crypto/tls: extend NPN support to the client.

R=bradfitzgo, rsc1, bradfitzwork
CC=golang-dev
https://golang.org/cl/4277085

src/pkg/crypto/tls/common.go

@@ -96,6 +96,7 @@ type ConnectionState struct {
 	HandshakeComplete          bool
 	CipherSuite                uint16
 	NegotiatedProtocol         string
+	NegotiatedProtocolIsMutual bool
 
 	// the certificate chain that was presented by the other side
 	PeerCertificates []*x509.Certificate
@@ -124,7 +125,6 @@ type Config struct {
 	RootCAs *CASet
 
 	// NextProtos is a list of supported, application level protocols.
-	// Currently only server-side handling is supported.
 	NextProtos []string
 
 	// ServerName is included in the client's handshake to support virtual

src/pkg/crypto/tls/conn.go

@@ -36,6 +36,7 @@ type Conn struct {
 	peerCertificates  []*x509.Certificate
 
 	clientProtocol         string
+	clientProtocolFallback bool
 
 	// first permanent error
 	errMutex sync.Mutex
@@ -761,6 +762,7 @@ func (c *Conn) ConnectionState() ConnectionState {
 	state.HandshakeComplete = c.handshakeComplete
 	if c.handshakeComplete {
 		state.NegotiatedProtocol = c.clientProtocol
+		state.NegotiatedProtocolIsMutual = !c.clientProtocolFallback
 		state.CipherSuite = c.cipherSuite
 		state.PeerCertificates = c.peerCertificates
 	}

src/pkg/crypto/tls/handshake_client.go

@@ -29,6 +29,7 @@ func (c *Conn) clientHandshake() os.Error {
 		serverName:         c.config.ServerName,
 		supportedCurves:    []uint16{curveP256, curveP384, curveP521},
 		supportedPoints:    []uint8{pointFormatUncompressed},
+		nextProtoNeg:       len(c.config.NextProtos) > 0,
 	}
 
 	t := uint32(c.config.time())
@@ -66,6 +67,11 @@ func (c *Conn) clientHandshake() os.Error {
 		return c.sendAlert(alertUnexpectedMessage)
 	}
 
+	if !hello.nextProtoNeg && serverHello.nextProtoNeg {
+		c.sendAlert(alertHandshakeFailure)
+		return os.ErrorString("server advertised unrequested NPN")
+	}
+
 	suite, suiteId := mutualCipherSuite(c.config.cipherSuites(), serverHello.cipherSuite)
 	if suite == nil {
 		return c.sendAlert(alertHandshakeFailure)
@@ -267,6 +273,17 @@ func (c *Conn) clientHandshake() os.Error {
 	c.out.prepareCipherSpec(clientCipher, clientHash)
 	c.writeRecord(recordTypeChangeCipherSpec, []byte{1})
 
+	if serverHello.nextProtoNeg {
+		nextProto := new(nextProtoMsg)
+		proto, fallback := mutualProtocol(c.config.NextProtos, serverHello.nextProtos)
+		nextProto.proto = proto
+		c.clientProtocol = proto
+		c.clientProtocolFallback = fallback
+
+		finishedHash.Write(nextProto.marshal())
+		c.writeRecord(recordTypeHandshake, nextProto.marshal())
+	}
+
 	finished := new(finishedMsg)
 	finished.verifyData = finishedHash.clientSum(masterSecret)
 	finishedHash.Write(finished.marshal())
@@ -299,3 +316,19 @@ func (c *Conn) clientHandshake() os.Error {
 	c.cipherSuite = suiteId
 	return nil
 }
+
+// mutualProtocol finds the mutual Next Protocol Negotiation protocol given the
+// set of client and server supported protocols. The set of client supported
+// protocols must not be empty. It returns the resulting protocol and flag
+// indicating if the fallback case was reached.
+func mutualProtocol(clientProtos, serverProtos []string) (string, bool) {
+	for _, s := range serverProtos {
+		for _, c := range clientProtos {
+			if s == c {
+				return s, false
+			}
+		}
+	}
+
+	return clientProtos[0], true
+}