Source file src/crypto/tls/handshake_client.go

Documentation: crypto/tls

  // Copyright 2009 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  package tls
  
  import (
  	"bytes"
  	"crypto"
  	"crypto/ecdsa"
  	"crypto/rsa"
  	"crypto/subtle"
  	"crypto/x509"
  	"errors"
  	"fmt"
  	"io"
  	"net"
  	"strconv"
  	"strings"
  	"sync/atomic"
  )
  
  type clientHandshakeState struct {
  	c            *Conn
  	serverHello  *serverHelloMsg
  	hello        *clientHelloMsg
  	suite        *cipherSuite
  	finishedHash finishedHash
  	masterSecret []byte
  	session      *ClientSessionState
  }
  
  func makeClientHello(config *Config) (*clientHelloMsg, error) {
  	if len(config.ServerName) == 0 && !config.InsecureSkipVerify {
  		return nil, errors.New("tls: either ServerName or InsecureSkipVerify must be specified in the tls.Config")
  	}
  
  	nextProtosLength := 0
  	for _, proto := range config.NextProtos {
  		if l := len(proto); l == 0 || l > 255 {
  			return nil, errors.New("tls: invalid NextProtos value")
  		} else {
  			nextProtosLength += 1 + l
  		}
  	}
  
  	if nextProtosLength > 0xffff {
  		return nil, errors.New("tls: NextProtos values too large")
  	}
  
  	hello := &clientHelloMsg{
  		vers:                         config.maxVersion(),
  		compressionMethods:           []uint8{compressionNone},
  		random:                       make([]byte, 32),
  		ocspStapling:                 true,
  		scts:                         true,
  		serverName:                   hostnameInSNI(config.ServerName),
  		supportedCurves:              config.curvePreferences(),
  		supportedPoints:              []uint8{pointFormatUncompressed},
  		nextProtoNeg:                 len(config.NextProtos) > 0,
  		secureRenegotiationSupported: true,
  		alpnProtocols:                config.NextProtos,
  	}
  	possibleCipherSuites := config.cipherSuites()
  	hello.cipherSuites = make([]uint16, 0, len(possibleCipherSuites))
  
  NextCipherSuite:
  	for _, suiteId := range possibleCipherSuites {
  		for _, suite := range cipherSuites {
  			if suite.id != suiteId {
  				continue
  			}
  			// Don't advertise TLS 1.2-only cipher suites unless
  			// we're attempting TLS 1.2.
  			if hello.vers < VersionTLS12 && suite.flags&suiteTLS12 != 0 {
  				continue
  			}
  			hello.cipherSuites = append(hello.cipherSuites, suiteId)
  			continue NextCipherSuite
  		}
  	}
  
  	_, err := io.ReadFull(config.rand(), hello.random)
  	if err != nil {
  		return nil, errors.New("tls: short read from Rand: " + err.Error())
  	}
  
  	if hello.vers >= VersionTLS12 {
  		hello.supportedSignatureAlgorithms = supportedSignatureAlgorithms
  	}
  
  	return hello, nil
  }
  
  func (c *Conn) clientHandshake() error {
  	if c.config == nil {
  		c.config = defaultConfig()
  	}
  
  	// This may be a renegotiation handshake, in which case some fields
  	// need to be reset.
  	c.didResume = false
  
  	hello, err := makeClientHello(c.config)
  	if err != nil {
  		return err
  	}
  
  	if c.handshakes > 0 {
  		hello.secureRenegotiation = c.clientFinished[:]
  	}
  
  	var session *ClientSessionState
  	var cacheKey string
  	sessionCache := c.config.ClientSessionCache
  	if c.config.SessionTicketsDisabled {
  		sessionCache = nil
  	}
  
  	if sessionCache != nil {
  		hello.ticketSupported = true
  	}
  
  	// Session resumption is not allowed if renegotiating because
  	// renegotiation is primarily used to allow a client to send a client
  	// certificate, which would be skipped if session resumption occurred.
  	if sessionCache != nil && c.handshakes == 0 {
  		// Try to resume a previously negotiated TLS session, if
  		// available.
  		cacheKey = clientSessionCacheKey(c.conn.RemoteAddr(), c.config)
  		candidateSession, ok := sessionCache.Get(cacheKey)
  		if ok {
  			// Check that the ciphersuite/version used for the
  			// previous session are still valid.
  			cipherSuiteOk := false
  			for _, id := range hello.cipherSuites {
  				if id == candidateSession.cipherSuite {
  					cipherSuiteOk = true
  					break
  				}
  			}
  
  			versOk := candidateSession.vers >= c.config.minVersion() &&
  				candidateSession.vers <= c.config.maxVersion()
  			if versOk && cipherSuiteOk {
  				session = candidateSession
  			}
  		}
  	}
  
  	if session != nil {
  		hello.sessionTicket = session.sessionTicket
  		// A random session ID is used to detect when the
  		// server accepted the ticket and is resuming a session
  		// (see RFC 5077).
  		hello.sessionId = make([]byte, 16)
  		if _, err := io.ReadFull(c.config.rand(), hello.sessionId); err != nil {
  			return errors.New("tls: short read from Rand: " + err.Error())
  		}
  	}
  
  	hs := &clientHandshakeState{
  		c:       c,
  		hello:   hello,
  		session: session,
  	}
  
  	if err = hs.handshake(); err != nil {
  		return err
  	}
  
  	// If we had a successful handshake and hs.session is different from
  	// the one already cached - cache a new one
  	if sessionCache != nil && hs.session != nil && session != hs.session {
  		sessionCache.Put(cacheKey, hs.session)
  	}
  
  	return nil
  }
  
  // Does the handshake, either a full one or resumes old session.
  // Requires hs.c, hs.hello, and, optionally, hs.session to be set.
  func (hs *clientHandshakeState) handshake() error {
  	c := hs.c
  
  	// send ClientHello
  	if _, err := c.writeRecord(recordTypeHandshake, hs.hello.marshal()); err != nil {
  		return err
  	}
  
  	msg, err := c.readHandshake()
  	if err != nil {
  		return err
  	}
  
  	var ok bool
  	if hs.serverHello, ok = msg.(*serverHelloMsg); !ok {
  		c.sendAlert(alertUnexpectedMessage)
  		return unexpectedMessageError(hs.serverHello, msg)
  	}
  
  	if err = hs.pickTLSVersion(); err != nil {
  		return err
  	}
  
  	if err = hs.pickCipherSuite(); err != nil {
  		return err
  	}
  
  	isResume, err := hs.processServerHello()
  	if err != nil {
  		return err
  	}
  
  	hs.finishedHash = newFinishedHash(c.vers, hs.suite)
  
  	// No signatures of the handshake are needed in a resumption.
  	// Otherwise, in a full handshake, if we don't have any certificates
  	// configured then we will never send a CertificateVerify message and
  	// thus no signatures are needed in that case either.
  	if isResume || (len(c.config.Certificates) == 0 && c.config.GetClientCertificate == nil) {
  		hs.finishedHash.discardHandshakeBuffer()
  	}
  
  	hs.finishedHash.Write(hs.hello.marshal())
  	hs.finishedHash.Write(hs.serverHello.marshal())
  
  	c.buffering = true
  	if isResume {
  		if err := hs.establishKeys(); err != nil {
  			return err
  		}
  		if err := hs.readSessionTicket(); err != nil {
  			return err
  		}
  		if err := hs.readFinished(c.serverFinished[:]); err != nil {
  			return err
  		}
  		c.clientFinishedIsFirst = false
  		if err := hs.sendFinished(c.clientFinished[:]); err != nil {
  			return err
  		}
  		if _, err := c.flush(); err != nil {
  			return err
  		}
  	} else {
  		if err := hs.doFullHandshake(); err != nil {
  			return err
  		}
  		if err := hs.establishKeys(); err != nil {
  			return err
  		}
  		if err := hs.sendFinished(c.clientFinished[:]); err != nil {
  			return err
  		}
  		if _, err := c.flush(); err != nil {
  			return err
  		}
  		c.clientFinishedIsFirst = true
  		if err := hs.readSessionTicket(); err != nil {
  			return err
  		}
  		if err := hs.readFinished(c.serverFinished[:]); err != nil {
  			return err
  		}
  	}
  
  	c.ekm = ekmFromMasterSecret(c.vers, hs.suite, hs.masterSecret, hs.hello.random, hs.serverHello.random)
  	c.didResume = isResume
  	atomic.StoreUint32(&c.handshakeStatus, 1)
  
  	return nil
  }
  
  func (hs *clientHandshakeState) pickTLSVersion() error {
  	vers, ok := hs.c.config.mutualVersion(hs.serverHello.vers)
  	if !ok || vers < VersionTLS10 {
  		// TLS 1.0 is the minimum version supported as a client.
  		hs.c.sendAlert(alertProtocolVersion)
  		return fmt.Errorf("tls: server selected unsupported protocol version %x", hs.serverHello.vers)
  	}
  
  	hs.c.vers = vers
  	hs.c.haveVers = true
  
  	return nil
  }
  
  func (hs *clientHandshakeState) pickCipherSuite() error {
  	if hs.suite = mutualCipherSuite(hs.hello.cipherSuites, hs.serverHello.cipherSuite); hs.suite == nil {
  		hs.c.sendAlert(alertHandshakeFailure)
  		return errors.New("tls: server chose an unconfigured cipher suite")
  	}
  
  	hs.c.cipherSuite = hs.suite.id
  	return nil
  }
  
  func (hs *clientHandshakeState) doFullHandshake() error {
  	c := hs.c
  
  	msg, err := c.readHandshake()
  	if err != nil {
  		return err
  	}
  	certMsg, ok := msg.(*certificateMsg)
  	if !ok || len(certMsg.certificates) == 0 {
  		c.sendAlert(alertUnexpectedMessage)
  		return unexpectedMessageError(certMsg, msg)
  	}
  	hs.finishedHash.Write(certMsg.marshal())
  
  	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))
  		for i, asn1Data := range certMsg.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)
  			}
  			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
  		// thus any previous trust decision is still valid.
  		//
  		// See https://mitls.org/pages/attacks/3SHAKE for the
  		// motivation behind this requirement.
  		if !bytes.Equal(c.peerCertificates[0].Raw, certMsg.certificates[0]) {
  			c.sendAlert(alertBadCertificate)
  			return errors.New("tls: server's identity changed during renegotiation")
  		}
  	}
  
  	msg, err = c.readHandshake()
  	if err != nil {
  		return err
  	}
  
  	cs, ok := msg.(*certificateStatusMsg)
  	if ok {
  		// RFC4366 on Certificate Status Request:
  		// The server MAY return a "certificate_status" message.
  
  		if !hs.serverHello.ocspStapling {
  			// If a server returns a "CertificateStatus" message, then the
  			// server MUST have included an extension of type "status_request"
  			// with empty "extension_data" in the extended server hello.
  
  			c.sendAlert(alertUnexpectedMessage)
  			return errors.New("tls: received unexpected CertificateStatus message")
  		}
  		hs.finishedHash.Write(cs.marshal())
  
  		if cs.statusType == statusTypeOCSP {
  			c.ocspResponse = cs.response
  		}
  
  		msg, err = c.readHandshake()
  		if err != nil {
  			return err
  		}
  	}
  
  	keyAgreement := hs.suite.ka(c.vers)
  
  	skx, ok := msg.(*serverKeyExchangeMsg)
  	if ok {
  		hs.finishedHash.Write(skx.marshal())
  		err = keyAgreement.processServerKeyExchange(c.config, hs.hello, hs.serverHello, c.peerCertificates[0], skx)
  		if err != nil {
  			c.sendAlert(alertUnexpectedMessage)
  			return err
  		}
  
  		msg, err = c.readHandshake()
  		if err != nil {
  			return err
  		}
  	}
  
  	var chainToSend *Certificate
  	var certRequested bool
  	certReq, ok := msg.(*certificateRequestMsg)
  	if ok {
  		certRequested = true
  		hs.finishedHash.Write(certReq.marshal())
  
  		if chainToSend, err = hs.getCertificate(certReq); err != nil {
  			c.sendAlert(alertInternalError)
  			return err
  		}
  
  		msg, err = c.readHandshake()
  		if err != nil {
  			return err
  		}
  	}
  
  	shd, ok := msg.(*serverHelloDoneMsg)
  	if !ok {
  		c.sendAlert(alertUnexpectedMessage)
  		return unexpectedMessageError(shd, msg)
  	}
  	hs.finishedHash.Write(shd.marshal())
  
  	// If the server requested a certificate then we have to send a
  	// Certificate message, even if it's empty because we don't have a
  	// certificate to send.
  	if certRequested {
  		certMsg = new(certificateMsg)
  		certMsg.certificates = chainToSend.Certificate
  		hs.finishedHash.Write(certMsg.marshal())
  		if _, err := c.writeRecord(recordTypeHandshake, certMsg.marshal()); err != nil {
  			return err
  		}
  	}
  
  	preMasterSecret, ckx, err := keyAgreement.generateClientKeyExchange(c.config, hs.hello, c.peerCertificates[0])
  	if err != nil {
  		c.sendAlert(alertInternalError)
  		return err
  	}
  	if ckx != nil {
  		hs.finishedHash.Write(ckx.marshal())
  		if _, err := c.writeRecord(recordTypeHandshake, ckx.marshal()); err != nil {
  			return err
  		}
  	}
  
  	if chainToSend != nil && len(chainToSend.Certificate) > 0 {
  		certVerify := &certificateVerifyMsg{
  			hasSignatureAndHash: c.vers >= VersionTLS12,
  		}
  
  		key, ok := chainToSend.PrivateKey.(crypto.Signer)
  		if !ok {
  			c.sendAlert(alertInternalError)
  			return fmt.Errorf("tls: client certificate private key of type %T does not implement crypto.Signer", chainToSend.PrivateKey)
  		}
  
  		signatureAlgorithm, sigType, hashFunc, err := pickSignatureAlgorithm(key.Public(), certReq.supportedSignatureAlgorithms, hs.hello.supportedSignatureAlgorithms, c.vers)
  		if err != nil {
  			c.sendAlert(alertInternalError)
  			return err
  		}
  		// SignatureAndHashAlgorithm was introduced in TLS 1.2.
  		if certVerify.hasSignatureAndHash {
  			certVerify.signatureAlgorithm = signatureAlgorithm
  		}
  		digest, err := hs.finishedHash.hashForClientCertificate(sigType, hashFunc, hs.masterSecret)
  		if err != nil {
  			c.sendAlert(alertInternalError)
  			return err
  		}
  		signOpts := crypto.SignerOpts(hashFunc)
  		if sigType == signatureRSAPSS {
  			signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: hashFunc}
  		}
  		certVerify.signature, err = key.Sign(c.config.rand(), digest, signOpts)
  		if err != nil {
  			c.sendAlert(alertInternalError)
  			return err
  		}
  
  		hs.finishedHash.Write(certVerify.marshal())
  		if _, err := c.writeRecord(recordTypeHandshake, certVerify.marshal()); err != nil {
  			return err
  		}
  	}
  
  	hs.masterSecret = masterFromPreMasterSecret(c.vers, hs.suite, preMasterSecret, hs.hello.random, hs.serverHello.random)
  	if err := c.config.writeKeyLog(hs.hello.random, hs.masterSecret); err != nil {
  		c.sendAlert(alertInternalError)
  		return errors.New("tls: failed to write to key log: " + err.Error())
  	}
  
  	hs.finishedHash.discardHandshakeBuffer()
  
  	return nil
  }
  
  func (hs *clientHandshakeState) establishKeys() error {
  	c := hs.c
  
  	clientMAC, serverMAC, clientKey, serverKey, clientIV, serverIV :=
  		keysFromMasterSecret(c.vers, hs.suite, hs.masterSecret, hs.hello.random, hs.serverHello.random, hs.suite.macLen, hs.suite.keyLen, hs.suite.ivLen)
  	var clientCipher, serverCipher interface{}
  	var clientHash, serverHash macFunction
  	if hs.suite.cipher != nil {
  		clientCipher = hs.suite.cipher(clientKey, clientIV, false /* not for reading */)
  		clientHash = hs.suite.mac(c.vers, clientMAC)
  		serverCipher = hs.suite.cipher(serverKey, serverIV, true /* for reading */)
  		serverHash = hs.suite.mac(c.vers, serverMAC)
  	} else {
  		clientCipher = hs.suite.aead(clientKey, clientIV)
  		serverCipher = hs.suite.aead(serverKey, serverIV)
  	}
  
  	c.in.prepareCipherSpec(c.vers, serverCipher, serverHash)
  	c.out.prepareCipherSpec(c.vers, clientCipher, clientHash)
  	return nil
  }
  
  func (hs *clientHandshakeState) serverResumedSession() bool {
  	// If the server responded with the same sessionId then it means the
  	// sessionTicket is being used to resume a TLS session.
  	return hs.session != nil && hs.hello.sessionId != nil &&
  		bytes.Equal(hs.serverHello.sessionId, hs.hello.sessionId)
  }
  
  func (hs *clientHandshakeState) processServerHello() (bool, error) {
  	c := hs.c
  
  	if hs.serverHello.compressionMethod != compressionNone {
  		c.sendAlert(alertUnexpectedMessage)
  		return false, errors.New("tls: server selected unsupported compression format")
  	}
  
  	if c.handshakes == 0 && hs.serverHello.secureRenegotiationSupported {
  		c.secureRenegotiation = true
  		if len(hs.serverHello.secureRenegotiation) != 0 {
  			c.sendAlert(alertHandshakeFailure)
  			return false, errors.New("tls: initial handshake had non-empty renegotiation extension")
  		}
  	}
  
  	if c.handshakes > 0 && c.secureRenegotiation {
  		var expectedSecureRenegotiation [24]byte
  		copy(expectedSecureRenegotiation[:], c.clientFinished[:])
  		copy(expectedSecureRenegotiation[12:], c.serverFinished[:])
  		if !bytes.Equal(hs.serverHello.secureRenegotiation, expectedSecureRenegotiation[:]) {
  			c.sendAlert(alertHandshakeFailure)
  			return false, errors.New("tls: incorrect renegotiation extension contents")
  		}
  	}
  
  	clientDidNPN := hs.hello.nextProtoNeg
  	clientDidALPN := len(hs.hello.alpnProtocols) > 0
  	serverHasNPN := hs.serverHello.nextProtoNeg
  	serverHasALPN := len(hs.serverHello.alpnProtocol) > 0
  
  	if !clientDidNPN && serverHasNPN {
  		c.sendAlert(alertHandshakeFailure)
  		return false, errors.New("tls: server advertised unrequested NPN extension")
  	}
  
  	if !clientDidALPN && serverHasALPN {
  		c.sendAlert(alertHandshakeFailure)
  		return false, errors.New("tls: server advertised unrequested ALPN extension")
  	}
  
  	if serverHasNPN && serverHasALPN {
  		c.sendAlert(alertHandshakeFailure)
  		return false, errors.New("tls: server advertised both NPN and ALPN extensions")
  	}
  
  	if serverHasALPN {
  		c.clientProtocol = hs.serverHello.alpnProtocol
  		c.clientProtocolFallback = false
  	}
  	c.scts = hs.serverHello.scts
  
  	if !hs.serverResumedSession() {
  		return false, nil
  	}
  
  	if hs.session.vers != c.vers {
  		c.sendAlert(alertHandshakeFailure)
  		return false, errors.New("tls: server resumed a session with a different version")
  	}
  
  	if hs.session.cipherSuite != hs.suite.id {
  		c.sendAlert(alertHandshakeFailure)
  		return false, errors.New("tls: server resumed a session with a different cipher suite")
  	}
  
  	// Restore masterSecret and peerCerts from previous state
  	hs.masterSecret = hs.session.masterSecret
  	c.peerCertificates = hs.session.serverCertificates
  	c.verifiedChains = hs.session.verifiedChains
  	return true, nil
  }
  
  func (hs *clientHandshakeState) readFinished(out []byte) error {
  	c := hs.c
  
  	c.readRecord(recordTypeChangeCipherSpec)
  	if c.in.err != nil {
  		return c.in.err
  	}
  
  	msg, err := c.readHandshake()
  	if err != nil {
  		return err
  	}
  	serverFinished, ok := msg.(*finishedMsg)
  	if !ok {
  		c.sendAlert(alertUnexpectedMessage)
  		return unexpectedMessageError(serverFinished, msg)
  	}
  
  	verify := hs.finishedHash.serverSum(hs.masterSecret)
  	if len(verify) != len(serverFinished.verifyData) ||
  		subtle.ConstantTimeCompare(verify, serverFinished.verifyData) != 1 {
  		c.sendAlert(alertHandshakeFailure)
  		return errors.New("tls: server's Finished message was incorrect")
  	}
  	hs.finishedHash.Write(serverFinished.marshal())
  	copy(out, verify)
  	return nil
  }
  
  func (hs *clientHandshakeState) readSessionTicket() error {
  	if !hs.serverHello.ticketSupported {
  		return nil
  	}
  
  	c := hs.c
  	msg, err := c.readHandshake()
  	if err != nil {
  		return err
  	}
  	sessionTicketMsg, ok := msg.(*newSessionTicketMsg)
  	if !ok {
  		c.sendAlert(alertUnexpectedMessage)
  		return unexpectedMessageError(sessionTicketMsg, msg)
  	}
  	hs.finishedHash.Write(sessionTicketMsg.marshal())
  
  	hs.session = &ClientSessionState{
  		sessionTicket:      sessionTicketMsg.ticket,
  		vers:               c.vers,
  		cipherSuite:        hs.suite.id,
  		masterSecret:       hs.masterSecret,
  		serverCertificates: c.peerCertificates,
  		verifiedChains:     c.verifiedChains,
  	}
  
  	return nil
  }
  
  func (hs *clientHandshakeState) sendFinished(out []byte) error {
  	c := hs.c
  
  	if _, err := c.writeRecord(recordTypeChangeCipherSpec, []byte{1}); err != nil {
  		return err
  	}
  	if hs.serverHello.nextProtoNeg {
  		nextProto := new(nextProtoMsg)
  		proto, fallback := mutualProtocol(c.config.NextProtos, hs.serverHello.nextProtos)
  		nextProto.proto = proto
  		c.clientProtocol = proto
  		c.clientProtocolFallback = fallback
  
  		hs.finishedHash.Write(nextProto.marshal())
  		if _, err := c.writeRecord(recordTypeHandshake, nextProto.marshal()); err != nil {
  			return err
  		}
  	}
  
  	finished := new(finishedMsg)
  	finished.verifyData = hs.finishedHash.clientSum(hs.masterSecret)
  	hs.finishedHash.Write(finished.marshal())
  	if _, err := c.writeRecord(recordTypeHandshake, finished.marshal()); err != nil {
  		return err
  	}
  	copy(out, finished.verifyData)
  	return nil
  }
  
  // tls11SignatureSchemes contains the signature schemes that we synthesise for
  // a TLS <= 1.1 connection, based on the supported certificate types.
  var tls11SignatureSchemes = []SignatureScheme{ECDSAWithP256AndSHA256, ECDSAWithP384AndSHA384, ECDSAWithP521AndSHA512, PKCS1WithSHA256, PKCS1WithSHA384, PKCS1WithSHA512, PKCS1WithSHA1}
  
  const (
  	// tls11SignatureSchemesNumECDSA is the number of initial elements of
  	// tls11SignatureSchemes that use ECDSA.
  	tls11SignatureSchemesNumECDSA = 3
  	// tls11SignatureSchemesNumRSA is the number of trailing elements of
  	// tls11SignatureSchemes that use RSA.
  	tls11SignatureSchemesNumRSA = 4
  )
  
  func (hs *clientHandshakeState) getCertificate(certReq *certificateRequestMsg) (*Certificate, error) {
  	c := hs.c
  
  	var rsaAvail, ecdsaAvail bool
  	for _, certType := range certReq.certificateTypes {
  		switch certType {
  		case certTypeRSASign:
  			rsaAvail = true
  		case certTypeECDSASign:
  			ecdsaAvail = true
  		}
  	}
  
  	if c.config.GetClientCertificate != nil {
  		var signatureSchemes []SignatureScheme
  
  		if !certReq.hasSignatureAndHash {
  			// Prior to TLS 1.2, the signature schemes were not
  			// included in the certificate request message. In this
  			// case we use a plausible list based on the acceptable
  			// certificate types.
  			signatureSchemes = tls11SignatureSchemes
  			if !ecdsaAvail {
  				signatureSchemes = signatureSchemes[tls11SignatureSchemesNumECDSA:]
  			}
  			if !rsaAvail {
  				signatureSchemes = signatureSchemes[:len(signatureSchemes)-tls11SignatureSchemesNumRSA]
  			}
  		} else {
  			signatureSchemes = certReq.supportedSignatureAlgorithms
  		}
  
  		return c.config.GetClientCertificate(&CertificateRequestInfo{
  			AcceptableCAs:    certReq.certificateAuthorities,
  			SignatureSchemes: signatureSchemes,
  		})
  	}
  
  	// RFC 4346 on the certificateAuthorities field: A list of the
  	// distinguished names of acceptable certificate authorities.
  	// These distinguished names may specify a desired
  	// distinguished name for a root CA or for a subordinate CA;
  	// thus, this message can be used to describe both known roots
  	// and a desired authorization space. If the
  	// certificate_authorities list is empty then the client MAY
  	// send any certificate of the appropriate
  	// ClientCertificateType, unless there is some external
  	// arrangement to the contrary.
  
  	// We need to search our list of client certs for one
  	// where SignatureAlgorithm is acceptable to the server and the
  	// Issuer is in certReq.certificateAuthorities
  findCert:
  	for i, chain := range c.config.Certificates {
  		if !rsaAvail && !ecdsaAvail {
  			continue
  		}
  
  		for j, cert := range chain.Certificate {
  			x509Cert := chain.Leaf
  			// parse the certificate if this isn't the leaf
  			// node, or if chain.Leaf was nil
  			if j != 0 || x509Cert == nil {
  				var err error
  				if x509Cert, err = x509.ParseCertificate(cert); err != nil {
  					c.sendAlert(alertInternalError)
  					return nil, errors.New("tls: failed to parse client certificate #" + strconv.Itoa(i) + ": " + err.Error())
  				}
  			}
  
  			switch {
  			case rsaAvail && x509Cert.PublicKeyAlgorithm == x509.RSA:
  			case ecdsaAvail && x509Cert.PublicKeyAlgorithm == x509.ECDSA:
  			default:
  				continue findCert
  			}
  
  			if len(certReq.certificateAuthorities) == 0 {
  				// they gave us an empty list, so just take the
  				// first cert from c.config.Certificates
  				return &chain, nil
  			}
  
  			for _, ca := range certReq.certificateAuthorities {
  				if bytes.Equal(x509Cert.RawIssuer, ca) {
  					return &chain, nil
  				}
  			}
  		}
  	}
  
  	// No acceptable certificate found. Don't send a certificate.
  	return new(Certificate), nil
  }
  
  // clientSessionCacheKey returns a key used to cache sessionTickets that could
  // be used to resume previously negotiated TLS sessions with a server.
  func clientSessionCacheKey(serverAddr net.Addr, config *Config) string {
  	if len(config.ServerName) > 0 {
  		return config.ServerName
  	}
  	return serverAddr.String()
  }
  
  // mutualProtocol finds the mutual Next Protocol Negotiation or ALPN protocol
  // given list of possible protocols and a list of the preference order. The
  // first list must not be empty. It returns the resulting protocol and flag
  // indicating if the fallback case was reached.
  func mutualProtocol(protos, preferenceProtos []string) (string, bool) {
  	for _, s := range preferenceProtos {
  		for _, c := range protos {
  			if s == c {
  				return s, false
  			}
  		}
  	}
  
  	return protos[0], true
  }
  
  // hostnameInSNI converts name into an approriate hostname for SNI.
  // Literal IP addresses and absolute FQDNs are not permitted as SNI values.
  // See https://tools.ietf.org/html/rfc6066#section-3.
  func hostnameInSNI(name string) string {
  	host := name
  	if len(host) > 0 && host[0] == '[' && host[len(host)-1] == ']' {
  		host = host[1 : len(host)-1]
  	}
  	if i := strings.LastIndex(host, "%"); i > 0 {
  		host = host[:i]
  	}
  	if net.ParseIP(host) != nil {
  		return ""
  	}
  	for len(name) > 0 && name[len(name)-1] == '.' {
  		name = name[:len(name)-1]
  	}
  	return name
  }
  

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