Subject Key Identifier is not recommended by CABF BR v2

With SC62, the CABF BR now lists SKI as not recommended.

Per discussion in zmap#762, zlint should provide two lints, one for rfc5280
behavior and one for CABF BR.

Both lint will conflict with each other, users are expected to select
(or ignore) which behavior they mean to follow.

Fixes zmap#749

v3/lints/cabf_br/lint_ext_subject_key_identifier_not_recommended_subscriber.go

@@ -0,0 +1,70 @@
+package cabf_br
+
+/*
+ * ZLint Copyright 2024 Regents of the University of Michigan
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy
+ * of the License at http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+ * implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ */
+
+import (
+	"github.com/zmap/zcrypto/x509"
+	"github.com/zmap/zlint/v3/lint"
+	"github.com/zmap/zlint/v3/util"
+)
+
+type subjectKeyIdNotRecommendedSubscriber struct{}
+
+/**********************************************************************
+RFC5280 suggested the addition of SKI extension, but CABF BR SC62
+marked the extension as NOT RECOMMENDED for subscriber certificates
+
+Warning:
+Users of zlint will trigger either
+`w_ext_subject_key_identifier_not_recommended_subscriber` (this lint)
+or `w_ext_subject_key_identifier_missing_sub_cert` the one enforcing
+RFC5280's behavior.
+
+Users are expected to specifically ignore one or the other lint
+depending on which one apply to them.
+
+See:
+ - https://github.com/zmap/zlint/issues/749
+ - https://github.com/zmap/zlint/issues/762
+**********************************************************************/
+
+func init() {
+	lint.RegisterCertificateLint(&lint.CertificateLint{
+		LintMetadata: lint.LintMetadata{
+			Name:          "w_ext_subject_key_identifier_not_recommended_subscriber",
+			Description:   "Subcriber certificates use of Subject Key Identifier is NOT RECOMMENDED",
+			Citation:      "BRs v2: 7.1.2.7.6",
+			Source:        lint.CABFBaselineRequirements,
+			EffectiveDate: util.SC62EffectiveDate,
+		},
+		Lint: NewSubjectKeyIdNotRecommendedSubscriber,
+	})
+}
+
+func NewSubjectKeyIdNotRecommendedSubscriber() lint.LintInterface {
+	return &subjectKeyIdNotRecommendedSubscriber{}
+}
+
+func (l *subjectKeyIdNotRecommendedSubscriber) CheckApplies(cert *x509.Certificate) bool {
+	return !util.IsCACert(cert)
+}
+
+func (l *subjectKeyIdNotRecommendedSubscriber) Execute(cert *x509.Certificate) *lint.LintResult {
+	if util.IsExtInCert(cert, util.SubjectKeyIdentityOID) {
+		return &lint.LintResult{Status: lint.Warn}
+	} else {
+		return &lint.LintResult{Status: lint.Pass}
+	}
+}