLS Certificates.<br>
</p>
<p>Please recall that a few years back multi-purpose Issuing CAs
existed, where the EKU was not present, and even if it was, it
allowed the inclusion of various KeyPurposeIds.</p>
<p>Is it ok for such an Issuing CA to create a single-purpose
client authentication TLS Certificate, one that is structured
according to RFC 5280 (thus can be successfully parsed by
Relying
Party RFC 5280-conformant software), contains an extKeyUsage
extension which contains the <i>id-kp-clientAuth</i> and DOES
NOT
include the <i>id-kp-serverAuth</i> KeyPurposeId?</p>
<p>My understanding is that these particular leaf certificates are
allowed to be issued by a server TLS capable CA and they are
considered out-of-scope of the BRs, in the sense that <b>they
are
not TLS Server Certificates</b>. The SCWG has accepted this
"risk"
with the client authentication certificates by allowing the
co-existence of <i>id-kp-clientAuth</i> and <i>id-kp-serverAuth</i>
KeyPurposeIds and the explicit dis-allowance of
<i>id-kp-emailProtection, id-kp-codeSigning, id-kp-timeStamping,
anyExtendedKeyUsage</i> in the CA Certificate profiles.<br>
</p>
<p>The first paragraph of the TLS BRs (section 1.1) states:</p>
<blockquote type="cite"><i>.....for the issuance and management of
Publicly-Trusted TLS Server Certificates;</i></blockquote>
Provided these certificates follow RFC 5280 and can be properly
parsed, Browsers should never consider such certificates server
TLS
certificates. They are by design "technically constrained".
<br>
<br>
Thoughts? Disagreements? I know that Apple has already publicly <a
href="https://bugzilla.mozilla.org/show_bug.cgi?id=1886467#c13"
moz-do-not-send="true">shared
an opinion</a> on this matter so I'm hoping to get more feedback
from Members
here :)
<br>
<br>
<br>
Thanks,
<br>
Dimitris.
<br>
<br>
<br>
<br>
<fieldset class="moz-mime-attachment-header"></fieldset>
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