crypto/tls: extract certificate validation for the client

Moved some code and added a comment in preparation for extending the TLS
1.3 client with certificate validation. No functional change.

handshake_client.go

@@ -350,6 +350,61 @@ func (hs *clientHandshakeState) pickCipherSuite() error {
 	return nil
 }
 
+// processCertsFromServer takes a chain of server certificates from a
+// Certificate message and verifies them.
+func (hs *clientHandshakeState) processCertsFromServer(certificates [][]byte) error {
+	c := hs.c
+	certs := make([]*x509.Certificate, len(certificates))
+	for i, asn1Data := range certificates {
+		cert, err := x509.ParseCertificate(asn1Data)
+		if err != nil {
+			c.sendAlert(alertBadCertificate)
+			return errors.New("tls: failed to parse certificate from server: " + err.Error())
+		}
+		certs[i] = cert
+	}
+
+	if !c.config.InsecureSkipVerify {
+		opts := x509.VerifyOptions{
+			Roots:         c.config.RootCAs,
+			CurrentTime:   c.config.time(),
+			DNSName:       c.config.ServerName,
+			Intermediates: x509.NewCertPool(),
+		}
+
+		for i, cert := range certs {
+			if i == 0 {
+				continue
+			}
+			opts.Intermediates.AddCert(cert)
+		}
+		var err error
+		c.verifiedChains, err = certs[0].Verify(opts)
+		if err != nil {
+			c.sendAlert(alertBadCertificate)
+			return err
+		}
+	}
+
+	if c.config.VerifyPeerCertificate != nil {
+		if err := c.config.VerifyPeerCertificate(certificates, c.verifiedChains); err != nil {
+			c.sendAlert(alertBadCertificate)
+			return err
+		}
+	}
+
+	switch certs[0].PublicKey.(type) {
+	case *rsa.PublicKey, *ecdsa.PublicKey:
+		break
+	default:
+		c.sendAlert(alertUnsupportedCertificate)
+		return fmt.Errorf("tls: server's certificate contains an unsupported type of public key: %T", certs[0].PublicKey)
+	}
+
+	c.peerCertificates = certs
+	return nil
+}
+
 func (hs *clientHandshakeState) doFullHandshake() error {
 	c := hs.c
 
@@ -367,53 +422,9 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 	if c.handshakes == 0 {
 		// If this is the first handshake on a connection, process and
 		// (optionally) verify the server's certificates.
-		certs := make([]*x509.Certificate, len(certMsg.certificates))
+		if err := hs.processCertsFromServer(certMsg.certificates); err != nil {
-		for i, asn1Data := range certMsg.certificates {
+			return err
-			cert, err := x509.ParseCertificate(asn1Data)
-			if err != nil {
-				c.sendAlert(alertBadCertificate)
-				return errors.New("tls: failed to parse certificate from server: " + err.Error())
-			}
-			certs[i] = cert
 		}
-
-		if !c.config.InsecureSkipVerify {
-			opts := x509.VerifyOptions{
-				Roots:         c.config.RootCAs,
-				CurrentTime:   c.config.time(),
-				DNSName:       c.config.ServerName,
-				Intermediates: x509.NewCertPool(),
-			}
-
-			for i, cert := range certs {
-				if i == 0 {
-					continue
-				}
-				opts.Intermediates.AddCert(cert)
-			}
-			c.verifiedChains, err = certs[0].Verify(opts)
-			if err != nil {
-				c.sendAlert(alertBadCertificate)
-				return err
-			}
-		}
-
-		if c.config.VerifyPeerCertificate != nil {
-			if err := c.config.VerifyPeerCertificate(certMsg.certificates, c.verifiedChains); err != nil {
-				c.sendAlert(alertBadCertificate)
-				return err
-			}
-		}
-
-		switch certs[0].PublicKey.(type) {
-		case *rsa.PublicKey, *ecdsa.PublicKey:
-			break
-		default:
-			c.sendAlert(alertUnsupportedCertificate)
-			return fmt.Errorf("tls: server's certificate contains an unsupported type of public key: %T", certs[0].PublicKey)
-		}
-
-		c.peerCertificates = certs
 	} else {
 		// This is a renegotiation handshake. We require that the
 		// server's identity (i.e. leaf certificate) is unchanged and