diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -1,2 +1,2 @@
-hello initia
+hello initial
 #############
diff --git a/example.go b/example.go
new file mode 100644
--- /dev/null
+++ b/example.go
@@ -0,0 +1,2130 @@
+// 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 runner
+
+import (
+	"bytes"
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rsa"
+	"crypto/subtle"
+	"crypto/x509"
+	"errors"
+	"fmt"
+	"io"
+	"math/big"
+	"time"
+
+	"./ed25519"
+)
+
+// 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
+	sessionState    *sessionState
+	finishedHash    finishedHash
+	masterSecret    []byte
+	certsFromClient [][]byte
+	cert            *Certificate
+	finishedBytes   []byte
+}
+
+// serverHandshake performs a TLS handshake as a server.
+func (c *Conn) serverHandshake() error {
+	config := c.config
+
+	// If this is the first server handshake, we generate a random key to
+	// encrypt the tickets with.
+	config.serverInitOnce.Do(config.serverInit)
+
+	c.sendHandshakeSeq = 0
+	c.recvHandshakeSeq = 0
+
+	hs := serverHandshakeState{
+		c: c,
+	}
+	if err := hs.readClientHello(); err != nil {
+		return err
+	}
+
+	if c.vers >= VersionTLS13 {
+		if err := hs.doTLS13Handshake(); err != nil {
+			return err
+		}
+	} else {
+		isResume, err := hs.processClientHello()
+		if err != nil {
+			return err
+		}
+
+		// For an overview of TLS handshaking, see https://tools.ietf.org/html/rfc5246#section-7.3
+		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
+			}
+			if c.config.Bugs.RenewTicketOnResume {
+				if err := hs.sendSessionTicket(); err != nil {
+					return err
+				}
+			}
+			if err := hs.sendFinished(c.firstFinished[:], isResume); err != nil {
+				return err
+			}
+			// Most retransmits are triggered by a timeout, but the final
+			// leg of the handshake is retransmited upon re-receiving a
+			// Finished.
+			if err := c.simulatePacketLoss(func() {
+				c.sendHandshakeSeq--
+				c.writeRecord(recordTypeHandshake, hs.finishedBytes)
+				c.flushHandshake()
+			}); err != nil {
+				return err
+			}
+			if err := hs.readFinished(nil, isResume); 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.firstFinished[:], isResume); err != nil {
+				return err
+			}
+			if c.config.Bugs.AlertBeforeFalseStartTest != 0 {
+				c.sendAlert(c.config.Bugs.AlertBeforeFalseStartTest)
+			}
+			if c.config.Bugs.ExpectFalseStart {
+				if err := c.readRecord(recordTypeApplicationData); err != nil {
+					return fmt.Errorf("tls: peer did not false start: %s", err)
+				}
+			}
+			if err := hs.sendSessionTicket(); err != nil {
+				return err
+			}
+			if err := hs.sendFinished(nil, isResume); err != nil {
+				return err
+			}
+		}
+
+		c.exporterSecret = hs.masterSecret
+	}
+	c.handshakeComplete = true
+	copy(c.clientRandom[:], hs.clientHello.random)
+	copy(c.serverRandom[:], hs.hello.random)
+
+	return nil
+}
+
+// readClientHello reads a ClientHello message from the client and determines
+// the protocol version.
+func (hs *serverHandshakeState) readClientHello() error {
+	config := hs.c.config
+	c := hs.c
+
+	if err := c.simulatePacketLoss(nil); err != nil {
+		return err
+	}
+	msg, err := c.readHandshake()
+	if err != nil {
+		return err
+	}
+	var ok bool
+	hs.clientHello, ok = msg.(*clientHelloMsg)
+	if !ok {
+		c.sendAlert(alertUnexpectedMessage)
+		return unexpectedMessageError(hs.clientHello, msg)
+	}
+	if size := config.Bugs.RequireClientHelloSize; size != 0 && len(hs.clientHello.raw) != size {
+		return fmt.Errorf("tls: ClientHello record size is %d, but expected %d", len(hs.clientHello.raw), size)
+	}
+
+	if c.isDTLS && !config.Bugs.SkipHelloVerifyRequest {
+		// Per RFC 6347, the version field in HelloVerifyRequest SHOULD
+		// be always DTLS 1.0
+		helloVerifyRequest := &helloVerifyRequestMsg{
+			vers:   VersionDTLS10,
+			cookie: make([]byte, 32),
+		}
+		if _, err := io.ReadFull(c.config.rand(), helloVerifyRequest.cookie); err != nil {
+			c.sendAlert(alertInternalError)
+			return errors.New("dtls: short read from Rand: " + err.Error())
+		}
+		c.writeRecord(recordTypeHandshake, helloVerifyRequest.marshal())
+		c.flushHandshake()
+
+		if err := c.simulatePacketLoss(nil); err != nil {
+			return err
+		}
+		msg, err := c.readHandshake()
+		if err != nil {
+			return err
+		}
+		newClientHello, ok := msg.(*clientHelloMsg)
+		if !ok {
+			c.sendAlert(alertUnexpectedMessage)
+			return unexpectedMessageError(hs.clientHello, msg)
+		}
+		if !bytes.Equal(newClientHello.cookie, helloVerifyRequest.cookie) {
+			return errors.New("dtls: invalid cookie")
+		}
+
+		// Apart from the cookie, the two ClientHellos must
+		// match. Note that clientHello.equal compares the
+		// serialization, so we make a copy.
+		oldClientHelloCopy := *hs.clientHello
+		oldClientHelloCopy.raw = nil
+		oldClientHelloCopy.cookie = nil
+		newClientHelloCopy := *newClientHello
+		newClientHelloCopy.raw = nil
+		newClientHelloCopy.cookie = nil
+		if !oldClientHelloCopy.equal(&newClientHelloCopy) {
+			return errors.New("dtls: retransmitted ClientHello does not match")
+		}
+		hs.clientHello = newClientHello
+	}
+
+	if config.Bugs.RequireSameRenegoClientVersion && c.clientVersion != 0 {
+		if c.clientVersion != hs.clientHello.vers {
+			return fmt.Errorf("tls: client offered different version on renego")
+		}
+	}
+
+	c.clientVersion = hs.clientHello.vers
+
+	// Use the versions extension if supplied, otherwise use the legacy ClientHello version.
+	if len(hs.clientHello.supportedVersions) == 0 {
+		if c.isDTLS {
+			if hs.clientHello.vers <= VersionDTLS12 {
+				hs.clientHello.supportedVersions = append(hs.clientHello.supportedVersions, VersionDTLS12)
+			}
+			if hs.clientHello.vers <= VersionDTLS10 {
+				hs.clientHello.supportedVersions = append(hs.clientHello.supportedVersions, VersionDTLS10)
+			}
+		} else {
+			if hs.clientHello.vers >= VersionTLS12 {
+				hs.clientHello.supportedVersions = append(hs.clientHello.supportedVersions, VersionTLS12)
+			}
+			if hs.clientHello.vers >= VersionTLS11 {
+				hs.clientHello.supportedVersions = append(hs.clientHello.supportedVersions, VersionTLS11)
+			}
+			if hs.clientHello.vers >= VersionTLS10 {
+				hs.clientHello.supportedVersions = append(hs.clientHello.supportedVersions, VersionTLS10)
+			}
+			if hs.clientHello.vers >= VersionSSL30 {
+				hs.clientHello.supportedVersions = append(hs.clientHello.supportedVersions, VersionSSL30)
+			}
+		}
+	} else if config.Bugs.ExpectGREASE && !containsGREASE(hs.clientHello.supportedVersions) {
+		return errors.New("tls: no GREASE version value found")
+	}
+
+	if !c.haveVers {
+		if config.Bugs.NegotiateVersion != 0 {
+			c.wireVersion = config.Bugs.NegotiateVersion
+		} else {
+			var found bool
+			for _, vers := range hs.clientHello.supportedVersions {
+				if _, ok := config.isSupportedVersion(vers, c.isDTLS); ok {
+					c.wireVersion = vers
+					found = true
+					break
+				}
+			}
+			if !found {
+				c.sendAlert(alertProtocolVersion)
+				return errors.New("tls: client did not offer any supported protocol versions")
+			}
+		}
+	} else if config.Bugs.NegotiateVersionOnRenego != 0 {
+		c.wireVersion = config.Bugs.NegotiateVersionOnRenego
+	}
+
+	c.vers, ok = wireToVersion(c.wireVersion, c.isDTLS)
+	if !ok {
+		panic("Could not map wire version")
+	}
+	c.haveVers = true
+
+	clientProtocol, ok := wireToVersion(c.clientVersion, c.isDTLS)
+
+	// Reject < 1.2 ClientHellos with signature_algorithms.
+	if ok && clientProtocol < VersionTLS12 && len(hs.clientHello.signatureAlgorithms) > 0 {
+		return fmt.Errorf("tls: client included signature_algorithms before TLS 1.2")
+	}
+
+	// Check the client cipher list is consistent with the version.
+	if ok && clientProtocol < VersionTLS12 {
+		for _, id := range hs.clientHello.cipherSuites {
+			if isTLS12Cipher(id) {
+				return fmt.Errorf("tls: client offered TLS 1.2 cipher before TLS 1.2")
+			}
+		}
+	}
+
+	if config.Bugs.ExpectNoTLS12Session {
+		if len(hs.clientHello.sessionId) > 0 && c.vers >= VersionTLS13 {
+			return fmt.Errorf("tls: client offered an unexpected session ID")
+		}
+		if len(hs.clientHello.sessionTicket) > 0 {
+			return fmt.Errorf("tls: client offered an unexpected session ticket")
+		}
+	}
+
+	if config.Bugs.ExpectNoTLS13PSK && len(hs.clientHello.pskIdentities) > 0 {
+		return fmt.Errorf("tls: client offered unexpected PSK identities")
+	}
+
+	var scsvFound bool
+	for _, cipherSuite := range hs.clientHello.cipherSuites {
+		if cipherSuite == fallbackSCSV {
+			scsvFound = true
+			break
+		}
+	}
+
+	if !scsvFound && config.Bugs.FailIfNotFallbackSCSV {
+		return errors.New("tls: no fallback SCSV found when expected")
+	} else if scsvFound && !config.Bugs.FailIfNotFallbackSCSV {
+		return errors.New("tls: fallback SCSV found when not expected")
+	}
+
+	if config.Bugs.ExpectGREASE && !containsGREASE(hs.clientHello.cipherSuites) {
+		return errors.New("tls: no GREASE cipher suite value found")
+	}
+
+	var greaseFound bool
+	for _, curve := range hs.clientHello.supportedCurves {
+		if isGREASEValue(uint16(curve)) {
+			greaseFound = true
+			break
+		}
+	}
+
+	if !greaseFound && config.Bugs.ExpectGREASE {
+		return errors.New("tls: no GREASE curve value found")
+	}
+
+	if len(hs.clientHello.keyShares) > 0 {
+		greaseFound = false
+		for _, keyShare := range hs.clientHello.keyShares {
+			if isGREASEValue(uint16(keyShare.group)) {
+				greaseFound = true
+				break
+			}
+		}
+
+		if !greaseFound && config.Bugs.ExpectGREASE {
+			return errors.New("tls: no GREASE curve value found")
+		}
+	}
+
+	if expected := hs.clientHello.dummyPQPaddingLen; expected != config.Bugs.ExpectDummyPQPaddingLength {
+		return fmt.Errorf("tls: expected dummy PQ padding extension of length %d, but got one of length %d", expected, config.Bugs.ExpectDummyPQPaddingLength)
+	}
+
+	applyBugsToClientHello(hs.clientHello, config)
+
+	return nil
+}
+
+func applyBugsToClientHello(clientHello *clientHelloMsg, config *Config) {
+	if config.Bugs.IgnorePeerSignatureAlgorithmPreferences {
+		clientHello.signatureAlgorithms = config.signSignatureAlgorithms()
+	}
+	if config.Bugs.IgnorePeerCurvePreferences {
+		clientHello.supportedCurves = config.curvePreferences()
+	}
+	if config.Bugs.IgnorePeerCipherPreferences {
+		clientHello.cipherSuites = config.cipherSuites()
+	}
+}
+
+func (hs *serverHandshakeState) doTLS13Handshake() error {
+	c := hs.c
+	config := c.config
+
+	// We've read the ClientHello, so the next record must be preceded with ChangeCipherSpec.
+	c.expectTLS13ChangeCipherSpec = true
+
+	hs.hello = &serverHelloMsg{
+		isDTLS:                c.isDTLS,
+		vers:                  c.wireVersion,
+		sessionId:             hs.clientHello.sessionId,
+		compressionMethod:     config.Bugs.SendCompressionMethod,
+		versOverride:          config.Bugs.SendServerHelloVersion,
+		supportedVersOverride: config.Bugs.SendServerSupportedExtensionVersion,
+		customExtension:       config.Bugs.CustomUnencryptedExtension,
+		unencryptedALPN:       config.Bugs.SendUnencryptedALPN,
+	}
+
+	hs.hello.random = make([]byte, 32)
+	if _, err := io.ReadFull(config.rand(), hs.hello.random); err != nil {
+		c.sendAlert(alertInternalError)
+		return err
+	}
+
+	// TLS 1.3 forbids clients from advertising any non-null compression.
+	if len(hs.clientHello.compressionMethods) != 1 || hs.clientHello.compressionMethods[0] != compressionNone {
+		return errors.New("tls: client sent compression method other than null for TLS 1.3")
+	}
+
+	// Prepare an EncryptedExtensions message, but do not send it yet.
+	encryptedExtensions := new(encryptedExtensionsMsg)
+	encryptedExtensions.empty = config.Bugs.EmptyEncryptedExtensions
+	if err := hs.processClientExtensions(&encryptedExtensions.extensions); err != nil {
+		return err
+	}
+
+	// Select the cipher suite.
+	var preferenceList, supportedList []uint16
+	if config.PreferServerCipherSuites {
+		preferenceList = config.cipherSuites()
+		supportedList = hs.clientHello.cipherSuites
+	} else {
+		preferenceList = hs.clientHello.cipherSuites
+		supportedList = config.cipherSuites()
+	}
+
+	for _, id := range preferenceList {
+		if hs.suite = c.tryCipherSuite(id, supportedList, c.vers, true, true); hs.suite != nil {
+			break
+		}
+	}
+
+	if hs.suite == nil {
+		c.sendAlert(alertHandshakeFailure)
+		return errors.New("tls: no cipher suite supported by both client and server")
+	}
+
+	hs.hello.cipherSuite = hs.suite.id
+	if c.config.Bugs.SendCipherSuite != 0 {
+		hs.hello.cipherSuite = c.config.Bugs.SendCipherSuite
+	}
+
+	hs.finishedHash = newFinishedHash(c.wireVersion, c.isDTLS, hs.suite)
+	hs.finishedHash.discardHandshakeBuffer()
+	hs.writeClientHash(hs.clientHello.marshal())
+
+	supportedCurve := false
+	var selectedCurve CurveID
+	preferredCurves := config.curvePreferences()
+Curves:
+	for _, curve := range hs.clientHello.supportedCurves {
+		for _, supported := range preferredCurves {
+			if supported == curve {
+				supportedCurve = true
+				selectedCurve = curve
+				break Curves
+			}
+		}
+	}
+
+	if !supportedCurve {
+		c.sendAlert(alertHandshakeFailure)
+		return errors.New("tls: no curve supported by both client and server")
+	}
+
+	pskIdentities := hs.clientHello.pskIdentities
+	pskKEModes := hs.clientHello.pskKEModes
+
+	if len(pskIdentities) == 0 && len(hs.clientHello.sessionTicket) > 0 && c.config.Bugs.AcceptAnySession {
+		psk := pskIdentity{
+			ticket: hs.clientHello.sessionTicket,
+		}
+		pskIdentities = []pskIdentity{psk}
+		pskKEModes = []byte{pskDHEKEMode}
+	}
+
+	var pskIndex int
+	foundKEMode := bytes.IndexByte(pskKEModes, pskDHEKEMode) >= 0
+	if foundKEMode && !config.SessionTicketsDisabled {
+		for i, pskIdentity := range pskIdentities {
+			// TODO(svaldez): Check the obfuscatedTicketAge before accepting 0-RTT.
+			sessionState, ok := c.decryptTicket(pskIdentity.ticket)
+			if !ok {
+				continue
+			}
+
+			if !config.Bugs.AcceptAnySession {
+				if sessionState.vers != c.vers {
+					continue
+				}
+				if sessionState.ticketExpiration.Before(c.config.time()) {
+					continue
+				}
+				sessionCipher := cipherSuiteFromID(sessionState.cipherSuite)
+				if sessionCipher == nil || sessionCipher.hash() != hs.suite.hash() {
+					continue
+				}
+			}
+
+			clientTicketAge := time.Duration(uint32(pskIdentity.obfuscatedTicketAge-sessionState.ticketAgeAdd)) * time.Millisecond
+			if config.Bugs.ExpectTicketAge != 0 && clientTicketAge != config.Bugs.ExpectTicketAge {
+				c.sendAlert(alertHandshakeFailure)
+				return errors.New("tls: invalid ticket age")
+			}
+
+			hs.sessionState = sessionState
+			hs.hello.hasPSKIdentity = true
+			hs.hello.pskIdentity = uint16(i)
+			pskIndex = i
+			if config.Bugs.SelectPSKIdentityOnResume != 0 {
+				hs.hello.pskIdentity = config.Bugs.SelectPSKIdentityOnResume
+			}
+			c.didResume = true
+			break
+		}
+	}
+
+	if config.Bugs.AlwaysSelectPSKIdentity {
+		hs.hello.hasPSKIdentity = true
+		hs.hello.pskIdentity = 0
+	}
+
+	// Verify the PSK binder. Note there may not be a PSK binder if
+	// AcceptAnyBinder is set. See https://crbug.com/boringssl/115.
+	if hs.sessionState != nil && !config.Bugs.AcceptAnySession {
+		binderToVerify := hs.clientHello.pskBinders[pskIndex]
+		if err := verifyPSKBinder(c.wireVersion, hs.clientHello, hs.sessionState, binderToVerify, []byte{}, []byte{}); err != nil {
+			return err
+		}
+	}
+
+	// Resolve PSK and compute the early secret.
+	if hs.sessionState != nil {
+		hs.finishedHash.addEntropy(hs.sessionState.masterSecret)
+	} else {
+		hs.finishedHash.addEntropy(hs.finishedHash.zeroSecret())
+	}
+
+	hs.hello.hasKeyShare = true
+	if hs.sessionState != nil && config.Bugs.NegotiatePSKResumption {
+		hs.hello.hasKeyShare = false
+	}
+	if config.Bugs.MissingKeyShare {
+		hs.hello.hasKeyShare = false
+	}
+
+	firstHelloRetryRequest := true
+
+ResendHelloRetryRequest:
+	var sendHelloRetryRequest bool
+	cipherSuite := hs.suite.id
+	if config.Bugs.SendHelloRetryRequestCipherSuite != 0 {
+		cipherSuite = config.Bugs.SendHelloRetryRequestCipherSuite
+	}
+	helloRetryRequest := &helloRetryRequestMsg{
+		vers:                c.wireVersion,
+		sessionId:           hs.clientHello.sessionId,
+		cipherSuite:         cipherSuite,
+		compressionMethod:   config.Bugs.SendCompressionMethod,
+		duplicateExtensions: config.Bugs.DuplicateHelloRetryRequestExtensions,
+	}
+
+	if config.Bugs.AlwaysSendHelloRetryRequest {
+		sendHelloRetryRequest = true
+	}
+
+	if config.Bugs.SendHelloRetryRequestCookie != nil {
+		sendHelloRetryRequest = true
+		helloRetryRequest.cookie = config.Bugs.SendHelloRetryRequestCookie
+	}
+
+	if len(config.Bugs.CustomHelloRetryRequestExtension) > 0 {
+		sendHelloRetryRequest = true
+		helloRetryRequest.customExtension = config.Bugs.CustomHelloRetryRequestExtension
+	}
+
+	var selectedKeyShare *keyShareEntry
+	if hs.hello.hasKeyShare {
+		// Look for the key share corresponding to our selected curve.
+		for i := range hs.clientHello.keyShares {
+			if hs.clientHello.keyShares[i].group == selectedCurve {
+				selectedKeyShare = &hs.clientHello.keyShares[i]
+				break
+			}
+		}
+
+		if config.Bugs.ExpectMissingKeyShare && selectedKeyShare != nil {
+			return errors.New("tls: expected missing key share")
+		}
+
+		if selectedKeyShare == nil {
+			helloRetryRequest.hasSelectedGroup = true
+			helloRetryRequest.selectedGroup = selectedCurve
+			sendHelloRetryRequest = true
+		}
+	}
+
+	if config.Bugs.SendHelloRetryRequestCurve != 0 {
+		helloRetryRequest.hasSelectedGroup = true
+		helloRetryRequest.selectedGroup = config.Bugs.SendHelloRetryRequestCurve
+		sendHelloRetryRequest = true
+	}
+
+	if config.Bugs.SkipHelloRetryRequest {
+		sendHelloRetryRequest = false
+	}
+
+	if sendHelloRetryRequest {
+		if err := hs.finishedHash.UpdateForHelloRetryRequest(); err != nil {
+			return err
+		}
+
+		oldClientHelloBytes := hs.clientHello.marshal()
+		hs.writeServerHash(helloRetryRequest.marshal())
+		c.writeRecord(recordTypeHandshake, helloRetryRequest.marshal())
+		c.flushHandshake()
+
+		if !c.config.Bugs.SkipChangeCipherSpec {
+			c.writeRecord(recordTypeChangeCipherSpec, []byte{1})
+		}
+
+		if hs.clientHello.hasEarlyData {
+			c.skipEarlyData = true
+		}
+
+		// Read new ClientHello.
+		newMsg, err := c.readHandshake()
+		if err != nil {
+			return err
+		}
+		newClientHello, ok := newMsg.(*clientHelloMsg)
+		if !ok {
+			c.sendAlert(alertUnexpectedMessage)
+			return unexpectedMessageError(newClientHello, newMsg)
+		}
+		hs.writeClientHash(newClientHello.marshal())
+
+		if config.Bugs.ExpectNoTLS13PSKAfterHRR && len(newClientHello.pskIdentities) > 0 {
+			return fmt.Errorf("tls: client offered unexpected PSK identities after HelloRetryRequest")
+		}
+
+		if newClientHello.hasEarlyData {
+			return errors.New("tls: EarlyData sent in new ClientHello")
+		}
+
+		applyBugsToClientHello(newClientHello, config)
+
+		// Check that the new ClientHello matches the old ClientHello,
+		// except for relevant modifications.
+		//
+		// TODO(davidben): Make this check more precise.
+		oldClientHelloCopy := *hs.clientHello
+		oldClientHelloCopy.raw = nil
+		oldClientHelloCopy.hasEarlyData = false
+		newClientHelloCopy := *newClientHello
+		newClientHelloCopy.raw = nil
+
+		if helloRetryRequest.hasSelectedGroup {
+			newKeyShares := newClientHelloCopy.keyShares
+			if len(newKeyShares) != 1 || newKeyShares[0].group != helloRetryRequest.selectedGroup {
+				return errors.New("tls: KeyShare from HelloRetryRequest not in new ClientHello")
+			}
+			selectedKeyShare = &newKeyShares[0]
+			newClientHelloCopy.keyShares = oldClientHelloCopy.keyShares
+		}
+
+		if len(helloRetryRequest.cookie) > 0 {
+			if !bytes.Equal(newClientHelloCopy.tls13Cookie, helloRetryRequest.cookie) {
+				return errors.New("tls: cookie from HelloRetryRequest not present in new ClientHello")
+			}
+			newClientHelloCopy.tls13Cookie = nil
+		}
+
+		// PSK binders and obfuscated ticket age are both updated in the
+		// second ClientHello.
+		if len(oldClientHelloCopy.pskIdentities) != len(newClientHelloCopy.pskIdentities) {
+			newClientHelloCopy.pskIdentities = oldClientHelloCopy.pskIdentities
+		} else {
+			if len(oldClientHelloCopy.pskIdentities) != len(newClientHelloCopy.pskIdentities) {
+				return errors.New("tls: PSK identity count from old and new ClientHello do not match")
+			}
+			for i, identity := range oldClientHelloCopy.pskIdentities {
+				newClientHelloCopy.pskIdentities[i].obfuscatedTicketAge = identity.obfuscatedTicketAge
+			}
+		}
+		newClientHelloCopy.pskBinders = oldClientHelloCopy.pskBinders
+		newClientHelloCopy.hasEarlyData = oldClientHelloCopy.hasEarlyData
+
+		if !oldClientHelloCopy.equal(&newClientHelloCopy) {
+			return errors.New("tls: new ClientHello does not match")
+		}
+
+		if firstHelloRetryRequest && config.Bugs.SecondHelloRetryRequest {
+			firstHelloRetryRequest = false
+			goto ResendHelloRetryRequest
+		}
+
+		// Verify the PSK binder. Note there may not be a PSK binder if
+		// AcceptAnyBinder is set. See https://crbug.com/115.
+		if hs.sessionState != nil && !config.Bugs.AcceptAnySession {
+			binderToVerify := newClientHello.pskBinders[pskIndex]
+			if err := verifyPSKBinder(c.wireVersion, newClientHello, hs.sessionState, binderToVerify, oldClientHelloBytes, helloRetryRequest.marshal()); err != nil {
+				return err
+			}
+		}
+	}
+
+	// Decide whether or not to accept early data.
+	if !sendHelloRetryRequest && hs.clientHello.hasEarlyData {
+		if !config.Bugs.AlwaysRejectEarlyData && hs.sessionState != nil {
+			if c.clientProtocol == string(hs.sessionState.earlyALPN) || config.Bugs.AlwaysAcceptEarlyData {
+				encryptedExtensions.extensions.hasEarlyData = true
+			}
+		}
+		if encryptedExtensions.extensions.hasEarlyData {
+			earlyTrafficSecret := hs.finishedHash.deriveSecret(earlyTrafficLabel)
+			c.earlyExporterSecret = hs.finishedHash.deriveSecret(earlyExporterLabel)
+
+			if err := c.useInTrafficSecret(c.wireVersion, hs.suite, earlyTrafficSecret); err != nil {
+				return err
+			}
+
+			c.earlyCipherSuite = hs.suite
+			expectEarlyData := config.Bugs.ExpectEarlyData
+			if n := config.Bugs.ExpectEarlyKeyingMaterial; n > 0 {
+				exporter, err := c.ExportEarlyKeyingMaterial(n, []byte(config.Bugs.ExpectEarlyKeyingLabel), []byte(config.Bugs.ExpectEarlyKeyingContext))
+				if err != nil {
+					return err
+				}
+				expectEarlyData = append([][]byte{exporter}, expectEarlyData...)
+			}
+
+			for _, expectedMsg := range expectEarlyData {
+				if err := c.readRecord(recordTypeApplicationData); err != nil {
+					return err
+				}
+				msg := c.input.data[c.input.off:]
+				if !bytes.Equal(msg, expectedMsg) {
+					return fmt.Errorf("tls: got early data record %x, wanted %x", msg, expectedMsg)
+				}
+				c.in.freeBlock(c.input)
+				c.input = nil
+			}
+		} else {
+			c.skipEarlyData = true
+		}
+	}
+
+	if config.Bugs.SendEarlyDataExtension {
+		encryptedExtensions.extensions.hasEarlyData = true
+	}
+
+	// Resolve ECDHE and compute the handshake secret.
+	if hs.hello.hasKeyShare {
+		// Once a curve has been selected and a key share identified,
+		// the server needs to generate a public value and send it in
+		// the ServerHello.
+		curve, ok := curveForCurveID(selectedCurve, config)
+		if !ok {
+			panic("tls: server failed to look up curve ID")
+		}
+		c.curveID = selectedCurve
+
+		var peerKey []byte
+		if config.Bugs.SkipHelloRetryRequest {
+			// If skipping HelloRetryRequest, use a random key to
+			// avoid crashing.
+			curve2, _ := curveForCurveID(selectedCurve, config)
+			var err error
+			peerKey, err = curve2.offer(config.rand())
+			if err != nil {
+				return err
+			}
+		} else {
+			peerKey = selectedKeyShare.keyExchange
+		}
+
+		publicKey, ecdheSecret, err := curve.accept(config.rand(), peerKey)
+		if err != nil {
+			c.sendAlert(alertHandshakeFailure)
+			return err
+		}
+		hs.finishedHash.nextSecret()
+		hs.finishedHash.addEntropy(ecdheSecret)
+		hs.hello.hasKeyShare = true
+
+		curveID := selectedCurve
+		if c.config.Bugs.SendCurve != 0 {
+			curveID = config.Bugs.SendCurve
+		}
+		if c.config.Bugs.InvalidECDHPoint {
+			publicKey[0] ^= 0xff
+		}
+
+		hs.hello.keyShare = keyShareEntry{
+			group:       curveID,
+			keyExchange: publicKey,
+		}
+
+		if config.Bugs.EncryptedExtensionsWithKeyShare {
+			encryptedExtensions.extensions.hasKeyShare = true
+			encryptedExtensions.extensions.keyShare = keyShareEntry{
+				group:       curveID,
+				keyExchange: publicKey,
+			}
+		}
+	} else {
+		hs.finishedHash.nextSecret()
+		hs.finishedHash.addEntropy(hs.finishedHash.zeroSecret())
+	}
+
+	// Send unencrypted ServerHello.
+	hs.writeServerHash(hs.hello.marshal())
+	if config.Bugs.PartialEncryptedExtensionsWithServerHello {
+		helloBytes := hs.hello.marshal()
+		toWrite := make([]byte, 0, len(helloBytes)+1)
+		toWrite = append(toWrite, helloBytes...)
+		toWrite = append(toWrite, typeEncryptedExtensions)
+		c.writeRecord(recordTypeHandshake, toWrite)
+	} else {
+		c.writeRecord(recordTypeHandshake, hs.hello.marshal())
+	}
+	c.flushHandshake()
+
+	if !c.config.Bugs.SkipChangeCipherSpec && !sendHelloRetryRequest {
+		c.writeRecord(recordTypeChangeCipherSpec, []byte{1})
+	}
+
+	for i := 0; i < c.config.Bugs.SendExtraChangeCipherSpec; i++ {
+		c.writeRecord(recordTypeChangeCipherSpec, []byte{1})
+	}
+
+	// Switch to handshake traffic keys.
+	serverHandshakeTrafficSecret := hs.finishedHash.deriveSecret(serverHandshakeTrafficLabel)
+	c.useOutTrafficSecret(c.wireVersion, hs.suite, serverHandshakeTrafficSecret)
+	// Derive handshake traffic read key, but don't switch yet.
+	clientHandshakeTrafficSecret := hs.finishedHash.deriveSecret(clientHandshakeTrafficLabel)
+
+	// Send EncryptedExtensions.
+	hs.writeServerHash(encryptedExtensions.marshal())
+	if config.Bugs.PartialEncryptedExtensionsWithServerHello {
+		// The first byte has already been sent.
+		c.writeRecord(recordTypeHandshake, encryptedExtensions.marshal()[1:])
+	} else {
+		c.writeRecord(recordTypeHandshake, encryptedExtensions.marshal())
+	}
+
+	if hs.sessionState == nil {
+		if config.ClientAuth >= RequestClientCert {
+			// Request a client certificate
+			certReq := &certificateRequestMsg{
+				vers: c.wireVersion,
+				hasSignatureAlgorithm: !config.Bugs.OmitCertificateRequestAlgorithms,
+				hasRequestContext:     true,
+				requestContext:        config.Bugs.SendRequestContext,
+				customExtension:       config.Bugs.SendCustomCertificateRequest,
+			}
+			if !config.Bugs.NoSignatureAlgorithms {
+				certReq.signatureAlgorithms = config.verifySignatureAlgorithms()
+			}
+
+			// 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 config.ClientCAs != nil {
+				certReq.certificateAuthorities = config.ClientCAs.Subjects()
+			}
+			hs.writeServerHash(certReq.marshal())
+			c.writeRecord(recordTypeHandshake, certReq.marshal())
+		}
+
+		certMsg := &certificateMsg{
+			hasRequestContext: true,
+		}
+		if !config.Bugs.EmptyCertificateList {
+			for i, certData := range hs.cert.Certificate {
+				cert := certificateEntry{
+					data: certData,
+				}
+				if i == 0 {
+					if hs.clientHello.ocspStapling {
+						cert.ocspResponse = hs.cert.OCSPStaple
+					}
+					if hs.clientHello.sctListSupported {
+						cert.sctList = hs.cert.SignedCertificateTimestampList
+					}
+					cert.duplicateExtensions = config.Bugs.SendDuplicateCertExtensions
+					cert.extraExtension = config.Bugs.SendExtensionOnCertificate
+				} else {
+					if config.Bugs.SendOCSPOnIntermediates != nil {
+						cert.ocspResponse = config.Bugs.SendOCSPOnIntermediates
+					}
+					if config.Bugs.SendSCTOnIntermediates != nil {
+						cert.sctList = config.Bugs.SendSCTOnIntermediates
+					}
+				}
+				certMsg.certificates = append(certMsg.certificates, cert)
+			}
+		}
+		certMsgBytes := certMsg.marshal()
+		hs.writeServerHash(certMsgBytes)
+		c.writeRecord(recordTypeHandshake, certMsgBytes)
+
+		certVerify := &certificateVerifyMsg{
+			hasSignatureAlgorithm: true,
+		}
+
+		// Determine the hash to sign.
+		privKey := hs.cert.PrivateKey
+
+		var err error
+		certVerify.signatureAlgorithm, err = selectSignatureAlgorithm(c.vers, privKey, config, hs.clientHello.signatureAlgorithms)
+		if err != nil {
+			c.sendAlert(alertInternalError)
+			return err
+		}
+
+		input := hs.finishedHash.certificateVerifyInput(serverCertificateVerifyContextTLS13)
+		certVerify.signature, err = signMessage(c.vers, privKey, c.config, certVerify.signatureAlgorithm, input)
+		if err != nil {
+			c.sendAlert(alertInternalError)
+			return err
+		}
+
+		if config.Bugs.SendSignatureAlgorithm != 0 {
+			certVerify.signatureAlgorithm = config.Bugs.SendSignatureAlgorithm
+		}
+
+		if !config.Bugs.SkipCertificateVerify {
+			hs.writeServerHash(certVerify.marshal())
+			c.writeRecord(recordTypeHandshake, certVerify.marshal())
+		}
+	} else if hs.sessionState != nil {
+		// Pick up certificates from the session instead.
+		if len(hs.sessionState.certificates) > 0 {
+			if _, err := hs.processCertsFromClient(hs.sessionState.certificates); err != nil {
+				return err
+			}
+		}
+	}
+
+	finished := new(finishedMsg)
+	finished.verifyData = hs.finishedHash.serverSum(serverHandshakeTrafficSecret)
+	if config.Bugs.BadFinished {
+		finished.verifyData[0]++
+	}
+	hs.writeServerHash(finished.marshal())
+	c.writeRecord(recordTypeHandshake, finished.marshal())
+	if c.config.Bugs.SendExtraFinished {
+		c.writeRecord(recordTypeHandshake, finished.marshal())
+	}
+	c.flushHandshake()
+
+	if encryptedExtensions.extensions.hasEarlyData && !c.skipEarlyData {
+		for _, expectedMsg := range config.Bugs.ExpectLateEarlyData {
+			if err := c.readRecord(recordTypeApplicationData); err != nil {
+				return err
+			}
+			if !bytes.Equal(c.input.data[c.input.off:], expectedMsg) {
+				return errors.New("ExpectLateEarlyData: did not get expected message")
+			}
+			c.in.freeBlock(c.input)
+			c.input = nil
+		}
+	}
+
+	// The various secrets do not incorporate the client's final leg, so
+	// derive them now before updating the handshake context.
+	hs.finishedHash.nextSecret()
+	hs.finishedHash.addEntropy(hs.finishedHash.zeroSecret())
+
+	clientTrafficSecret := hs.finishedHash.deriveSecret(clientApplicationTrafficLabel)
+	serverTrafficSecret := hs.finishedHash.deriveSecret(serverApplicationTrafficLabel)
+	c.exporterSecret = hs.finishedHash.deriveSecret(exporterLabel)
+
+	// Switch to application data keys on write. In particular, any alerts
+	// from the client certificate are sent over these keys.
+	c.useOutTrafficSecret(c.wireVersion, hs.suite, serverTrafficSecret)
+
+	// Send 0.5-RTT messages.
+	for _, halfRTTMsg := range config.Bugs.SendHalfRTTData {
+		if _, err := c.writeRecord(recordTypeApplicationData, halfRTTMsg); err != nil {
+			return err
+		}
+	}
+
+	// Read end_of_early_data.
+	if encryptedExtensions.extensions.hasEarlyData {
+		msg, err := c.readHandshake()
+		if err != nil {
+			return err
+		}
+
+		endOfEarlyData, ok := msg.(*endOfEarlyDataMsg)
+		if !ok {
+			c.sendAlert(alertUnexpectedMessage)
+			return unexpectedMessageError(endOfEarlyData, msg)
+		}
+		hs.writeClientHash(endOfEarlyData.marshal())
+	}
+
+	// Switch input stream to handshake traffic keys.
+	if err := c.useInTrafficSecret(c.wireVersion, hs.suite, clientHandshakeTrafficSecret); err != nil {
+		return err
+	}
+
+	// If we requested a client certificate, then the client must send a
+	// certificate message, even if it's empty.
+	if config.ClientAuth >= RequestClientCert {
+		msg, err := c.readHandshake()
+		if err != nil {
+			return err
+		}
+
+		certMsg, ok := msg.(*certificateMsg)
+		if !ok {
+			c.sendAlert(alertUnexpectedMessage)
+			return unexpectedMessageError(certMsg, msg)
+		}
+		hs.writeClientHash(certMsg.marshal())
+
+		if len(certMsg.certificates) == 0 {
+			// The client didn't actually send a certificate
+			switch config.ClientAuth {
+			case RequireAnyClientCert, RequireAndVerifyClientCert:
+				c.sendAlert(alertCertificateRequired)
+				return errors.New("tls: client didn't provide a certificate")
+			}
+		}
+
+		var certs [][]byte
+		for _, cert := range certMsg.certificates {
+			certs = append(certs, cert.data)
+			// OCSP responses and SCT lists are not negotiated in
+			// client certificates.
+			if cert.ocspResponse != nil || cert.sctList != nil {
+				c.sendAlert(alertUnsupportedExtension)
+				return errors.New("tls: unexpected extensions in the client certificate")
+			}
+		}
+		pub, err := hs.processCertsFromClient(certs)
+		if err != nil {
+			return err
+		}
+
+		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)
+			}
+
+			c.peerSignatureAlgorithm = certVerify.signatureAlgorithm
+			input := hs.finishedHash.certificateVerifyInput(clientCertificateVerifyContextTLS13)
+			if err := verifyMessage(c.vers, pub, config, certVerify.signatureAlgorithm, input, certVerify.signature); err != nil {
+				c.sendAlert(alertBadCertificate)
+				return err
+			}
+			hs.writeClientHash(certVerify.marshal())
+		}
+	}
+
+	if encryptedExtensions.extensions.channelIDRequested {
+		msg, err := c.readHandshake()
+		if err != nil {
+			return err
+		}
+		channelIDMsg, ok := msg.(*channelIDMsg)
+		if !ok {
+			c.sendAlert(alertUnexpectedMessage)
+			return unexpectedMessageError(channelIDMsg, msg)
+		}
+		channelIDHash := crypto.SHA256.New()
+		channelIDHash.Write(hs.finishedHash.certificateVerifyInput(channelIDContextTLS13))
+		channelID, err := verifyChannelIDMessage(channelIDMsg, channelIDHash.Sum(nil))
+		if err != nil {
+			return err
+		}
+		c.channelID = channelID
+
+		hs.writeClientHash(channelIDMsg.marshal())
+	}
+
+	// Read the client Finished message.
+	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(clientHandshakeTrafficSecret)
+	if len(verify) != len(clientFinished.verifyData) ||
+		subtle.ConstantTimeCompare(verify, clientFinished.verifyData) != 1 {
+		c.sendAlert(alertHandshakeFailure)
+		return errors.New("tls: client's Finished message was incorrect")
+	}
+	hs.writeClientHash(clientFinished.marshal())
+
+	// Switch to application data keys on read.
+	if err := c.useInTrafficSecret(c.wireVersion, hs.suite, clientTrafficSecret); err != nil {
+		return err
+	}
+
+	c.cipherSuite = hs.suite
+	c.resumptionSecret = hs.finishedHash.deriveSecret(resumptionLabel)
+
+	// TODO(davidben): Allow configuring the number of tickets sent for
+	// testing.
+	if !c.config.SessionTicketsDisabled && foundKEMode {
+		ticketCount := 2
+		for i := 0; i < ticketCount; i++ {
+			c.SendNewSessionTicket([]byte{byte(i)})
+		}
+	}
+	return nil
+}
+
+// processClientHello processes the ClientHello message from the client and
+// decides whether we will perform session resumption.
+func (hs *serverHandshakeState) processClientHello() (isResume bool, err error) {
+	config := hs.c.config
+	c := hs.c
+
+	hs.hello = &serverHelloMsg{
+		isDTLS:            c.isDTLS,
+		vers:              c.wireVersion,
+		versOverride:      config.Bugs.SendServerHelloVersion,
+		compressionMethod: config.Bugs.SendCompressionMethod,
+		extensions: serverExtensions{
+			supportedVersion: config.Bugs.SendServerSupportedExtensionVersion,
+		},
+		omitExtensions:  config.Bugs.OmitExtensions,
+		emptyExtensions: config.Bugs.EmptyExtensions,
+	}
+
+	hs.hello.random = make([]byte, 32)
+	_, err = io.ReadFull(config.rand(), hs.hello.random)
+	if err != nil {
+		c.sendAlert(alertInternalError)
+		return false, err
+	}
+	// Signal downgrades in the server random, per draft-ietf-tls-tls13-16,
+	// section 4.1.3.
+	if c.vers <= VersionTLS12 && config.maxVersion(c.isDTLS) >= VersionTLS13 {
+		copy(hs.hello.random[len(hs.hello.random)-8:], downgradeTLS13)
+	}
+	if c.vers <= VersionTLS11 && config.maxVersion(c.isDTLS) == VersionTLS12 {
+		copy(hs.hello.random[len(hs.hello.random)-8:], downgradeTLS12)
+	}
+	if config.Bugs.SendDraftTLS13DowngradeRandom {
+		copy(hs.hello.random[len(hs.hello.random)-8:], downgradeTLS13Draft)
+	}
+
+	if len(hs.clientHello.sessionId) == 0 && c.config.Bugs.ExpectClientHelloSessionID {
+		return false, errors.New("tls: expected non-empty session ID from client")
+	}
+
+	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")
+	}
+
+	if err := hs.processClientExtensions(&hs.hello.extensions); err != nil {
+		return false, err
+	}
+
+	supportedCurve := false
+	preferredCurves := 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
+
+	_, hs.ecdsaOk = hs.cert.PrivateKey.(*ecdsa.PrivateKey)
+	// Ed25519 also uses ECDSA certificates.
+	_, ed25519Ok := hs.cert.PrivateKey.(ed25519.PrivateKey)
+	hs.ecdsaOk = hs.ecdsaOk || ed25519Ok
+
+	// For test purposes, check that the peer never offers a session when
+	// renegotiating.
+	if c.cipherSuite != nil && len(hs.clientHello.sessionId) > 0 && c.config.Bugs.FailIfResumeOnRenego {
+		return false, errors.New("tls: offered resumption on renegotiation")
+	}
+
+	if c.config.Bugs.FailIfSessionOffered && (len(hs.clientHello.sessionTicket) > 0 || len(hs.clientHello.sessionId) > 0) {
+		return false, errors.New("tls: client offered a session ticket or ID")
+	}
+
+	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.suite = c.tryCipherSuite(id, supportedList, c.vers, hs.ellipticOk, hs.ecdsaOk); hs.suite != nil {
+			break
+		}
+	}
+
+	if hs.suite == nil {
+		c.sendAlert(alertHandshakeFailure)
+		return false, errors.New("tls: no cipher suite supported by both client and server")
+	}
+
+	return false, nil
+}
+
+// processClientExtensions processes all ClientHello extensions not directly
+// related to cipher suite negotiation and writes responses in serverExtensions.
+func (hs *serverHandshakeState) processClientExtensions(serverExtensions *serverExtensions) error {
+	config := hs.c.config
+	c := hs.c
+
+	if c.vers < VersionTLS13 || config.Bugs.NegotiateRenegotiationInfoAtAllVersions {
+		if !bytes.Equal(c.clientVerify, hs.clientHello.secureRenegotiation) {
+			c.sendAlert(alertHandshakeFailure)
+			return errors.New("tls: renegotiation mismatch")
+		}
+
+		if len(c.clientVerify) > 0 && !c.config.Bugs.EmptyRenegotiationInfo {
+			serverExtensions.secureRenegotiation = append(serverExtensions.secureRenegotiation, c.clientVerify...)
+			serverExtensions.secureRenegotiation = append(serverExtensions.secureRenegotiation, c.serverVerify...)
+			if c.config.Bugs.BadRenegotiationInfo {
+				serverExtensions.secureRenegotiation[0] ^= 0x80
+			}
+			if c.config.Bugs.BadRenegotiationInfoEnd {
+				serverExtensions.secureRenegotiation[len(serverExtensions.secureRenegotiation)-1] ^= 0x80
+			}
+		} else {
+			serverExtensions.secureRenegotiation = hs.clientHello.secureRenegotiation
+		}
+
+		if c.noRenegotiationInfo() {
+			serverExtensions.secureRenegotiation = nil
+		}
+	}
+
+	serverExtensions.duplicateExtension = c.config.Bugs.DuplicateExtension
+
+	if len(hs.clientHello.serverName) > 0 {
+		c.serverName = hs.clientHello.serverName
+	}
+	if len(config.Certificates) == 0 {
+		c.sendAlert(alertInternalError)
+		return errors.New("tls: no certificates configured")
+	}
+	hs.cert = &config.Certificates[0]
+	if len(hs.clientHello.serverName) > 0 {
+		hs.cert = config.getCertificateForName(hs.clientHello.serverName)
+	}
+	if expected := c.config.Bugs.ExpectServerName; expected != "" && expected != hs.clientHello.serverName {
+		return errors.New("tls: unexpected server name")
+	}
+
+	if cert := config.Bugs.RenegotiationCertificate; c.cipherSuite != nil && cert != nil {
+		hs.cert = cert
+	}
+
+	if len(hs.clientHello.alpnProtocols) > 0 {
+		// We will never offer ALPN as a client on renegotiation
+		// handshakes.
+		if len(c.clientVerify) > 0 {
+			return errors.New("tls: offered ALPN on renegotiation")
+		}
+		if proto := c.config.Bugs.ALPNProtocol; proto != nil {
+			serverExtensions.alpnProtocol = *proto
+			serverExtensions.alpnProtocolEmpty = len(*proto) == 0
+			c.clientProtocol = *proto
+			c.usedALPN = true
+		} else if selectedProto, fallback := mutualProtocol(hs.clientHello.alpnProtocols, c.config.NextProtos); !fallback {
+			serverExtensions.alpnProtocol = selectedProto
+			c.clientProtocol = selectedProto
+			c.usedALPN = true
+		}
+	}
+
+	if len(c.config.Bugs.SendALPN) > 0 {
+		serverExtensions.alpnProtocol = c.config.Bugs.SendALPN
+	}
+
+	if c.vers < VersionTLS13 || config.Bugs.NegotiateNPNAtAllVersions {
+		if len(hs.clientHello.alpnProtocols) == 0 || c.config.Bugs.NegotiateALPNAndNPN {
+			// Although sending an empty NPN extension is reasonable, Firefox has
+			// had a bug around this. Best to send nothing at all if
+			// config.NextProtos is empty. See
+			// https://code.google.com/p/go/issues/detail?id=5445.
+			if hs.clientHello.nextProtoNeg && len(config.NextProtos) > 0 {
+				serverExtensions.nextProtoNeg = true
+				serverExtensions.nextProtos = config.NextProtos
+				serverExtensions.npnAfterAlpn = config.Bugs.SwapNPNAndALPN
+			}
+		}
+	}
+
+	if len(hs.clientHello.quicTransportParams) > 0 {
+		c.quicTransportParams = hs.clientHello.quicTransportParams
+		serverExtensions.quicTransportParams = c.config.QUICTransportParams
+	}
+
+	if c.vers < VersionTLS13 || config.Bugs.NegotiateEMSAtAllVersions {
+		disableEMS := config.Bugs.NoExtendedMasterSecret
+		if c.cipherSuite != nil {
+			disableEMS = config.Bugs.NoExtendedMasterSecretOnRenegotiation
+		}
+		serverExtensions.extendedMasterSecret = c.vers >= VersionTLS10 && hs.clientHello.extendedMasterSecret && !disableEMS
+	}
+
+	if hs.clientHello.channelIDSupported && config.RequestChannelID {
+		serverExtensions.channelIDRequested = true
+	}
+
+	if config.TokenBindingParams != nil {
+		if !bytes.Equal(config.ExpectTokenBindingParams, hs.clientHello.tokenBindingParams) {
+			return errors.New("client did not send expected token binding params")
+		}
+
+		// For testing, blindly send whatever is set in config, even if
+		// it is invalid.
+		serverExtensions.tokenBindingParams = config.TokenBindingParams
+		serverExtensions.tokenBindingVersion = config.TokenBindingVersion
+	}
+
+	if len(hs.clientHello.tokenBindingParams) > 0 && (!hs.clientHello.extendedMasterSecret || hs.clientHello.secureRenegotiation == nil) {
+		return errors.New("client sent Token Binding without EMS and/or RI")
+	}
+
+	if hs.clientHello.srtpProtectionProfiles != nil {
+	SRTPLoop:
+		for _, p1 := range c.config.SRTPProtectionProfiles {
+			for _, p2 := range hs.clientHello.srtpProtectionProfiles {
+				if p1 == p2 {
+					serverExtensions.srtpProtectionProfile = p1
+					c.srtpProtectionProfile = p1
+					break SRTPLoop
+				}
+			}
+		}
+	}
+
+	if c.config.Bugs.SendSRTPProtectionProfile != 0 {
+		serverExtensions.srtpProtectionProfile = c.config.Bugs.SendSRTPProtectionProfile
+	}
+
+	if expected := c.config.Bugs.ExpectedCustomExtension; expected != nil {
+		if hs.clientHello.customExtension != *expected {
+			return fmt.Errorf("tls: bad custom extension contents %q", hs.clientHello.customExtension)
+		}
+	}
+	serverExtensions.customExtension = config.Bugs.CustomExtension
+
+	if c.config.Bugs.AdvertiseTicketExtension {
+		serverExtensions.ticketSupported = true
+	}
+
+	if c.config.Bugs.SendSupportedPointFormats != nil {
+		serverExtensions.supportedPoints = c.config.Bugs.SendSupportedPointFormats
+	}
+
+	if c.config.Bugs.SendServerSupportedCurves {
+		serverExtensions.supportedCurves = c.config.curvePreferences()
+	}
+
+	if !hs.clientHello.hasGREASEExtension && config.Bugs.ExpectGREASE {
+		return errors.New("tls: no GREASE extension found")
+	}
+
+	if l := hs.clientHello.dummyPQPaddingLen; l != 0 {
+		serverExtensions.dummyPQPaddingLen = l
+	}
+
+	serverExtensions.serverNameAck = c.config.Bugs.SendServerNameAck
+
+	return nil
+}
+
+// checkForResumption returns true if we should perform resumption on this connection.
+func (hs *serverHandshakeState) checkForResumption() bool {
+	c := hs.c
+
+	ticket := hs.clientHello.sessionTicket
+	if len(ticket) == 0 && len(hs.clientHello.pskIdentities) > 0 && c.config.Bugs.AcceptAnySession {
+		ticket = hs.clientHello.pskIdentities[0].ticket
+	}
+	if len(ticket) > 0 {
+		if c.config.SessionTicketsDisabled {
+			return false
+		}
+
+		var ok bool
+		if hs.sessionState, ok = c.decryptTicket(ticket); !ok {
+			return false
+		}
+	} else {
+		if c.config.ServerSessionCache == nil {
+			return false
+		}
+
+		var ok bool
+		sessionId := string(hs.clientHello.sessionId)
+		if hs.sessionState, ok = c.config.ServerSessionCache.Get(sessionId); !ok {
+			return false
+		}
+	}
+
+	if c.config.Bugs.AcceptAnySession {
+		// Replace the cipher suite with one known to work, to test
+		// cross-version resumption attempts.
+		hs.sessionState.cipherSuite = TLS_RSA_WITH_AES_128_CBC_SHA
+	} else {
+		// Never resume a session for a different SSL 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.
+	hs.suite = c.tryCipherSuite(hs.sessionState.cipherSuite, c.config.cipherSuites(), c.vers, hs.ellipticOk, hs.ecdsaOk)
+
+	if hs.suite == nil {
+		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
+	if c.config.Bugs.SendCipherSuite != 0 {
+		hs.hello.cipherSuite = c.config.Bugs.SendCipherSuite
+	}
+	// 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.extensions.ticketSupported = c.config.Bugs.RenewTicketOnResume
+
+	if c.config.Bugs.SendSCTListOnResume != nil {
+		hs.hello.extensions.sctList = c.config.Bugs.SendSCTListOnResume
+	}
+
+	if c.config.Bugs.SendOCSPResponseOnResume != nil {
+		// There is no way, syntactically, to send an OCSP response on a
+		// resumption handshake.
+		hs.hello.extensions.ocspStapling = true
+	}
+
+	hs.finishedHash = newFinishedHash(c.wireVersion, c.isDTLS, hs.suite)
+	hs.finishedHash.discardHandshakeBuffer()
+	hs.writeClientHash(hs.clientHello.marshal())
+	hs.writeServerHash(hs.hello.marshal())
+
+	c.writeRecord(recordTypeHandshake, hs.hello.marshal())
+
+	if len(hs.sessionState.certificates) > 0 {
+		if _, err := hs.processCertsFromClient(hs.sessionState.certificates); err != nil {
+			return err
+		}
+	}
+
+	hs.masterSecret = hs.sessionState.masterSecret
+	c.extendedMasterSecret = hs.sessionState.extendedMasterSecret
+
+	return nil
+}
+
+func (hs *serverHandshakeState) doFullHandshake() error {
+	config := hs.c.config
+	c := hs.c
+
+	isPSK := hs.suite.flags&suitePSK != 0
+	if !isPSK && hs.clientHello.ocspStapling && len(hs.cert.OCSPStaple) > 0 {
+		hs.hello.extensions.ocspStapling = true
+	}
+
+	if hs.clientHello.sctListSupported && len(hs.cert.SignedCertificateTimestampList) > 0 {
+		hs.hello.extensions.sctList = hs.cert.SignedCertificateTimestampList
+	}
+
+	if len(c.clientVerify) > 0 && config.Bugs.SendSCTListOnRenegotiation != nil {
+		hs.hello.extensions.sctList = config.Bugs.SendSCTListOnRenegotiation
+	}
+
+	hs.hello.extensions.ticketSupported = hs.clientHello.ticketSupported && !config.SessionTicketsDisabled && c.vers > VersionSSL30
+	hs.hello.cipherSuite = hs.suite.id
+	if config.Bugs.SendCipherSuite != 0 {
+		hs.hello.cipherSuite = config.Bugs.SendCipherSuite
+	}
+	c.extendedMasterSecret = hs.hello.extensions.extendedMasterSecret
+
+	// Generate a session ID if we're to save the session.
+	if !hs.hello.extensions.ticketSupported && config.ServerSessionCache != nil {
+		hs.hello.sessionId = make([]byte, 32)
+		if _, err := io.ReadFull(config.rand(), hs.hello.sessionId); err != nil {
+			c.sendAlert(alertInternalError)
+			return errors.New("tls: short read from Rand: " + err.Error())
+		}
+	}
+	if config.Bugs.EchoSessionIDInFullHandshake {
+		hs.hello.sessionId = hs.clientHello.sessionId
+	}
+
+	hs.finishedHash = newFinishedHash(c.wireVersion, c.isDTLS, hs.suite)
+	hs.writeClientHash(hs.clientHello.marshal())
+	hs.writeServerHash(hs.hello.marshal())
+
+	if config.Bugs.SendSNIWarningAlert {
+		c.SendAlert(alertLevelWarning, alertUnrecognizedName)
+	}
+
+	c.writeRecord(recordTypeHandshake, hs.hello.marshal())
+
+	if !isPSK {
+		certMsg := new(certificateMsg)
+		if !config.Bugs.EmptyCertificateList {
+			for _, certData := range hs.cert.Certificate {
+				certMsg.certificates = append(certMsg.certificates, certificateEntry{
+					data: certData,
+				})
+			}
+		}
+		if !config.Bugs.UnauthenticatedECDH {
+			certMsgBytes := certMsg.marshal()
+			hs.writeServerHash(certMsgBytes)
+			c.writeRecord(recordTypeHandshake, certMsgBytes)
+		}
+	}
+
+	if hs.hello.extensions.ocspStapling && !c.config.Bugs.SkipCertificateStatus {
+		certStatus := new(certificateStatusMsg)
+		certStatus.statusType = statusTypeOCSP
+		certStatus.response = hs.cert.OCSPStaple
+		if len(c.clientVerify) > 0 && config.Bugs.SendOCSPResponseOnRenegotiation != nil {
+			certStatus.response = config.Bugs.SendOCSPResponseOnRenegotiation
+		}
+		hs.writeServerHash(certStatus.marshal())
+		c.writeRecord(recordTypeHandshake, certStatus.marshal())
+	}
+
+	keyAgreement := hs.suite.ka(c.vers)
+	skx, err := keyAgreement.generateServerKeyExchange(config, hs.cert, hs.clientHello, hs.hello)
+	if err != nil {
+		c.sendAlert(alertHandshakeFailure)
+		return err
+	}
+	if ecdhe, ok := keyAgreement.(*ecdheKeyAgreement); ok {
+		c.curveID = ecdhe.curveID
+	}
+	if skx != nil && !config.Bugs.SkipServerKeyExchange {
+		hs.writeServerHash(skx.marshal())
+		c.writeRecord(recordTypeHandshake, skx.marshal())
+	}
+
+	if config.ClientAuth >= RequestClientCert {
+		// Request a client certificate
+		certReq := &certificateRequestMsg{
+			vers:             c.wireVersion,
+			certificateTypes: config.ClientCertificateTypes,
+		}
+		if certReq.certificateTypes == nil {
+			certReq.certificateTypes = []byte{
+				byte(CertTypeRSASign),
+				byte(CertTypeECDSASign),
+			}
+		}
+		if c.vers >= VersionTLS12 {
+			certReq.hasSignatureAlgorithm = true
+			if !config.Bugs.NoSignatureAlgorithms {
+				certReq.signatureAlgorithms = config.verifySignatureAlgorithms()
+			}
+		}
+
+		// 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 config.ClientCAs != nil {
+			certReq.certificateAuthorities = config.ClientCAs.Subjects()
+		}
+		hs.writeServerHash(certReq.marshal())
+		c.writeRecord(recordTypeHandshake, certReq.marshal())
+	}
+
+	helloDone := new(serverHelloDoneMsg)
+	hs.writeServerHash(helloDone.marshal())
+	c.writeRecord(recordTypeHandshake, helloDone.marshal())
+	c.flushHandshake()
+
+	var pub crypto.PublicKey // public key for client auth, if any
+
+	if err := c.simulatePacketLoss(nil); err != nil {
+		return err
+	}
+	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 config.ClientAuth >= RequestClientCert {
+		var certMsg *certificateMsg
+		var certificates [][]byte
+		if certMsg, ok = msg.(*certificateMsg); ok {
+			if c.vers == VersionSSL30 && len(certMsg.certificates) == 0 {
+				return errors.New("tls: empty certificate message in SSL 3.0")
+			}
+
+			hs.writeClientHash(certMsg.marshal())
+			for _, cert := range certMsg.certificates {
+				certificates = append(certificates, cert.data)
+			}
+		} else if c.vers == VersionSSL30 {
+			// In SSL 3.0, no certificate is signaled by a warning
+			// alert which we translate to ssl3NoCertificateMsg.
+			if _, ok := msg.(*ssl3NoCertificateMsg); !ok {
+				return errors.New("tls: client provided neither a certificate nor no_certificate warning alert")
+			}
+		} else {
+			// In TLS, the Certificate message is required. In SSL
+			// 3.0, the peer skips it when sending no certificates.
+			c.sendAlert(alertUnexpectedMessage)
+			return unexpectedMessageError(certMsg, msg)
+		}
+
+		if len(certificates) == 0 {
+			// The client didn't actually send a certificate
+			switch config.ClientAuth {
+			case RequireAnyClientCert, RequireAndVerifyClientCert:
+				c.sendAlert(alertBadCertificate)
+				return errors.New("tls: client didn't provide a certificate")
+			}
+		}
+
+		pub, err = hs.processCertsFromClient(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.writeClientHash(ckx.marshal())
+
+	preMasterSecret, err := keyAgreement.processClientKeyExchange(config, hs.cert, ckx, c.vers)
+	if err != nil {
+		c.sendAlert(alertHandshakeFailure)
+		return err
+	}
+	if c.extendedMasterSecret {
+		hs.masterSecret = extendedMasterFromPreMasterSecret(c.vers, hs.suite, preMasterSecret, hs.finishedHash)
+	} else {
+		if c.config.Bugs.RequireExtendedMasterSecret {
+			return errors.New("tls: extended master secret required but not supported by peer")
+		}
+		hs.masterSecret = masterFromPreMasterSecret(c.vers, hs.suite, preMasterSecret, hs.clientHello.random, hs.hello.random)
+	}
+
+	// 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 sigAlg signatureAlgorithm
+		if certVerify.hasSignatureAlgorithm {
+			sigAlg = certVerify.signatureAlgorithm
+			c.peerSignatureAlgorithm = sigAlg
+		}
+
+		if c.vers > VersionSSL30 {
+			err = verifyMessage(c.vers, pub, c.config, sigAlg, hs.finishedHash.buffer, certVerify.signature)
+		} else {
+			// SSL 3.0's client certificate construction is
+			// incompatible with signatureAlgorithm.
+			rsaPub, ok := pub.(*rsa.PublicKey)
+			if !ok {
+				err = errors.New("unsupported key type for client certificate")
+			} else {
+				digest := hs.finishedHash.hashForClientCertificateSSL3(hs.masterSecret)
+				err = rsa.VerifyPKCS1v15(rsaPub, crypto.MD5SHA1, digest, certVerify.signature)
+			}
+		}
+		if err != nil {
+			c.sendAlert(alertBadCertificate)
+			return errors.New("could not validate signature of connection nonces: " + err.Error())
+		}
+
+		hs.writeClientHash(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(c.vers))
+
+	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(c.vers, clientKey, clientIV)
+		serverCipher = hs.suite.aead(c.vers, serverKey, serverIV)
+	}
+
+	c.in.prepareCipherSpec(c.wireVersion, clientCipher, clientHash)
+	c.out.prepareCipherSpec(c.wireVersion, serverCipher, serverHash)
+
+	return nil
+}
+
+func (hs *serverHandshakeState) readFinished(out []byte, isResume bool) error {
+	c := hs.c
+
+	c.readRecord(recordTypeChangeCipherSpec)
+	if err := c.in.error(); err != nil {
+		return err
+	}
+
+	if hs.hello.extensions.nextProtoNeg {
+		msg, err := c.readHandshake()
+		if err != nil {
+			return err
+		}
+		nextProto, ok := msg.(*nextProtoMsg)
+		if !ok {
+			c.sendAlert(alertUnexpectedMessage)
+			return unexpectedMessageError(nextProto, msg)
+		}
+		hs.writeClientHash(nextProto.marshal())
+		c.clientProtocol = nextProto.proto
+	}
+
+	if hs.hello.extensions.channelIDRequested {
+		msg, err := c.readHandshake()
+		if err != nil {
+			return err
+		}
+		channelIDMsg, ok := msg.(*channelIDMsg)
+		if !ok {
+			c.sendAlert(alertUnexpectedMessage)
+			return unexpectedMessageError(channelIDMsg, msg)
+		}
+		var resumeHash []byte
+		if isResume {
+			resumeHash = hs.sessionState.handshakeHash
+		}
+		channelID, err := verifyChannelIDMessage(channelIDMsg, hs.finishedHash.hashForChannelID(resumeHash))
+		if err != nil {
+			return err
+		}
+		c.channelID = channelID
+
+		hs.writeClientHash(channelIDMsg.marshal())
+	}
+
+	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")
+	}
+	c.clientVerify = append(c.clientVerify[:0], clientFinished.verifyData...)
+	copy(out, clientFinished.verifyData)
+
+	hs.writeClientHash(clientFinished.marshal())
+	return nil
+}
+
+func (hs *serverHandshakeState) sendSessionTicket() error {
+	c := hs.c
+	state := sessionState{
+		vers:          c.vers,
+		cipherSuite:   hs.suite.id,
+		masterSecret:  hs.masterSecret,
+		certificates:  hs.certsFromClient,
+		handshakeHash: hs.finishedHash.Sum(),
+	}
+
+	if !hs.hello.extensions.ticketSupported || hs.c.config.Bugs.SkipNewSessionTicket {
+		if c.config.ServerSessionCache != nil && len(hs.hello.sessionId) != 0 {
+			c.config.ServerSessionCache.Put(string(hs.hello.sessionId), &state)
+		}
+		return nil
+	}
+
+	m := new(newSessionTicketMsg)
+	m.vers = c.wireVersion
+	m.isDTLS = c.isDTLS
+	if c.config.Bugs.SendTicketLifetime != 0 {
+		m.ticketLifetime = uint32(c.config.Bugs.SendTicketLifetime / time.Second)
+	}
+
+	if !c.config.Bugs.SendEmptySessionTicket {
+		var err error
+		m.ticket, err = c.encryptTicket(&state)
+		if err != nil {
+			return err
+		}
+	}
+
+	hs.writeServerHash(m.marshal())
+	c.writeRecord(recordTypeHandshake, m.marshal())
+
+	return nil
+}
+
+func (hs *serverHandshakeState) sendFinished(out []byte, isResume bool) error {
+	c := hs.c
+
+	finished := new(finishedMsg)
+	finished.verifyData = hs.finishedHash.serverSum(hs.masterSecret)
+	copy(out, finished.verifyData)
+	if c.config.Bugs.BadFinished {
+		finished.verifyData[0]++
+	}
+	c.serverVerify = append(c.serverVerify[:0], finished.verifyData...)
+	hs.finishedBytes = finished.marshal()
+	hs.writeServerHash(hs.finishedBytes)
+	postCCSBytes := hs.finishedBytes
+
+	if c.config.Bugs.FragmentAcrossChangeCipherSpec {
+		c.writeRecord(recordTypeHandshake, postCCSBytes[:5])
+		postCCSBytes = postCCSBytes[5:]
+	} else if c.config.Bugs.SendUnencryptedFinished {
+		c.writeRecord(recordTypeHandshake, postCCSBytes)
+		postCCSBytes = nil
+	}
+
+	if !c.config.Bugs.SkipChangeCipherSpec {
+		ccs := []byte{1}
+		if c.config.Bugs.BadChangeCipherSpec != nil {
+			ccs = c.config.Bugs.BadChangeCipherSpec
+		}
+		c.writeRecord(recordTypeChangeCipherSpec, ccs)
+	}
+
+	if c.config.Bugs.AppDataAfterChangeCipherSpec != nil {
+		c.writeRecord(recordTypeApplicationData, c.config.Bugs.AppDataAfterChangeCipherSpec)
+	}
+	if c.config.Bugs.AlertAfterChangeCipherSpec != 0 {
+		c.sendAlert(c.config.Bugs.AlertAfterChangeCipherSpec)
+		return errors.New("tls: simulating post-CCS alert")
+	}
+
+	if !c.config.Bugs.SkipFinished && len(postCCSBytes) > 0 {
+		c.writeRecord(recordTypeHandshake, postCCSBytes)
+		if c.config.Bugs.SendExtraFinished {
+			c.writeRecord(recordTypeHandshake, finished.marshal())
+		}
+	}
+
+	if isResume || (!c.config.Bugs.PackHelloRequestWithFinished && !c.config.Bugs.PackAppDataWithHandshake) {
+		// Defer flushing until Renegotiate() or Write().
+		c.flushHandshake()
+	}
+
+	c.cipherSuite = hs.suite
+
+	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())
+		}
+
+		ok := false
+		for _, ku := range certs[0].ExtKeyUsage {
+			if ku == x509.ExtKeyUsageClientAuth {
+				ok = true
+				break
+			}
+		}
+		if !ok {
+			c.sendAlert(alertHandshakeFailure)
+			return nil, errors.New("tls: client's certificate's extended key usage doesn't permit it to be used for client authentication")
+		}
+
+		c.verifiedChains = chains
+	}
+
+	if len(certs) > 0 {
+		pub := getCertificatePublicKey(certs[0])
+		switch pub.(type) {
+		case *ecdsa.PublicKey, *rsa.PublicKey, ed25519.PublicKey:
+			break
+		default:
+			c.sendAlert(alertUnsupportedCertificate)
+			return nil, fmt.Errorf("tls: client's certificate contains an unsupported public key of type %T", pub)
+		}
+		c.peerCertificates = certs
+		return pub, nil
+	}
+
+	return nil, nil
+}
+
+func verifyChannelIDMessage(channelIDMsg *channelIDMsg, channelIDHash []byte) (*ecdsa.PublicKey, error) {
+	x := new(big.Int).SetBytes(channelIDMsg.channelID[0:32])
+	y := new(big.Int).SetBytes(channelIDMsg.channelID[32:64])
+	r := new(big.Int).SetBytes(channelIDMsg.channelID[64:96])
+	s := new(big.Int).SetBytes(channelIDMsg.channelID[96:128])
+	if !elliptic.P256().IsOnCurve(x, y) {
+		return nil, errors.New("tls: invalid channel ID public key")
+	}
+	channelID := &ecdsa.PublicKey{Curve: elliptic.P256(), X: x, Y: y}
+	if !ecdsa.Verify(channelID, channelIDHash, r, s) {
+		return nil, errors.New("tls: invalid channel ID signature")
+	}
+	return channelID, nil
+}
+
+func (hs *serverHandshakeState) writeServerHash(msg []byte) {
+	// writeServerHash is called before writeRecord.
+	hs.writeHash(msg, hs.c.sendHandshakeSeq)
+}
+
+func (hs *serverHandshakeState) writeClientHash(msg []byte) {
+	// writeClientHash is called after readHandshake.
+	hs.writeHash(msg, hs.c.recvHandshakeSeq-1)
+}
+
+func (hs *serverHandshakeState) writeHash(msg []byte, seqno uint16) {
+	if hs.c.isDTLS {
+		// This is somewhat hacky. DTLS hashes a slightly different format.
+		// First, the TLS header.
+		hs.finishedHash.Write(msg[:4])
+		// Then the sequence number and reassembled fragment offset (always 0).
+		hs.finishedHash.Write([]byte{byte(seqno >> 8), byte(seqno), 0, 0, 0})
+		// Then the reassembled fragment (always equal to the message length).
+		hs.finishedHash.Write(msg[1:4])
+		// And then the message body.
+		hs.finishedHash.Write(msg[4:])
+	} else {
+		hs.finishedHash.Write(msg)
+	}
+}
+
+// tryCipherSuite returns a cipherSuite with the given id if that cipher suite
+// is acceptable to use.
+func (c *Conn) tryCipherSuite(id uint16, supportedCipherSuites []uint16, version uint16, ellipticOk, ecdsaOk bool) *cipherSuite {
+	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 version >= VersionTLS13 || candidate.flags&suiteTLS13 != 0 {
+				if version < VersionTLS13 || candidate.flags&suiteTLS13 == 0 {
+					continue
+				}
+				return candidate
+			}
+			if (candidate.flags&suiteECDHE != 0) && !ellipticOk {
+				continue
+			}
+			if (candidate.flags&suiteECDSA != 0) != ecdsaOk {
+				continue
+			}
+			if version < VersionTLS12 && candidate.flags&suiteTLS12 != 0 {
+				continue
+			}
+			return candidate
+		}
+	}
+
+	return nil
+}
+
+func isTLS12Cipher(id uint16) bool {
+	for _, cipher := range cipherSuites {
+		if cipher.id != id {
+			continue
+		}
+		return cipher.flags&suiteTLS12 != 0
+	}
+	// Unknown cipher.
+	return false
+}
+
+func isGREASEValue(val uint16) bool {
+	return val&0x0f0f == 0x0a0a && val&0xff == val>>8
+}
+
+func verifyPSKBinder(version uint16, clientHello *clientHelloMsg, sessionState *sessionState, binderToVerify, firstClientHello, helloRetryRequest []byte) error {
+	binderLen := 2
+	for _, binder := range clientHello.pskBinders {
+		binderLen += 1 + len(binder)
+	}
+
+	truncatedHello := clientHello.marshal()
+	truncatedHello = truncatedHello[:len(truncatedHello)-binderLen]
+	pskCipherSuite := cipherSuiteFromID(sessionState.cipherSuite)
+	if pskCipherSuite == nil {
+		return errors.New("tls: Unknown cipher suite for PSK in session")
+	}
+
+	binder := computePSKBinder(sessionState.masterSecret, version, resumptionPSKBinderLabel, pskCipherSuite, firstClientHello, helloRetryRequest, truncatedHello)
+	if !bytes.Equal(binder, binderToVerify) {
+		return errors.New("tls: PSK binder does not verify")
+	}
+
+	return nil
+}
