diff --git a/testing/certutil/certutil.go b/testing/certutil/certutil.go
index 030a5be..b2495dd 100644
--- a/testing/certutil/certutil.go
+++ b/testing/certutil/certutil.go
@@ -24,6 +24,7 @@ import (
 	"crypto/elliptic"
 	"crypto/rand"
 	"crypto/rsa"
+	"crypto/sha256"
 	"crypto/tls"
 	"crypto/x509"
 	"crypto/x509/pkix"
@@ -349,6 +350,7 @@ func newRootCert(priv crypto.PrivateKey, pub crypto.PublicKey, opts ...Option) (
 		KeyUsage:              x509.KeyUsageDigitalSignature | x509.KeyUsageCertSign,
 		BasicConstraintsValid: true,
 		IsCA:                  true,
+		SubjectKeyId:          generateSubjectKeyID(pub),
 	}
 
 	rootCertRawBytes, err := x509.CreateCertificate(
@@ -406,6 +408,23 @@ func getCgf(opts []Option) configs {
 	return cfg
 }
 
+func generateSubjectKeyID(pub crypto.PublicKey) []byte {
+	// SubjectKeyId generated using method 1 in RFC 7093, Section 2:
+	//   1) The keyIdentifier is composed of the leftmost 160-bits of the
+	//   SHA-256 hash of the value of the BIT STRING subjectPublicKey
+	//   (excluding the tag, length, and number of unused bits).
+	var publicKeyBytes []byte
+	switch publicKey := pub.(type) {
+	case *rsa.PublicKey:
+		publicKeyBytes = x509.MarshalPKCS1PublicKey(publicKey)
+	case *ecdsa.PublicKey:
+		//nolint:staticcheck // no alternative
+		publicKeyBytes = elliptic.Marshal(publicKey.Curve, publicKey.X, publicKey.Y)
+	}
+	h := sha256.Sum256(publicKeyBytes)
+	return h[:20]
+}
+
 // defaultChildCert generates a child certificate for localhost and 127.0.0.1.
 // It returns the certificate and its key as a Pair and an error if any happens.
 func defaultChildCert(
diff --git a/transport/httpcommon/diag_test.go b/transport/httpcommon/diag_test.go
index 767ebe8..a5e07c1 100644
--- a/transport/httpcommon/diag_test.go
+++ b/transport/httpcommon/diag_test.go
@@ -21,6 +21,7 @@ import (
 	"context"
 	"crypto/rand"
 	"crypto/rsa"
+	"crypto/sha256"
 	"crypto/tls"
 	"crypto/x509"
 	"crypto/x509/pkix"
@@ -313,6 +314,8 @@ func genCA(t *testing.T) tls.Certificate {
 	caKey, err := rsa.GenerateKey(rand.Reader, 2048) // less secure key for quicker testing.
 	require.NoError(t, err)
 
+	ca.SubjectKeyId = generateSubjectKeyID(&caKey.PublicKey)
+
 	caBytes, err := x509.CreateCertificate(rand.Reader, ca, ca, &caKey.PublicKey, caKey)
 	require.NoError(t, err)
 
@@ -378,6 +381,10 @@ func genSignedCert(
 	certKey, err := rsa.GenerateKey(rand.Reader, 2048)
 	require.NoError(t, err)
 
+	if isCA {
+		cert.SubjectKeyId = generateSubjectKeyID(&certKey.PublicKey)
+	}
+
 	certBytes, err := x509.CreateCertificate(
 		rand.Reader,
 		cert,
@@ -401,3 +408,13 @@ func serial() *big.Int {
 	ser = ser + 1
 	return big.NewInt(ser)
 }
+
+func generateSubjectKeyID(publicKey *rsa.PublicKey) []byte {
+	// SubjectKeyId generated using method 1 in RFC 7093, Section 2:
+	//   1) The keyIdentifier is composed of the leftmost 160-bits of the
+	//   SHA-256 hash of the value of the BIT STRING subjectPublicKey
+	//   (excluding the tag, length, and number of unused bits).
+	publicKeyBytes := x509.MarshalPKCS1PublicKey(publicKey)
+	h := sha256.Sum256(publicKeyBytes)
+	return h[:20]
+}
diff --git a/transport/tlscommon/ca_pinning_test.go b/transport/tlscommon/ca_pinning_test.go
index cdd63ac..e1f44ef 100644
--- a/transport/tlscommon/ca_pinning_test.go
+++ b/transport/tlscommon/ca_pinning_test.go
@@ -22,6 +22,7 @@ import (
 	"context"
 	"crypto/rand"
 	"crypto/rsa"
+	"crypto/sha256"
 	"crypto/tls"
 	"crypto/x509"
 	"crypto/x509/pkix"
@@ -335,6 +336,8 @@ func genCA() (tls.Certificate, error) {
 		return tls.Certificate{}, fmt.Errorf("fail to generate RSA key: %w", err)
 	}
 
+	ca.SubjectKeyId = generateSubjectKeyID(&caKey.PublicKey)
+
 	caBytes, err := x509.CreateCertificate(rand.Reader, ca, ca, &caKey.PublicKey, caKey)
 	if err != nil {
 		return tls.Certificate{}, fmt.Errorf("fail to create certificate: %w", err)
@@ -404,6 +407,10 @@ func genSignedCert(
 		return tls.Certificate{}, fmt.Errorf("fail to generate RSA key: %w", err)
 	}
 
+	if isCA {
+		cert.SubjectKeyId = generateSubjectKeyID(&certKey.PublicKey)
+	}
+
 	certBytes, err := x509.CreateCertificate(
 		rand.Reader,
 		cert,
@@ -432,3 +439,13 @@ func serial() *big.Int {
 	ser = ser + 1
 	return big.NewInt(ser)
 }
+
+func generateSubjectKeyID(publicKey *rsa.PublicKey) []byte {
+	// SubjectKeyId generated using method 1 in RFC 7093, Section 2:
+	//   1) The keyIdentifier is composed of the leftmost 160-bits of the
+	//   SHA-256 hash of the value of the BIT STRING subjectPublicKey
+	//   (excluding the tag, length, and number of unused bits).
+	publicKeyBytes := x509.MarshalPKCS1PublicKey(publicKey)
+	h := sha256.Sum256(publicKeyBytes)
+	return h[:20]
+}