handshake_client.go

/*
Copyright Suzhou Tongji Fintech Research Institute 2017 All Rights Reserved.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package gmtls

import (
	"bytes"
	"crypto"
	"crypto/ecdsa"
	"crypto/rsa"
	"crypto/subtle"
	"errors"
	"fmt"
	"io"
	"net"
	"strconv"
	"strings"

	"github.com/tjfoc/gmsm/sm2"
)

type clientHandshakeState struct {
	c            *Conn
	serverHello  *serverHelloMsg
	hello        *clientHelloMsg
	suite        *cipherSuite
	finishedHash finishedHash
	masterSecret []byte
	session      *ClientSessionState
}

// c.out.Mutex <= L; c.handshakeMutex <= L.
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

	if len(c.config.ServerName) == 0 && !c.config.InsecureSkipVerify {
		return errors.New("tls: either ServerName or InsecureSkipVerify must be specified in the tls.Config")
	}

	nextProtosLength := 0
	for _, proto := range c.config.NextProtos {
		if l := len(proto); l == 0 || l > 255 {
			return errors.New("tls: invalid NextProtos value")
		} else {
			nextProtosLength += 1 + l
		}
	}
	if nextProtosLength > 0xffff {
		return errors.New("tls: NextProtos values too large")
	}

	hello := &clientHelloMsg{
		vers:                         c.config.maxVersion(),
		compressionMethods:           []uint8{compressionNone},
		random:                       make([]byte, 32),
		ocspStapling:                 true,
		scts:                         true,
		serverName:                   hostnameInSNI(c.config.ServerName),
		supportedCurves:              c.config.curvePreferences(),
		supportedPoints:              []uint8{pointFormatUncompressed},
		nextProtoNeg:                 len(c.config.NextProtos) > 0,
		secureRenegotiationSupported: true,
		alpnProtocols:                c.config.NextProtos,
	}

	if c.handshakes > 0 {
		hello.secureRenegotiation = c.clientFinished[:]
	}

	possibleCipherSuites := c.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(c.config.rand(), hello.random)
	if err != nil {
		c.sendAlert(alertInternalError)
		return errors.New("tls: short read from Rand: " + err.Error())
	}

	if hello.vers >= VersionTLS12 {
		hello.signatureAndHashes = supportedSignatureAlgorithms
	}

	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 {
			c.sendAlert(alertInternalError)
			return errors.New("tls: short read from Rand: " + err.Error())
		}
	}

	if _, err := c.writeRecord(recordTypeHandshake, hello.marshal()); err != nil {
		return err
	}

	msg, err := c.readHandshake()
	if err != nil {
		return err
	}
	serverHello, ok := msg.(*serverHelloMsg)
	if !ok {
		c.sendAlert(alertUnexpectedMessage)
		return unexpectedMessageError(serverHello, msg)
	}

	vers, ok := c.config.mutualVersion(serverHello.vers)
	if !ok || vers < VersionTLS10 {
		// TLS 1.0 is the minimum version supported as a client.
		c.sendAlert(alertProtocolVersion)
		return fmt.Errorf("tls: server selected unsupported protocol version %x", serverHello.vers)
	}
	c.vers = vers
	c.haveVers = true

	suite := mutualCipherSuite(hello.cipherSuites, serverHello.cipherSuite)
	if suite == nil {
		c.sendAlert(alertHandshakeFailure)
		return errors.New("tls: server chose an unconfigured cipher suite")
	}

	hs := &clientHandshakeState{
		c:            c,
		serverHello:  serverHello,
		hello:        hello,
		suite:        suite,
		finishedHash: newFinishedHash(c.vers, suite),
		session:      session,
	}

	isResume, err := hs.processServerHello()
	if err != nil {
		return err
	}

	// 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
		}
	}

	if sessionCache != nil && hs.session != nil && session != hs.session {
		sessionCache.Put(cacheKey, hs.session)
	}

	c.didResume = isResume
	c.handshakeComplete = true
	c.cipherSuite = 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([]*sm2.Certificate, len(certMsg.certificates))
		for i, asn1Data := range certMsg.certificates {
			cert, err := sm2.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 := sm2.VerifyOptions{
				Roots:         c.config.RootCAs,
				CurrentTime:   c.config.time(),
				DNSName:       c.config.ServerName,
				Intermediates: sm2.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, *sm2.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")
		}
	}

	if hs.serverHello.ocspStapling {
		msg, err = c.readHandshake()
		if err != nil {
			return err
		}
		cs, ok := msg.(*certificateStatusMsg)
		if !ok {
			c.sendAlert(alertUnexpectedMessage)
			return unexpectedMessageError(cs, msg)
		}
		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)
		}

		var signatureType uint8
		switch key.Public().(type) {
		case *ecdsa.PublicKey, *sm2.PublicKey:
			signatureType = signatureECDSA
		case *rsa.PublicKey:
			signatureType = signatureRSA
		default:
			c.sendAlert(alertInternalError)
			return fmt.Errorf("tls: failed to sign handshake with client certificate: unknown client certificate key type: %T", key)
		}

		certVerify.signatureAndHash, err = hs.finishedHash.selectClientCertSignatureAlgorithm(certReq.signatureAndHashes, signatureType)
		if err != nil {
			c.sendAlert(alertInternalError)
			return err
		}
		digest, hashFunc, err := hs.finishedHash.hashForClientCertificate(certVerify.signatureAndHash, hs.masterSecret)
		if err != nil {
			c.sendAlert(alertInternalError)
			return err
		}
		certVerify.signature, err = key.Sign(c.config.rand(), digest, hashFunc)
		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 = make([]SignatureScheme, 0, len(certReq.signatureAndHashes))
			for _, sah := range certReq.signatureAndHashes {
				signatureSchemes = append(signatureSchemes, SignatureScheme(sah.hash)<<8+SignatureScheme(sah.signature))
			}
		}

		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 = sm2.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 == sm2.RSA:
			case ecdsaAvail && x509Cert.PublicKeyAlgorithm == sm2.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 ""
	}
	if len(name) > 0 && name[len(name)-1] == '.' {
		name = name[:len(name)-1]
	}
	return name
}