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Source file src/crypto/tls/handshake_server.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 (
  	"crypto"
  	"crypto/ecdsa"
  	"crypto/rsa"
  	"crypto/subtle"
  	"crypto/x509"
  	"encoding/asn1"
  	"errors"
  	"fmt"
  	"io"
  )
  
  // serverHandshakeState contains details of a server handshake in progress.
  // It's discarded once the handshake has completed.
  type serverHandshakeState struct {
  	c                     *Conn
  	clientHello           *clientHelloMsg
  	hello                 *serverHelloMsg
  	suite                 *cipherSuite
  	ellipticOk            bool
  	ecdsaOk               bool
  	rsaDecryptOk          bool
  	rsaSignOk             bool
  	sessionState          *sessionState
  	finishedHash          finishedHash
  	masterSecret          []byte
  	certsFromClient       [][]byte
  	cert                  *Certificate
  	cachedClientHelloInfo *ClientHelloInfo
  }
  
  // serverHandshake performs a TLS handshake as a server.
  // c.out.Mutex <= L; c.handshakeMutex <= L.
  func (c *Conn) serverHandshake() error {
  	// If this is the first server handshake, we generate a random key to
  	// encrypt the tickets with.
  	c.config.serverInitOnce.Do(func() { c.config.serverInit(nil) })
  
  	hs := serverHandshakeState{
  		c: c,
  	}
  	isResume, err := hs.readClientHello()
  	if err != nil {
  		return err
  	}
  
  	// For an overview of TLS handshaking, see https://tools.ietf.org/html/rfc5246#section-7.3
  	c.buffering = true
  	if isResume {
  		// The client has included a session ticket and so we do an abbreviated handshake.
  		if err := hs.doResumeHandshake(); err != nil {
  			return err
  		}
  		if err := hs.establishKeys(); err != nil {
  			return err
  		}
  		// ticketSupported is set in a resumption handshake if the
  		// ticket from the client was encrypted with an old session
  		// ticket key and thus a refreshed ticket should be sent.
  		if hs.hello.ticketSupported {
  			if err := hs.sendSessionTicket(); err != nil {
  				return err
  			}
  		}
  		if err := hs.sendFinished(c.serverFinished[:]); err != nil {
  			return err
  		}
  		if _, err := c.flush(); err != nil {
  			return err
  		}
  		c.clientFinishedIsFirst = false
  		if err := hs.readFinished(nil); err != nil {
  			return err
  		}
  		c.didResume = true
  	} else {
  		// The client didn't include a session ticket, or it wasn't
  		// valid so we do a full handshake.
  		if err := hs.doFullHandshake(); err != nil {
  			return err
  		}
  		if err := hs.establishKeys(); err != nil {
  			return err
  		}
  		if err := hs.readFinished(c.clientFinished[:]); err != nil {
  			return err
  		}
  		c.clientFinishedIsFirst = true
  		c.buffering = true
  		if err := hs.sendSessionTicket(); err != nil {
  			return err
  		}
  		if err := hs.sendFinished(nil); err != nil {
  			return err
  		}
  		if _, err := c.flush(); err != nil {
  			return err
  		}
  	}
  	c.handshakeComplete = true
  
  	return nil
  }
  
  // readClientHello reads a ClientHello message from the client and decides
  // whether we will perform session resumption.
  func (hs *serverHandshakeState) readClientHello() (isResume bool, err error) {
  	c := hs.c
  
  	msg, err := c.readHandshake()
  	if err != nil {
  		return false, err
  	}
  	var ok bool
  	hs.clientHello, ok = msg.(*clientHelloMsg)
  	if !ok {
  		c.sendAlert(alertUnexpectedMessage)
  		return false, unexpectedMessageError(hs.clientHello, msg)
  	}
  
  	if c.config.GetConfigForClient != nil {
  		if newConfig, err := c.config.GetConfigForClient(hs.clientHelloInfo()); err != nil {
  			c.sendAlert(alertInternalError)
  			return false, err
  		} else if newConfig != nil {
  			newConfig.serverInitOnce.Do(func() { newConfig.serverInit(c.config) })
  			c.config = newConfig
  		}
  	}
  
  	c.vers, ok = c.config.mutualVersion(hs.clientHello.vers)
  	if !ok {
  		c.sendAlert(alertProtocolVersion)
  		return false, fmt.Errorf("tls: client offered an unsupported, maximum protocol version of %x", hs.clientHello.vers)
  	}
  	c.haveVers = true
  
  	hs.hello = new(serverHelloMsg)
  
  	supportedCurve := false
  	preferredCurves := c.config.curvePreferences()
  Curves:
  	for _, curve := range hs.clientHello.supportedCurves {
  		for _, supported := range preferredCurves {
  			if supported == curve {
  				supportedCurve = true
  				break Curves
  			}
  		}
  	}
  
  	supportedPointFormat := false
  	for _, pointFormat := range hs.clientHello.supportedPoints {
  		if pointFormat == pointFormatUncompressed {
  			supportedPointFormat = true
  			break
  		}
  	}
  	hs.ellipticOk = supportedCurve && supportedPointFormat
  
  	foundCompression := false
  	// We only support null compression, so check that the client offered it.
  	for _, compression := range hs.clientHello.compressionMethods {
  		if compression == compressionNone {
  			foundCompression = true
  			break
  		}
  	}
  
  	if !foundCompression {
  		c.sendAlert(alertHandshakeFailure)
  		return false, errors.New("tls: client does not support uncompressed connections")
  	}
  
  	hs.hello.vers = c.vers
  	hs.hello.random = make([]byte, 32)
  	_, err = io.ReadFull(c.config.rand(), hs.hello.random)
  	if err != nil {
  		c.sendAlert(alertInternalError)
  		return false, err
  	}
  
  	if len(hs.clientHello.secureRenegotiation) != 0 {
  		c.sendAlert(alertHandshakeFailure)
  		return false, errors.New("tls: initial handshake had non-empty renegotiation extension")
  	}
  
  	hs.hello.secureRenegotiationSupported = hs.clientHello.secureRenegotiationSupported
  	hs.hello.compressionMethod = compressionNone
  	if len(hs.clientHello.serverName) > 0 {
  		c.serverName = hs.clientHello.serverName
  	}
  
  	if len(hs.clientHello.alpnProtocols) > 0 {
  		if selectedProto, fallback := mutualProtocol(hs.clientHello.alpnProtocols, c.config.NextProtos); !fallback {
  			hs.hello.alpnProtocol = selectedProto
  			c.clientProtocol = selectedProto
  		}
  	} else {
  		// Although sending an empty NPN extension is reasonable, Firefox has
  		// had a bug around this. Best to send nothing at all if
  		// c.config.NextProtos is empty. See
  		// https://golang.org/issue/5445.
  		if hs.clientHello.nextProtoNeg && len(c.config.NextProtos) > 0 {
  			hs.hello.nextProtoNeg = true
  			hs.hello.nextProtos = c.config.NextProtos
  		}
  	}
  
  	hs.cert, err = c.config.getCertificate(hs.clientHelloInfo())
  	if err != nil {
  		c.sendAlert(alertInternalError)
  		return false, err
  	}
  	if hs.clientHello.scts {
  		hs.hello.scts = hs.cert.SignedCertificateTimestamps
  	}
  
  	if priv, ok := hs.cert.PrivateKey.(crypto.Signer); ok {
  		switch priv.Public().(type) {
  		case *ecdsa.PublicKey:
  			hs.ecdsaOk = true
  		case *rsa.PublicKey:
  			hs.rsaSignOk = true
  		default:
  			c.sendAlert(alertInternalError)
  			return false, fmt.Errorf("tls: unsupported signing key type (%T)", priv.Public())
  		}
  	}
  	if priv, ok := hs.cert.PrivateKey.(crypto.Decrypter); ok {
  		switch priv.Public().(type) {
  		case *rsa.PublicKey:
  			hs.rsaDecryptOk = true
  		default:
  			c.sendAlert(alertInternalError)
  			return false, fmt.Errorf("tls: unsupported decryption key type (%T)", priv.Public())
  		}
  	}
  
  	if hs.checkForResumption() {
  		return true, nil
  	}
  
  	var preferenceList, supportedList []uint16
  	if c.config.PreferServerCipherSuites {
  		preferenceList = c.config.cipherSuites()
  		supportedList = hs.clientHello.cipherSuites
  	} else {
  		preferenceList = hs.clientHello.cipherSuites
  		supportedList = c.config.cipherSuites()
  	}
  
  	for _, id := range preferenceList {
  		if hs.setCipherSuite(id, supportedList, c.vers) {
  			break
  		}
  	}
  
  	if hs.suite == nil {
  		c.sendAlert(alertHandshakeFailure)
  		return false, errors.New("tls: no cipher suite supported by both client and server")
  	}
  
  	// See https://tools.ietf.org/html/rfc7507.
  	for _, id := range hs.clientHello.cipherSuites {
  		if id == TLS_FALLBACK_SCSV {
  			// The client is doing a fallback connection.
  			if hs.clientHello.vers < c.config.maxVersion() {
  				c.sendAlert(alertInappropriateFallback)
  				return false, errors.New("tls: client using inappropriate protocol fallback")
  			}
  			break
  		}
  	}
  
  	return false, nil
  }
  
  // checkForResumption reports whether we should perform resumption on this connection.
  func (hs *serverHandshakeState) checkForResumption() bool {
  	c := hs.c
  
  	if c.config.SessionTicketsDisabled {
  		return false
  	}
  
  	var ok bool
  	var sessionTicket = append([]uint8{}, hs.clientHello.sessionTicket...)
  	if hs.sessionState, ok = c.decryptTicket(sessionTicket); !ok {
  		return false
  	}
  
  	// Never resume a session for a different TLS version.
  	if c.vers != hs.sessionState.vers {
  		return false
  	}
  
  	cipherSuiteOk := false
  	// Check that the client is still offering the ciphersuite in the session.
  	for _, id := range hs.clientHello.cipherSuites {
  		if id == hs.sessionState.cipherSuite {
  			cipherSuiteOk = true
  			break
  		}
  	}
  	if !cipherSuiteOk {
  		return false
  	}
  
  	// Check that we also support the ciphersuite from the session.
  	if !hs.setCipherSuite(hs.sessionState.cipherSuite, c.config.cipherSuites(), hs.sessionState.vers) {
  		return false
  	}
  
  	sessionHasClientCerts := len(hs.sessionState.certificates) != 0
  	needClientCerts := c.config.ClientAuth == RequireAnyClientCert || c.config.ClientAuth == RequireAndVerifyClientCert
  	if needClientCerts && !sessionHasClientCerts {
  		return false
  	}
  	if sessionHasClientCerts && c.config.ClientAuth == NoClientCert {
  		return false
  	}
  
  	return true
  }
  
  func (hs *serverHandshakeState) doResumeHandshake() error {
  	c := hs.c
  
  	hs.hello.cipherSuite = hs.suite.id
  	// We echo the client's session ID in the ServerHello to let it know
  	// that we're doing a resumption.
  	hs.hello.sessionId = hs.clientHello.sessionId
  	hs.hello.ticketSupported = hs.sessionState.usedOldKey
  	hs.finishedHash = newFinishedHash(c.vers, hs.suite)
  	hs.finishedHash.discardHandshakeBuffer()
  	hs.finishedHash.Write(hs.clientHello.marshal())
  	hs.finishedHash.Write(hs.hello.marshal())
  	if _, err := c.writeRecord(recordTypeHandshake, hs.hello.marshal()); err != nil {
  		return err
  	}
  
  	if len(hs.sessionState.certificates) > 0 {
  		if _, err := hs.processCertsFromClient(hs.sessionState.certificates); err != nil {
  			return err
  		}
  	}
  
  	hs.masterSecret = hs.sessionState.masterSecret
  
  	return nil
  }
  
  func (hs *serverHandshakeState) doFullHandshake() error {
  	c := hs.c
  
  	if hs.clientHello.ocspStapling && len(hs.cert.OCSPStaple) > 0 {
  		hs.hello.ocspStapling = true
  	}
  
  	hs.hello.ticketSupported = hs.clientHello.ticketSupported && !c.config.SessionTicketsDisabled
  	hs.hello.cipherSuite = hs.suite.id
  
  	hs.finishedHash = newFinishedHash(hs.c.vers, hs.suite)
  	if c.config.ClientAuth == NoClientCert {
  		// No need to keep a full record of the handshake if client
  		// certificates won't be used.
  		hs.finishedHash.discardHandshakeBuffer()
  	}
  	hs.finishedHash.Write(hs.clientHello.marshal())
  	hs.finishedHash.Write(hs.hello.marshal())
  	if _, err := c.writeRecord(recordTypeHandshake, hs.hello.marshal()); err != nil {
  		return err
  	}
  
  	certMsg := new(certificateMsg)
  	certMsg.certificates = hs.cert.Certificate
  	hs.finishedHash.Write(certMsg.marshal())
  	if _, err := c.writeRecord(recordTypeHandshake, certMsg.marshal()); err != nil {
  		return err
  	}
  
  	if hs.hello.ocspStapling {
  		certStatus := new(certificateStatusMsg)
  		certStatus.statusType = statusTypeOCSP
  		certStatus.response = hs.cert.OCSPStaple
  		hs.finishedHash.Write(certStatus.marshal())
  		if _, err := c.writeRecord(recordTypeHandshake, certStatus.marshal()); err != nil {
  			return err
  		}
  	}
  
  	keyAgreement := hs.suite.ka(c.vers)
  	skx, err := keyAgreement.generateServerKeyExchange(c.config, hs.cert, hs.clientHello, hs.hello)
  	if err != nil {
  		c.sendAlert(alertHandshakeFailure)
  		return err
  	}
  	if skx != nil {
  		hs.finishedHash.Write(skx.marshal())
  		if _, err := c.writeRecord(recordTypeHandshake, skx.marshal()); err != nil {
  			return err
  		}
  	}
  
  	if c.config.ClientAuth >= RequestClientCert {
  		// Request a client certificate
  		certReq := new(certificateRequestMsg)
  		certReq.certificateTypes = []byte{
  			byte(certTypeRSASign),
  			byte(certTypeECDSASign),
  		}
  		if c.vers >= VersionTLS12 {
  			certReq.hasSignatureAndHash = true
  			certReq.signatureAndHashes = supportedSignatureAlgorithms
  		}
  
  		// An empty list of certificateAuthorities signals to
  		// the client that it may send any certificate in response
  		// to our request. When we know the CAs we trust, then
  		// we can send them down, so that the client can choose
  		// an appropriate certificate to give to us.
  		if c.config.ClientCAs != nil {
  			certReq.certificateAuthorities = c.config.ClientCAs.Subjects()
  		}
  		hs.finishedHash.Write(certReq.marshal())
  		if _, err := c.writeRecord(recordTypeHandshake, certReq.marshal()); err != nil {
  			return err
  		}
  	}
  
  	helloDone := new(serverHelloDoneMsg)
  	hs.finishedHash.Write(helloDone.marshal())
  	if _, err := c.writeRecord(recordTypeHandshake, helloDone.marshal()); err != nil {
  		return err
  	}
  
  	if _, err := c.flush(); err != nil {
  		return err
  	}
  
  	var pub crypto.PublicKey // public key for client auth, if any
  
  	msg, err := c.readHandshake()
  	if err != nil {
  		return err
  	}
  
  	var ok bool
  	// If we requested a client certificate, then the client must send a
  	// certificate message, even if it's empty.
  	if c.config.ClientAuth >= RequestClientCert {
  		if certMsg, ok = msg.(*certificateMsg); !ok {
  			c.sendAlert(alertUnexpectedMessage)
  			return unexpectedMessageError(certMsg, msg)
  		}
  		hs.finishedHash.Write(certMsg.marshal())
  
  		if len(certMsg.certificates) == 0 {
  			// The client didn't actually send a certificate
  			switch c.config.ClientAuth {
  			case RequireAnyClientCert, RequireAndVerifyClientCert:
  				c.sendAlert(alertBadCertificate)
  				return errors.New("tls: client didn't provide a certificate")
  			}
  		}
  
  		pub, err = hs.processCertsFromClient(certMsg.certificates)
  		if err != nil {
  			return err
  		}
  
  		msg, err = c.readHandshake()
  		if err != nil {
  			return err
  		}
  	}
  
  	// Get client key exchange
  	ckx, ok := msg.(*clientKeyExchangeMsg)
  	if !ok {
  		c.sendAlert(alertUnexpectedMessage)
  		return unexpectedMessageError(ckx, msg)
  	}
  	hs.finishedHash.Write(ckx.marshal())
  
  	preMasterSecret, err := keyAgreement.processClientKeyExchange(c.config, hs.cert, ckx, c.vers)
  	if err != nil {
  		c.sendAlert(alertHandshakeFailure)
  		return err
  	}
  	hs.masterSecret = masterFromPreMasterSecret(c.vers, hs.suite, preMasterSecret, hs.clientHello.random, hs.hello.random)
  	if err := c.config.writeKeyLog(hs.clientHello.random, hs.masterSecret); err != nil {
  		c.sendAlert(alertInternalError)
  		return err
  	}
  
  	// If we received a client cert in response to our certificate request message,
  	// the client will send us a certificateVerifyMsg immediately after the
  	// clientKeyExchangeMsg. This message is a digest of all preceding
  	// handshake-layer messages that is signed using the private key corresponding
  	// to the client's certificate. This allows us to verify that the client is in
  	// possession of the private key of the certificate.
  	if len(c.peerCertificates) > 0 {
  		msg, err = c.readHandshake()
  		if err != nil {
  			return err
  		}
  		certVerify, ok := msg.(*certificateVerifyMsg)
  		if !ok {
  			c.sendAlert(alertUnexpectedMessage)
  			return unexpectedMessageError(certVerify, msg)
  		}
  
  		// Determine the signature type.
  		var signatureAndHash signatureAndHash
  		if certVerify.hasSignatureAndHash {
  			signatureAndHash = certVerify.signatureAndHash
  			if !isSupportedSignatureAndHash(signatureAndHash, supportedSignatureAlgorithms) {
  				return errors.New("tls: unsupported hash function for client certificate")
  			}
  		} else {
  			// Before TLS 1.2 the signature algorithm was implicit
  			// from the key type, and only one hash per signature
  			// algorithm was possible. Leave the hash as zero.
  			switch pub.(type) {
  			case *ecdsa.PublicKey:
  				signatureAndHash.signature = signatureECDSA
  			case *rsa.PublicKey:
  				signatureAndHash.signature = signatureRSA
  			}
  		}
  
  		switch key := pub.(type) {
  		case *ecdsa.PublicKey:
  			if signatureAndHash.signature != signatureECDSA {
  				err = errors.New("tls: bad signature type for client's ECDSA certificate")
  				break
  			}
  			ecdsaSig := new(ecdsaSignature)
  			if _, err = asn1.Unmarshal(certVerify.signature, ecdsaSig); err != nil {
  				break
  			}
  			if ecdsaSig.R.Sign() <= 0 || ecdsaSig.S.Sign() <= 0 {
  				err = errors.New("tls: ECDSA signature contained zero or negative values")
  				break
  			}
  			var digest []byte
  			if digest, _, err = hs.finishedHash.hashForClientCertificate(signatureAndHash, hs.masterSecret); err != nil {
  				break
  			}
  			if !ecdsa.Verify(key, digest, ecdsaSig.R, ecdsaSig.S) {
  				err = errors.New("tls: ECDSA verification failure")
  			}
  		case *rsa.PublicKey:
  			if signatureAndHash.signature != signatureRSA {
  				err = errors.New("tls: bad signature type for client's RSA certificate")
  				break
  			}
  			var digest []byte
  			var hashFunc crypto.Hash
  			if digest, hashFunc, err = hs.finishedHash.hashForClientCertificate(signatureAndHash, hs.masterSecret); err != nil {
  				break
  			}
  			err = rsa.VerifyPKCS1v15(key, hashFunc, digest, certVerify.signature)
  		}
  		if err != nil {
  			c.sendAlert(alertBadCertificate)
  			return errors.New("tls: could not validate signature of connection nonces: " + err.Error())
  		}
  
  		hs.finishedHash.Write(certVerify.marshal())
  	}
  
  	hs.finishedHash.discardHandshakeBuffer()
  
  	return nil
  }
  
  func (hs *serverHandshakeState) establishKeys() error {
  	c := hs.c
  
  	clientMAC, serverMAC, clientKey, serverKey, clientIV, serverIV :=
  		keysFromMasterSecret(c.vers, hs.suite, hs.masterSecret, hs.clientHello.random, hs.hello.random, hs.suite.macLen, hs.suite.keyLen, hs.suite.ivLen)
  
  	var clientCipher, serverCipher interface{}
  	var clientHash, serverHash macFunction
  
  	if hs.suite.aead == nil {
  		clientCipher = hs.suite.cipher(clientKey, clientIV, true /* for reading */)
  		clientHash = hs.suite.mac(c.vers, clientMAC)
  		serverCipher = hs.suite.cipher(serverKey, serverIV, false /* not 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, clientCipher, clientHash)
  	c.out.prepareCipherSpec(c.vers, serverCipher, serverHash)
  
  	return nil
  }
  
  func (hs *serverHandshakeState) readFinished(out []byte) error {
  	c := hs.c
  
  	c.readRecord(recordTypeChangeCipherSpec)
  	if c.in.err != nil {
  		return c.in.err
  	}
  
  	if hs.hello.nextProtoNeg {
  		msg, err := c.readHandshake()
  		if err != nil {
  			return err
  		}
  		nextProto, ok := msg.(*nextProtoMsg)
  		if !ok {
  			c.sendAlert(alertUnexpectedMessage)
  			return unexpectedMessageError(nextProto, msg)
  		}
  		hs.finishedHash.Write(nextProto.marshal())
  		c.clientProtocol = nextProto.proto
  	}
  
  	msg, err := c.readHandshake()
  	if err != nil {
  		return err
  	}
  	clientFinished, ok := msg.(*finishedMsg)
  	if !ok {
  		c.sendAlert(alertUnexpectedMessage)
  		return unexpectedMessageError(clientFinished, msg)
  	}
  
  	verify := hs.finishedHash.clientSum(hs.masterSecret)
  	if len(verify) != len(clientFinished.verifyData) ||
  		subtle.ConstantTimeCompare(verify, clientFinished.verifyData) != 1 {
  		c.sendAlert(alertHandshakeFailure)
  		return errors.New("tls: client's Finished message is incorrect")
  	}
  
  	hs.finishedHash.Write(clientFinished.marshal())
  	copy(out, verify)
  	return nil
  }
  
  func (hs *serverHandshakeState) sendSessionTicket() error {
  	if !hs.hello.ticketSupported {
  		return nil
  	}
  
  	c := hs.c
  	m := new(newSessionTicketMsg)
  
  	var err error
  	state := sessionState{
  		vers:         c.vers,
  		cipherSuite:  hs.suite.id,
  		masterSecret: hs.masterSecret,
  		certificates: hs.certsFromClient,
  	}
  	m.ticket, err = c.encryptTicket(&state)
  	if err != nil {
  		return err
  	}
  
  	hs.finishedHash.Write(m.marshal())
  	if _, err := c.writeRecord(recordTypeHandshake, m.marshal()); err != nil {
  		return err
  	}
  
  	return nil
  }
  
  func (hs *serverHandshakeState) sendFinished(out []byte) error {
  	c := hs.c
  
  	if _, err := c.writeRecord(recordTypeChangeCipherSpec, []byte{1}); err != nil {
  		return err
  	}
  
  	finished := new(finishedMsg)
  	finished.verifyData = hs.finishedHash.serverSum(hs.masterSecret)
  	hs.finishedHash.Write(finished.marshal())
  	if _, err := c.writeRecord(recordTypeHandshake, finished.marshal()); err != nil {
  		return err
  	}
  
  	c.cipherSuite = hs.suite.id
  	copy(out, finished.verifyData)
  
  	return nil
  }
  
  // processCertsFromClient takes a chain of client certificates either from a
  // Certificates message or from a sessionState and verifies them. It returns
  // the public key of the leaf certificate.
  func (hs *serverHandshakeState) processCertsFromClient(certificates [][]byte) (crypto.PublicKey, error) {
  	c := hs.c
  
  	hs.certsFromClient = certificates
  	certs := make([]*x509.Certificate, len(certificates))
  	var err error
  	for i, asn1Data := range certificates {
  		if certs[i], err = x509.ParseCertificate(asn1Data); err != nil {
  			c.sendAlert(alertBadCertificate)
  			return nil, errors.New("tls: failed to parse client certificate: " + err.Error())
  		}
  	}
  
  	if c.config.ClientAuth >= VerifyClientCertIfGiven && len(certs) > 0 {
  		opts := x509.VerifyOptions{
  			Roots:         c.config.ClientCAs,
  			CurrentTime:   c.config.time(),
  			Intermediates: x509.NewCertPool(),
  			KeyUsages:     []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
  		}
  
  		for _, cert := range certs[1:] {
  			opts.Intermediates.AddCert(cert)
  		}
  
  		chains, err := certs[0].Verify(opts)
  		if err != nil {
  			c.sendAlert(alertBadCertificate)
  			return nil, errors.New("tls: failed to verify client's certificate: " + err.Error())
  		}
  
  		c.verifiedChains = chains
  	}
  
  	if c.config.VerifyPeerCertificate != nil {
  		if err := c.config.VerifyPeerCertificate(certificates, c.verifiedChains); err != nil {
  			c.sendAlert(alertBadCertificate)
  			return nil, err
  		}
  	}
  
  	if len(certs) == 0 {
  		return nil, nil
  	}
  
  	var pub crypto.PublicKey
  	switch key := certs[0].PublicKey.(type) {
  	case *ecdsa.PublicKey, *rsa.PublicKey:
  		pub = key
  	default:
  		c.sendAlert(alertUnsupportedCertificate)
  		return nil, fmt.Errorf("tls: client's certificate contains an unsupported public key of type %T", certs[0].PublicKey)
  	}
  	c.peerCertificates = certs
  	return pub, nil
  }
  
  // setCipherSuite sets a cipherSuite with the given id as the serverHandshakeState
  // suite if that cipher suite is acceptable to use.
  // It returns a bool indicating if the suite was set.
  func (hs *serverHandshakeState) setCipherSuite(id uint16, supportedCipherSuites []uint16, version uint16) bool {
  	for _, supported := range supportedCipherSuites {
  		if id == supported {
  			var candidate *cipherSuite
  
  			for _, s := range cipherSuites {
  				if s.id == id {
  					candidate = s
  					break
  				}
  			}
  			if candidate == nil {
  				continue
  			}
  			// Don't select a ciphersuite which we can't
  			// support for this client.
  			if candidate.flags&suiteECDHE != 0 {
  				if !hs.ellipticOk {
  					continue
  				}
  				if candidate.flags&suiteECDSA != 0 {
  					if !hs.ecdsaOk {
  						continue
  					}
  				} else if !hs.rsaSignOk {
  					continue
  				}
  			} else if !hs.rsaDecryptOk {
  				continue
  			}
  			if version < VersionTLS12 && candidate.flags&suiteTLS12 != 0 {
  				continue
  			}
  			hs.suite = candidate
  			return true
  		}
  	}
  	return false
  }
  
  // suppVersArray is the backing array of ClientHelloInfo.SupportedVersions
  var suppVersArray = [...]uint16{VersionTLS12, VersionTLS11, VersionTLS10, VersionSSL30}
  
  func (hs *serverHandshakeState) clientHelloInfo() *ClientHelloInfo {
  	if hs.cachedClientHelloInfo != nil {
  		return hs.cachedClientHelloInfo
  	}
  
  	var supportedVersions []uint16
  	if hs.clientHello.vers > VersionTLS12 {
  		supportedVersions = suppVersArray[:]
  	} else if hs.clientHello.vers >= VersionSSL30 {
  		supportedVersions = suppVersArray[VersionTLS12-hs.clientHello.vers:]
  	}
  
  	signatureSchemes := make([]SignatureScheme, 0, len(hs.clientHello.signatureAndHashes))
  	for _, sah := range hs.clientHello.signatureAndHashes {
  		signatureSchemes = append(signatureSchemes, SignatureScheme(sah.hash)<<8+SignatureScheme(sah.signature))
  	}
  
  	hs.cachedClientHelloInfo = &ClientHelloInfo{
  		CipherSuites:      hs.clientHello.cipherSuites,
  		ServerName:        hs.clientHello.serverName,
  		SupportedCurves:   hs.clientHello.supportedCurves,
  		SupportedPoints:   hs.clientHello.supportedPoints,
  		SignatureSchemes:  signatureSchemes,
  		SupportedProtos:   hs.clientHello.alpnProtocols,
  		SupportedVersions: supportedVersions,
  		Conn:              hs.c.conn,
  	}
  
  	return hs.cachedClientHelloInfo
  }
  

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