crypto/tls: select only compatible chains from Certificates

Now that we have a full implementation of the logic to check certificate
compatibility, we can let applications just list multiple chains in
Certificates (for example, an RSA and an ECDSA one) and choose the most
appropriate automatically.

NameToCertificate only maps each name to one chain, so simply deprecate
it, and while at it simplify its implementation by not stripping
trailing dots from the SNI (which is specified not to have any, see RFC
6066, Section 3) and by not supporting multi-level wildcards, which are
not a thing in the WebPKI (and in crypto/x509).

The performance of SupportsCertificate without Leaf is poor, but doesn't
affect current users. For now document that, and address it properly in
the next cycle. See #35504.

While cleaning up the Certificates/GetCertificate/GetConfigForClient
behavior, also support leaving Certificates/GetCertificate nil if
GetConfigForClient is set, and send unrecognized_name when there are no
available certificates.

Fixes #29139
Fixes #18377

Change-Id: I26604db48806fe4d608388e55da52f34b7ca4566
Reviewed-on: https://go-review.googlesource.com/c/go/+/205059
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>

src/crypto/tls/alert.go

@@ -40,6 +40,7 @@ const (
 	alertNoRenegotiation        alert = 100
 	alertMissingExtension       alert = 109
 	alertUnsupportedExtension   alert = 110
+	alertUnrecognizedName       alert = 112
 	alertNoApplicationProtocol  alert = 120
 )
 
@@ -69,6 +70,7 @@ var alertText = map[alert]string{
 	alertNoRenegotiation:        "no renegotiation",
 	alertMissingExtension:       "missing extension",
 	alertUnsupportedExtension:   "unsupported extension",
+	alertUnrecognizedName:       "unrecognized name",
 	alertNoApplicationProtocol:  "no application protocol",
 }
 

src/crypto/tls/common.go

@@ -457,19 +457,26 @@ type Config struct {
 	// If Time is nil, TLS uses time.Now.
 	Time func() time.Time
 
-	// Certificates contains one or more certificate chains to present to
-	// the other side of the connection. Server configurations must include
-	// at least one certificate or else set GetCertificate. Clients doing
-	// client-authentication may set either Certificates or
-	// GetClientCertificate.
+	// Certificates contains one or more certificate chains to present to the
+	// other side of the connection. The first certificate compatible with the
+	// peer's requirements is selected automatically.
+	//
+	// Server configurations must set one of Certificates, GetCertificate or
+	// GetConfigForClient. Clients doing client-authentication may set either
+	// Certificates or GetClientCertificate.
+	//
+	// Note: if there are multiple Certificates, and they don't have the
+	// optional field Leaf set, certificate selection will incur a significant
+	// per-handshake performance cost.
 	Certificates []Certificate
 
 	// NameToCertificate maps from a certificate name to an element of
 	// Certificates. Note that a certificate name can be of the form
 	// '*.example.com' and so doesn't have to be a domain name as such.
-	// See Config.BuildNameToCertificate
-	// The nil value causes the first element of Certificates to be used
-	// for all connections.
+	//
+	// Deprecated: NameToCertificate only allows associating a single
+	// certificate with a given name. Leave this field nil to let the library
+	// select the first compatible chain from Certificates.
 	NameToCertificate map[string]*Certificate
 
 	// GetCertificate returns a Certificate based on the given
@@ -478,7 +485,7 @@ type Config struct {
 	//
 	// If GetCertificate is nil or returns nil, then the certificate is
 	// retrieved from NameToCertificate. If NameToCertificate is nil, the
-	// first element of Certificates will be used.
+	// best element of Certificates will be used.
 	GetCertificate func(*ClientHelloInfo) (*Certificate, error)
 
 	// GetClientCertificate, if not nil, is called when a server requests a
@@ -869,6 +876,8 @@ func (c *Config) mutualVersion(peerVersions []uint16) (uint16, bool) {
 	return 0, false
 }
 
+var errNoCertificates = errors.New("tls: no certificates configured")
+
 // getCertificate returns the best certificate for the given ClientHelloInfo,
 // defaulting to the first element of c.Certificates.
 func (c *Config) getCertificate(clientHello *ClientHelloInfo) (*Certificate, error) {
@@ -881,31 +890,32 @@ func (c *Config) getCertificate(clientHello *ClientHelloInfo) (*Certificate, err
 	}
 
 	if len(c.Certificates) == 0 {
-		return nil, errors.New("tls: no certificates configured")
+		return nil, errNoCertificates
 	}
 
-	if len(c.Certificates) == 1 || c.NameToCertificate == nil {
+	if len(c.Certificates) == 1 {
 		// There's only one choice, so no point doing any work.
 		return &c.Certificates[0], nil
 	}
 
-	name := strings.ToLower(clientHello.ServerName)
-	for len(name) > 0 && name[len(name)-1] == '.' {
-		name = name[:len(name)-1]
-	}
-
-	if cert, ok := c.NameToCertificate[name]; ok {
-		return cert, nil
+	if c.NameToCertificate != nil {
+		name := strings.ToLower(clientHello.ServerName)
+		if cert, ok := c.NameToCertificate[name]; ok {
+			return cert, nil
+		}
+		if len(name) > 0 {
+			labels := strings.Split(name, ".")
+			labels[0] = "*"
+			wildcardName := strings.Join(labels, ".")
+			if cert, ok := c.NameToCertificate[wildcardName]; ok {
+				return cert, nil
+			}
+		}
 	}
 
-	// try replacing labels in the name with wildcards until we get a
-	// match.
-	labels := strings.Split(name, ".")
-	for i := range labels {
-		labels[i] = "*"
-		candidate := strings.Join(labels, ".")
-		if cert, ok := c.NameToCertificate[candidate]; ok {
-			return cert, nil
+	for _, cert := range c.Certificates {
+		if err := clientHello.SupportsCertificate(&cert); err == nil {
+			return &cert, nil
 		}
 	}
 
@@ -1109,6 +1119,10 @@ func (cri *CertificateRequestInfo) SupportsCertificate(c *Certificate) error {
 // BuildNameToCertificate parses c.Certificates and builds c.NameToCertificate
 // from the CommonName and SubjectAlternateName fields of each of the leaf
 // certificates.
+//
+// Deprecated: NameToCertificate only allows associating a single certificate
+// with a given name. Leave that field nil to let the library select the first
+// compatible chain from Certificates.
 func (c *Config) BuildNameToCertificate() {
 	c.NameToCertificate = make(map[string]*Certificate)
 	for i := range c.Certificates {

src/crypto/tls/conn_test.go

@@ -72,8 +72,6 @@ var certWildcardExampleCom = `308201743082011ea003020102021100a7aa6297c9416a4633
 
 var certFooExampleCom = `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`
 
-var certDoubleWildcardExampleCom = `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`
-
 func TestCertificateSelection(t *testing.T) {
 	config := Config{
 		Certificates: []Certificate{
@@ -86,9 +84,6 @@ func TestCertificateSelection(t *testing.T) {
 			{
 				Certificate: [][]byte{fromHex(certFooExampleCom)},
 			},
-			{
-				Certificate: [][]byte{fromHex(certDoubleWildcardExampleCom)},
-			},
 		},
 	}
 
@@ -124,11 +119,8 @@ func TestCertificateSelection(t *testing.T) {
 	if n := pointerToIndex(certificateForName("foo.example.com")); n != 2 {
 		t.Errorf("foo.example.com returned certificate %d, not 2", n)
 	}
-	if n := pointerToIndex(certificateForName("foo.bar.example.com")); n != 3 {
-		t.Errorf("foo.bar.example.com returned certificate %d, not 3", n)
-	}
-	if n := pointerToIndex(certificateForName("foo.bar.baz.example.com")); n != 0 {
-		t.Errorf("foo.bar.baz.example.com returned certificate %d, not 0", n)
+	if n := pointerToIndex(certificateForName("foo.bar.example.com")); n != 0 {
+		t.Errorf("foo.bar.example.com returned certificate %d, not 0", n)
 	}
 }
 

src/crypto/tls/handshake_server.go

@@ -227,7 +227,11 @@ func (hs *serverHandshakeState) processClientHello() error {
 
 	hs.cert, err = c.config.getCertificate(clientHelloInfo(c, hs.clientHello))
 	if err != nil {
-		c.sendAlert(alertInternalError)
+		if err == errNoCertificates {
+			c.sendAlert(alertUnrecognizedName)
+		} else {
+			c.sendAlert(alertInternalError)
+		}
 		return err
 	}
 	if hs.clientHello.scts {

src/crypto/tls/handshake_server_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"crypto"
 	"crypto/elliptic"
+	"crypto/x509"
 	"encoding/pem"
 	"errors"
 	"fmt"
@@ -1662,3 +1663,26 @@ T+E0J8wlH24pgwQHzy7Ko2qLwn1b5PW8ecrlvP1g
 	serverConfig.MaxVersion = VersionTLS12
 	testHandshake(t, testConfig, serverConfig)
 }
+
+func TestMultipleCertificates(t *testing.T) {
+	clientConfig := testConfig.Clone()
+	clientConfig.CipherSuites = []uint16{TLS_RSA_WITH_AES_128_GCM_SHA256}
+	clientConfig.MaxVersion = VersionTLS12
+
+	serverConfig := testConfig.Clone()
+	serverConfig.Certificates = []Certificate{{
+		Certificate: [][]byte{testECDSACertificate},
+		PrivateKey:  testECDSAPrivateKey,
+	}, {
+		Certificate: [][]byte{testRSACertificate},
+		PrivateKey:  testRSAPrivateKey,
+	}}
+
+	_, clientState, err := testHandshake(t, clientConfig, serverConfig)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := clientState.PeerCertificates[0].PublicKeyAlgorithm; got != x509.RSA {
+		t.Errorf("expected RSA certificate, got %v", got)
+	}
+}

src/crypto/tls/handshake_server_tls13.go

@@ -361,16 +361,13 @@ func (hs *serverHandshakeStateTLS13) pickCertificate() error {
 		return c.sendAlert(alertMissingExtension)
 	}
 
-	// This implements a very simplistic certificate selection strategy for now:
-	// getCertificate delegates to the application Config.GetCertificate, or
-	// selects based on the server_name only. If the selected certificate's
-	// public key does not match the client signature_algorithms, the handshake
-	// is aborted. No attention is given to signature_algorithms_cert, and it is
-	// not passed to the application Config.GetCertificate. This will need to
-	// improve according to RFC 8446, sections 4.4.2.2 and 4.2.3.
 	certificate, err := c.config.getCertificate(clientHelloInfo(c, hs.clientHello))
 	if err != nil {
-		c.sendAlert(alertInternalError)
+		if err == errNoCertificates {
+			c.sendAlert(alertUnrecognizedName)
+		} else {
+			c.sendAlert(alertInternalError)
+		}
 		return err
 	}
 	hs.sigAlg, err = selectSignatureScheme(c.vers, certificate, hs.clientHello.supportedSignatureAlgorithms)

src/crypto/tls/tls.go

@@ -75,8 +75,9 @@ func NewListener(inner net.Listener, config *Config) net.Listener {
 // The configuration config must be non-nil and must include
 // at least one certificate or else set GetCertificate.
 func Listen(network, laddr string, config *Config) (net.Listener, error) {
-	if config == nil || (len(config.Certificates) == 0 && config.GetCertificate == nil) {
-		return nil, errors.New("tls: neither Certificates nor GetCertificate set in Config")
+	if config == nil || len(config.Certificates) == 0 &&
+		config.GetCertificate == nil && config.GetConfigForClient == nil {
+		return nil, errors.New("tls: neither Certificates, GetCertificate, nor GetConfigForClient set in Config")
 	}
 	l, err := net.Listen(network, laddr)
 	if err != nil {