crypto/tls: implement TLS 1.3 minimal server

13.go

@@ -0,0 +1,334 @@
+package tls
+
+import (
+	"bytes"
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/hmac"
+	"crypto/rsa"
+	"crypto/subtle"
+	"errors"
+	"io"
+
+	"golang_org/x/crypto/curve25519"
+)
+
+func (hs *serverHandshakeState) doTLS13Handshake() error {
+	config := hs.c.config
+	c := hs.c
+
+	hs.c.cipherSuite, hs.hello13.cipherSuite = hs.suite.id, hs.suite.id
+
+	// When picking the group for the handshake, priority is given to groups
+	// that the client provided a keyShare for, so to avoid a round-trip.
+	// After that the order of CurvePreferences is respected.
+	var ks keyShare
+	for _, curveID := range config.curvePreferences() {
+		for _, keyShare := range hs.clientHello.keyShares {
+			if curveID == keyShare.group {
+				ks = keyShare
+				break
+			}
+		}
+	}
+	if ks.group == 0 {
+		c.sendAlert(alertInternalError)
+		return errors.New("tls: HelloRetryRequest not implemented") // TODO(filippo)
+	}
+
+	privateKey, serverKS, err := config.generateKeyShare(ks.group)
+	if err != nil {
+		c.sendAlert(alertInternalError)
+		return err
+	}
+	hs.hello13.keyShare = serverKS
+
+	hash := crypto.SHA256
+	if hs.suite.flags&suiteSHA384 != 0 {
+		hash = crypto.SHA384
+	}
+	hashSize := hash.Size()
+
+	ecdheSecret := deriveECDHESecret(ks, privateKey)
+	if ecdheSecret == nil {
+		c.sendAlert(alertIllegalParameter)
+		return errors.New("tls: bad ECDHE client share")
+	}
+
+	hs.finishedHash = newFinishedHash(hs.c.vers, hs.suite)
+	hs.finishedHash.discardHandshakeBuffer()
+	hs.finishedHash.Write(hs.clientHello.marshal())
+	hs.finishedHash.Write(hs.hello13.marshal())
+	if _, err := c.writeRecord(recordTypeHandshake, hs.hello13.marshal()); err != nil {
+		return err
+	}
+
+	earlySecret := hkdfExtract(hash, nil, nil)
+	handshakeSecret := hkdfExtract(hash, ecdheSecret, earlySecret)
+
+	handshakeCtx := hs.finishedHash.Sum()
+
+	cHandshakeTS := hkdfExpandLabel(hash, handshakeSecret, handshakeCtx, "client handshake traffic secret", hashSize)
+	cKey := hkdfExpandLabel(hash, cHandshakeTS, nil, "key", hs.suite.keyLen)
+	cIV := hkdfExpandLabel(hash, cHandshakeTS, nil, "iv", 12)
+	sHandshakeTS := hkdfExpandLabel(hash, handshakeSecret, handshakeCtx, "server handshake traffic secret", hashSize)
+	sKey := hkdfExpandLabel(hash, sHandshakeTS, nil, "key", hs.suite.keyLen)
+	sIV := hkdfExpandLabel(hash, sHandshakeTS, nil, "iv", 12)
+
+	clientCipher := hs.suite.aead(cKey, cIV)
+	c.in.setCipher(c.vers, clientCipher)
+	serverCipher := hs.suite.aead(sKey, sIV)
+	c.out.setCipher(c.vers, serverCipher)
+
+	hs.finishedHash.Write(hs.hello13Enc.marshal())
+	if _, err := c.writeRecord(recordTypeHandshake, hs.hello13Enc.marshal()); err != nil {
+		return err
+	}
+
+	certMsg := &certificateMsg13{
+		certificates: hs.cert.Certificate,
+	}
+	hs.finishedHash.Write(certMsg.marshal())
+	if _, err := c.writeRecord(recordTypeHandshake, certMsg.marshal()); err != nil {
+		return err
+	}
+
+	sigScheme, err := hs.selectTLS13SignatureScheme()
+	if err != nil {
+		c.sendAlert(alertInternalError)
+		return err
+	}
+
+	sigHash := hashForSignatureScheme(sigScheme)
+	opts := crypto.SignerOpts(sigHash)
+	if signatureSchemeIsPSS(sigScheme) {
+		opts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: sigHash}
+	}
+
+	toSign := prepareDigitallySigned(sigHash, "TLS 1.3, server CertificateVerify", hs.finishedHash.Sum())
+	signature, err := hs.cert.PrivateKey.(crypto.Signer).Sign(config.rand(), toSign[:], opts)
+	if err != nil {
+		c.sendAlert(alertInternalError)
+		return err
+	}
+
+	verifyMsg := &certificateVerifyMsg{
+		hasSignatureAndHash: true,
+		signatureAndHash:    sigSchemeToSigAndHash(sigScheme),
+		signature:           signature,
+	}
+	hs.finishedHash.Write(verifyMsg.marshal())
+	if _, err := c.writeRecord(recordTypeHandshake, verifyMsg.marshal()); err != nil {
+		return err
+	}
+
+	serverFinishedKey := hkdfExpandLabel(hash, sHandshakeTS, nil, "finished", hashSize)
+	clientFinishedKey := hkdfExpandLabel(hash, cHandshakeTS, nil, "finished", hashSize)
+
+	h := hmac.New(hash.New, serverFinishedKey)
+	h.Write(hs.finishedHash.Sum())
+	verifyData := h.Sum(nil)
+	serverFinished := &finishedMsg{
+		verifyData: verifyData,
+	}
+	hs.finishedHash.Write(serverFinished.marshal())
+	if _, err := c.writeRecord(recordTypeHandshake, serverFinished.marshal()); err != nil {
+		return err
+	}
+
+	if _, err := c.flush(); err != nil {
+		return err
+	}
+
+	msg, err := c.readHandshake()
+	if err != nil {
+		return err
+	}
+
+	clientFinished, ok := msg.(*finishedMsg)
+	if !ok {
+		c.sendAlert(alertUnexpectedMessage)
+		return unexpectedMessageError(clientFinished, msg)
+	}
+	h = hmac.New(hash.New, clientFinishedKey)
+	h.Write(hs.finishedHash.Sum())
+	expectedVerifyData := h.Sum(nil)
+	if len(expectedVerifyData) != len(clientFinished.verifyData) ||
+		subtle.ConstantTimeCompare(expectedVerifyData, clientFinished.verifyData) != 1 {
+		c.sendAlert(alertHandshakeFailure)
+		return errors.New("tls: client's Finished message is incorrect")
+	}
+
+	masterSecret := hkdfExtract(hash, nil, handshakeSecret)
+	handshakeCtx = hs.finishedHash.Sum()
+
+	cTrafficSecret0 := hkdfExpandLabel(hash, masterSecret, handshakeCtx, "client application traffic secret", hashSize)
+	cKey = hkdfExpandLabel(hash, cTrafficSecret0, nil, "key", hs.suite.keyLen)
+	cIV = hkdfExpandLabel(hash, cTrafficSecret0, nil, "iv", 12)
+	sTrafficSecret0 := hkdfExpandLabel(hash, masterSecret, handshakeCtx, "server application traffic secret", hashSize)
+	sKey = hkdfExpandLabel(hash, sTrafficSecret0, nil, "key", hs.suite.keyLen)
+	sIV = hkdfExpandLabel(hash, sTrafficSecret0, nil, "iv", 12)
+
+	clientCipher = hs.suite.aead(cKey, cIV)
+	c.in.setCipher(c.vers, clientCipher)
+	serverCipher = hs.suite.aead(sKey, sIV)
+	c.out.setCipher(c.vers, serverCipher)
+
+	return nil
+}
+
+// selectTLS13SignatureScheme chooses the SignatureScheme for the CertificateVerify
+// based on the certificate type and client supported schemes. If no overlap is found,
+// a fallback is selected.
+//
+// See https://tools.ietf.org/html/draft-ietf-tls-tls13-18#section-4.4.1.2
+func (hs *serverHandshakeState) selectTLS13SignatureScheme() (sigScheme SignatureScheme, err error) {
+	var supportedSchemes []SignatureScheme
+	signer, ok := hs.cert.PrivateKey.(crypto.Signer)
+	if !ok {
+		return 0, errors.New("tls: certificate private key does not implement crypto.Signer")
+	}
+	pk := signer.Public()
+	if _, ok := pk.(*rsa.PublicKey); ok {
+		sigScheme = PSSWithSHA256
+		supportedSchemes = []SignatureScheme{PSSWithSHA256, PSSWithSHA384, PSSWithSHA512}
+	} else if pk, ok := pk.(*ecdsa.PublicKey); ok {
+		switch pk.Curve {
+		case elliptic.P256():
+			sigScheme = ECDSAWithP256AndSHA256
+			supportedSchemes = []SignatureScheme{ECDSAWithP256AndSHA256}
+		case elliptic.P384():
+			sigScheme = ECDSAWithP384AndSHA384
+			supportedSchemes = []SignatureScheme{ECDSAWithP384AndSHA384}
+		case elliptic.P521():
+			sigScheme = ECDSAWithP521AndSHA512
+			supportedSchemes = []SignatureScheme{ECDSAWithP521AndSHA512}
+		default:
+			return 0, errors.New("tls: unknown ECDSA certificate curve")
+		}
+	} else {
+		return 0, errors.New("tls: unknown certificate key type")
+	}
+
+	for _, ss := range supportedSchemes {
+		for _, cs := range hs.clientHello.signatureAndHashes {
+			if ss == sigAndHashToSigScheme(cs) {
+				return ss, nil
+			}
+		}
+	}
+
+	return sigScheme, nil
+}
+
+func sigSchemeToSigAndHash(s SignatureScheme) (sah signatureAndHash) {
+	sah.hash = byte(s >> 8)
+	sah.signature = byte(s)
+	return
+}
+
+func sigAndHashToSigScheme(sah signatureAndHash) SignatureScheme {
+	return SignatureScheme(sah.hash)<<8 | SignatureScheme(sah.signature)
+}
+
+func signatureSchemeIsPSS(s SignatureScheme) bool {
+	return s == PSSWithSHA256 || s == PSSWithSHA384 || s == PSSWithSHA512
+}
+
+// hashForSignatureScheme returns the Hash used by a SignatureScheme which is
+// supported by selectTLS13SignatureScheme.
+func hashForSignatureScheme(ss SignatureScheme) crypto.Hash {
+	switch ss {
+	case PSSWithSHA256, ECDSAWithP256AndSHA256:
+		return crypto.SHA256
+	case PSSWithSHA384, ECDSAWithP384AndSHA384:
+		return crypto.SHA384
+	case PSSWithSHA512, ECDSAWithP521AndSHA512:
+		return crypto.SHA512
+	default:
+		panic("unsupported SignatureScheme passed to hashForSignatureScheme")
+	}
+}
+
+func prepareDigitallySigned(hash crypto.Hash, context string, data []byte) []byte {
+	message := bytes.Repeat([]byte{32}, 64)
+	message = append(message, context...)
+	message = append(message, 0)
+	message = append(message, data...)
+	h := hash.New()
+	h.Write(message)
+	return h.Sum(nil)
+}
+
+func (c *Config) generateKeyShare(curveID CurveID) ([]byte, keyShare, error) {
+	if curveID == X25519 {
+		var scalar, public [32]byte
+		if _, err := io.ReadFull(c.rand(), scalar[:]); err != nil {
+			return nil, keyShare{}, err
+		}
+
+		curve25519.ScalarBaseMult(&public, &scalar)
+		return scalar[:], keyShare{group: curveID, data: public[:]}, nil
+	}
+
+	curve, ok := curveForCurveID(curveID)
+	if !ok {
+		return nil, keyShare{}, errors.New("tls: preferredCurves includes unsupported curve")
+	}
+
+	privateKey, x, y, err := elliptic.GenerateKey(curve, c.rand())
+	if err != nil {
+		return nil, keyShare{}, err
+	}
+	ecdhePublic := elliptic.Marshal(curve, x, y)
+
+	return privateKey, keyShare{group: curveID, data: ecdhePublic}, nil
+}
+
+func deriveECDHESecret(ks keyShare, pk []byte) []byte {
+	if ks.group == X25519 {
+		if len(ks.data) != 32 {
+			return nil
+		}
+
+		var theirPublic, sharedKey, scalar [32]byte
+		copy(theirPublic[:], ks.data)
+		copy(scalar[:], pk)
+		curve25519.ScalarMult(&sharedKey, &scalar, &theirPublic)
+		return sharedKey[:]
+	}
+
+	curve, ok := curveForCurveID(ks.group)
+	if !ok {
+		return nil
+	}
+	x, y := elliptic.Unmarshal(curve, ks.data)
+	if x == nil {
+		return nil
+	}
+	x, _ = curve.ScalarMult(x, y, pk)
+	xBytes := x.Bytes()
+	curveSize := (curve.Params().BitSize + 8 - 1) >> 3
+	if len(xBytes) == curveSize {
+		return xBytes
+	}
+	buf := make([]byte, curveSize)
+	copy(buf[len(buf)-len(xBytes):], xBytes)
+	return buf
+}
+
+func hkdfExpandLabel(hash crypto.Hash, secret, hashValue []byte, label string, L int) []byte {
+	hkdfLabel := make([]byte, 4+len("TLS 1.3, ")+len(label)+len(hashValue))
+	hkdfLabel[0] = byte(L >> 8)
+	hkdfLabel[1] = byte(L)
+	hkdfLabel[2] = byte(len("TLS 1.3, ") + len(label))
+	copy(hkdfLabel[3:], "TLS 1.3, ")
+	z := hkdfLabel[3+len("TLS 1.3, "):]
+	copy(z, label)
+	z = z[len(label):]
+	z[0] = byte(len(hashValue))
+	copy(z[1:], hashValue)
+
+	return hkdfExpand(hash, secret, hkdfLabel, L)
+}

_dev/tris-localserver/server.go

@@ -10,12 +10,11 @@ import (
 )
 
 var tlsVersionToName = map[uint16]string{
-	tls.VersionTLS10: "1.0",
-	tls.VersionTLS11: "1.1",
-	tls.VersionTLS12: "1.2",
-	tls.VersionTLS13: "1.3",
-	0x7f00 | 16:      "1.3 (draft 16)",
-	0x7f00 | 18:      "1.3 (draft 18)",
+	tls.VersionTLS10:        "1.0",
+	tls.VersionTLS11:        "1.1",
+	tls.VersionTLS12:        "1.2",
+	tls.VersionTLS13:        "1.3",
+	tls.VersionTLS13Draft18: "1.3 (draft 18)",
 }
 
 func main() {

common.go

@@ -22,11 +22,12 @@ import (
 )
 
 const (
-	VersionSSL30 = 0x0300
-	VersionTLS10 = 0x0301
-	VersionTLS11 = 0x0302
-	VersionTLS12 = 0x0303
-	VersionTLS13 = 0x0304
+	VersionSSL30        = 0x0300
+	VersionTLS10        = 0x0301
+	VersionTLS11        = 0x0302
+	VersionTLS12        = 0x0303
+	VersionTLS13        = 0x0304
+	VersionTLS13Draft18 = 0x7f00 | 18
 )
 
 const (

conn.go

@@ -185,6 +185,14 @@ func (hc *halfConn) changeCipherSpec() error {
 	return nil
 }
 
+func (hc *halfConn) setCipher(version uint16, cipher interface{}) {
+	hc.version = version
+	hc.cipher = cipher
+	for i := range hc.seq {
+		hc.seq[i] = 0
+	}
+}
+
 // incSeq increments the sequence number.
 func (hc *halfConn) incSeq() {
 	for i := 7; i >= 0; i-- {

handshake_server.go

@@ -22,6 +22,8 @@ type serverHandshakeState struct {
 	c                     *Conn
 	clientHello           *clientHelloMsg
 	hello                 *serverHelloMsg
+	hello13               *serverHelloMsg13
+	hello13Enc            *encryptedExtensionsMsg
 	suite                 *cipherSuite
 	ellipticOk            bool
 	ecdsaOk               bool
@@ -52,7 +54,11 @@ func (c *Conn) serverHandshake() error {
 
 	// For an overview of TLS handshaking, see https://tools.ietf.org/html/rfc5246#section-7.3
 	c.buffering = true
-	if isResume {
+	if hs.hello13 != nil {
+		if err := hs.doTLS13Handshake(); err != nil {
+			return err
+		}
+	} else if isResume {
 		// The client has included a session ticket and so we do an abbreviated handshake.
 		if err := hs.doResumeHandshake(); err != nil {
 			return err
@@ -134,14 +140,31 @@ func (hs *serverHandshakeState) readClientHello() (isResume bool, err error) {
 		}
 	}
 
-	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)
+	var keyShares []CurveID
+	for _, ks := range hs.clientHello.keyShares {
+		keyShares = append(keyShares, ks.group)
 	}
-	c.haveVers = true
 
-	hs.hello = new(serverHelloMsg)
+	if hs.clientHello.supportedVersions != nil {
+		for _, v := range hs.clientHello.supportedVersions {
+			if (v >= c.config.minVersion() && v <= c.config.maxVersion()) ||
+				v == VersionTLS13Draft18 {
+				c.vers = v
+				break
+			}
+		}
+		if c.vers == 0 {
+			c.sendAlert(alertProtocolVersion)
+			return false, fmt.Errorf("tls: none of the client versions (%x) are supported", hs.clientHello.supportedVersions)
+		}
+	} else {
+		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
 
 	supportedCurve := false
 	preferredCurves := c.config.curvePreferences()
@@ -162,6 +185,8 @@ Curves:
 			break
 		}
 	}
+	// TLS 1.3 has removed point format negotiation.
+	supportedPointFormat = supportedPointFormat || c.vers >= VersionTLS13
 	hs.ellipticOk = supportedCurve && supportedPointFormat
 
 	foundCompression := false
@@ -177,13 +202,9 @@ Curves:
 		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.compressionMethods) != 1 && c.vers >= VersionTLS13 {
+		c.sendAlert(alertIllegalParameter)
+		return false, errors.New("tls: 1.3 client offered compression")
 	}
 
 	if len(hs.clientHello.secureRenegotiation) != 0 {
@@ -191,15 +212,40 @@ Curves:
 		return false, errors.New("tls: initial handshake had non-empty renegotiation extension")
 	}
 
-	hs.hello.secureRenegotiationSupported = hs.clientHello.secureRenegotiationSupported
-	hs.hello.compressionMethod = compressionNone
+	if c.vers < VersionTLS13 {
+		hs.hello = new(serverHelloMsg)
+		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
+		}
+		hs.hello.secureRenegotiationSupported = hs.clientHello.secureRenegotiationSupported
+		hs.hello.compressionMethod = compressionNone
+	} else {
+		hs.hello13 = new(serverHelloMsg13)
+		hs.hello13Enc = new(encryptedExtensionsMsg)
+		hs.hello13.vers = c.vers
+		hs.hello13.random = make([]byte, 32)
+		_, err = io.ReadFull(c.config.rand(), hs.hello13.random)
+		if err != nil {
+			c.sendAlert(alertInternalError)
+			return false, err
+		}
+	}
+
 	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
+			if hs.hello != nil {
+				hs.hello.alpnProtocol = selectedProto
+			} else {
+				hs.hello13Enc.alpnProtocol = selectedProto
+			}
 			c.clientProtocol = selectedProto
 		}
 	} else {
@@ -207,7 +253,7 @@ Curves:
 		// 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 {
+		if hs.clientHello.nextProtoNeg && len(c.config.NextProtos) > 0 && c.vers < VersionTLS13 {
 			hs.hello.nextProtoNeg = true
 			hs.hello.nextProtos = c.config.NextProtos
 		}
@@ -218,7 +264,7 @@ Curves:
 		c.sendAlert(alertInternalError)
 		return false, err
 	}
-	if hs.clientHello.scts {
+	if hs.clientHello.scts && hs.hello != nil { // TODO: TLS 1.3 SCTs
 		hs.hello.scts = hs.cert.SignedCertificateTimestamps
 	}
 
@@ -243,7 +289,7 @@ Curves:
 		}
 	}
 
-	if hs.checkForResumption() {
+	if c.vers != VersionTLS13 && hs.checkForResumption() {
 		return true, nil
 	}
 

hkdf.go

@@ -0,0 +1,58 @@
+// Copyright 2014 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
+
+// Mostly derived from golang.org/x/crypto/hkdf, but with an exposed
+// Extract API.
+//
+// HKDF is a cryptographic key derivation function (KDF) with the goal of
+// expanding limited input keying material into one or more cryptographically
+// strong secret keys.
+//
+// RFC 5869: https://tools.ietf.org/html/rfc5869
+
+import (
+	"crypto"
+	"crypto/hmac"
+)
+
+func hkdfExpand(hash crypto.Hash, prk, info []byte, l int) []byte {
+	var (
+		expander = hmac.New(hash.New, prk)
+		res      = make([]byte, l)
+		counter  = byte(1)
+		prev     []byte
+	)
+
+	if l > 255*expander.Size() {
+		panic("hkdf: requested too much output")
+	}
+
+	p := res
+	for len(p) > 0 {
+		expander.Reset()
+		expander.Write(prev)
+		expander.Write(info)
+		expander.Write([]byte{counter})
+		prev = expander.Sum(prev[:0])
+		counter++
+		n := copy(p, prev)
+		p = p[n:]
+	}
+
+	return res
+}
+
+func hkdfExtract(hash crypto.Hash, secret, salt []byte) []byte {
+	if salt == nil {
+		salt = make([]byte, hash.Size())
+	}
+	if secret == nil {
+		secret = make([]byte, hash.Size())
+	}
+	extractor := hmac.New(hash.New, salt)
+	extractor.Write(secret)
+	return extractor.Sum(nil)
+}

key_agreement.go

@@ -323,14 +323,14 @@ func (ka *ecdheKeyAgreement) processClientKeyExchange(config *Config, cert *Cert
 	if x == nil {
 		return nil, errClientKeyExchange
 	}
-	if !curve.IsOnCurve(x, y) {
-		return nil, errClientKeyExchange
-	}
 	x, _ = curve.ScalarMult(x, y, ka.privateKey)
-	preMasterSecret := make([]byte, (curve.Params().BitSize+7)>>3)
+	curveSize := (curve.Params().BitSize + 7) >> 3
 	xBytes := x.Bytes()
+	if len(xBytes) == curveSize {
+		return xBytes, nil
+	}
+	preMasterSecret := make([]byte, curveSize)
 	copy(preMasterSecret[len(preMasterSecret)-len(xBytes):], xBytes)
-
 	return preMasterSecret, nil
 }
 

prf.go

@@ -122,6 +122,13 @@ var clientFinishedLabel = []byte("client finished")
 var serverFinishedLabel = []byte("server finished")
 
 func prfAndHashForVersion(version uint16, suite *cipherSuite) (func(result, secret, label, seed []byte), crypto.Hash) {
+	if version >= VersionTLS13 {
+		if suite.flags&suiteSHA384 != 0 {
+			return prf12(sha512.New384), crypto.SHA384
+		}
+		return prf12(sha256.New), crypto.SHA256
+	}
+
 	switch version {
 	case VersionSSL30:
 		return prf30, crypto.Hash(0)