crypto/tls: replace signatureAndHash by SignatureScheme.

Consolidate the signature and hash fields (SignatureAndHashAlgorithm in
TLS 1.2) into a single uint16 (SignatureScheme in TLS 1.3 draft 21).
This makes it easier to add RSASSA-PSS for TLS 1.2 in the future.

Fields were named like "signatureAlgorithm" rather than
"signatureScheme" since that name is also used throughout the 1.3 draft.

The only new public symbol is ECDSAWithSHA1, other than that this is an
internal change with no new functionality.

Change-Id: Iba63d262ab1af895420583ac9e302d9705a7e0f0
Reviewed-on: https://go-review.googlesource.com/62210Reviewed-by: Adam Langley <agl@golang.org>

src/crypto/tls/common.go

@@ -126,35 +126,23 @@ const (
 	// Rest of these are reserved by the TLS spec
 )
 
-// Hash functions for TLS 1.2 (See RFC 5246, section A.4.1)
-const (
-	hashSHA1   uint8 = 2
-	hashSHA256 uint8 = 4
-	hashSHA384 uint8 = 5
-)
-
 // Signature algorithms for TLS 1.2 (See RFC 5246, section A.4.1)
 const (
 	signatureRSA   uint8 = 1
 	signatureECDSA uint8 = 3
 )
 
-// signatureAndHash mirrors the TLS 1.2, SignatureAndHashAlgorithm struct. See
-// RFC 5246, section A.4.1.
-type signatureAndHash struct {
-	hash, signature uint8
-}
-
 // supportedSignatureAlgorithms contains the signature and hash algorithms that
 // the code advertises as supported in a TLS 1.2 ClientHello and in a TLS 1.2
-// CertificateRequest.
-var supportedSignatureAlgorithms = []signatureAndHash{
-	{hashSHA256, signatureRSA},
-	{hashSHA256, signatureECDSA},
-	{hashSHA384, signatureRSA},
-	{hashSHA384, signatureECDSA},
-	{hashSHA1, signatureRSA},
-	{hashSHA1, signatureECDSA},
+// CertificateRequest. The two fields are merged to match with TLS 1.3.
+// Note that in TLS 1.2, the ECDSA algorithms are not constrained to P-256, etc.
+var supportedSignatureAlgorithms = []SignatureScheme{
+	PKCS1WithSHA256,
+	ECDSAWithP256AndSHA256,
+	PKCS1WithSHA384,
+	ECDSAWithP384AndSHA384,
+	PKCS1WithSHA1,
+	ECDSAWithSHA1,
 }
 
 // ConnectionState records basic TLS details about the connection.
@@ -234,6 +222,9 @@ const (
 	ECDSAWithP256AndSHA256 SignatureScheme = 0x0403
 	ECDSAWithP384AndSHA384 SignatureScheme = 0x0503
 	ECDSAWithP521AndSHA512 SignatureScheme = 0x0603
+
+	// Legacy signature and hash algorithms for TLS 1.2.
+	ECDSAWithSHA1 SignatureScheme = 0x0203
 )
 
 // ClientHelloInfo contains information from a ClientHello message in order to
@@ -961,11 +952,24 @@ func unexpectedMessageError(wanted, got interface{}) error {
 	return fmt.Errorf("tls: received unexpected handshake message of type %T when waiting for %T", got, wanted)
 }
 
-func isSupportedSignatureAndHash(sigHash signatureAndHash, sigHashes []signatureAndHash) bool {
-	for _, s := range sigHashes {
-		if s == sigHash {
+func isSupportedSignatureAlgorithm(sigAlg SignatureScheme, supportedSignatureAlgorithms []SignatureScheme) bool {
+	for _, s := range supportedSignatureAlgorithms {
+		if s == sigAlg {
 			return true
 		}
 	}
 	return false
 }
+
+// signatureFromSignatureScheme maps a signature algorithm to the underlying
+// signature method (without hash function).
+func signatureFromSignatureScheme(signatureAlgorithm SignatureScheme) uint8 {
+	switch signatureAlgorithm {
+	case PKCS1WithSHA1, PKCS1WithSHA256, PKCS1WithSHA384, PKCS1WithSHA512:
+		return signatureRSA
+	case ECDSAWithSHA1, ECDSAWithP256AndSHA256, ECDSAWithP384AndSHA384, ECDSAWithP521AndSHA512:
+		return signatureECDSA
+	default:
+		return 0
+	}
+}

src/crypto/tls/handshake_client.go

@@ -85,7 +85,7 @@ NextCipherSuite:
 	}
 
 	if hello.vers >= VersionTLS12 {
-		hello.signatureAndHashes = supportedSignatureAlgorithms
+		hello.supportedSignatureAlgorithms = supportedSignatureAlgorithms
 	}
 
 	return hello, nil
@@ -482,12 +482,15 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 			return fmt.Errorf("tls: failed to sign handshake with client certificate: unknown client certificate key type: %T", key)
 		}
 
-		certVerify.signatureAndHash, err = hs.finishedHash.selectClientCertSignatureAlgorithm(certReq.signatureAndHashes, signatureType)
-		if err != nil {
-			c.sendAlert(alertInternalError)
-			return err
+		// SignatureAndHashAlgorithm was introduced in TLS 1.2.
+		if certVerify.hasSignatureAndHash {
+			certVerify.signatureAlgorithm, err = hs.finishedHash.selectClientCertSignatureAlgorithm(certReq.supportedSignatureAlgorithms, signatureType)
+			if err != nil {
+				c.sendAlert(alertInternalError)
+				return err
+			}
 		}
-		digest, hashFunc, err := hs.finishedHash.hashForClientCertificate(certVerify.signatureAndHash, hs.masterSecret)
+		digest, hashFunc, err := hs.finishedHash.hashForClientCertificate(signatureType, certVerify.signatureAlgorithm, hs.masterSecret)
 		if err != nil {
 			c.sendAlert(alertInternalError)
 			return err
@@ -746,10 +749,7 @@ func (hs *clientHandshakeState) getCertificate(certReq *certificateRequestMsg) (
 				signatureSchemes = signatureSchemes[:len(signatureSchemes)-tls11SignatureSchemesNumRSA]
 			}
 		} else {
-			signatureSchemes = make([]SignatureScheme, 0, len(certReq.signatureAndHashes))
-			for _, sah := range certReq.signatureAndHashes {
-				signatureSchemes = append(signatureSchemes, SignatureScheme(sah.hash)<<8+SignatureScheme(sah.signature))
-			}
+			signatureSchemes = certReq.supportedSignatureAlgorithms
 		}
 
 		return c.config.GetClientCertificate(&CertificateRequestInfo{

src/crypto/tls/handshake_messages.go

@@ -24,7 +24,7 @@ type clientHelloMsg struct {
 	supportedPoints              []uint8
 	ticketSupported              bool
 	sessionTicket                []uint8
-	signatureAndHashes           []signatureAndHash
+	supportedSignatureAlgorithms []SignatureScheme
 	secureRenegotiation          []byte
 	secureRenegotiationSupported bool
 	alpnProtocols                []string
@@ -50,7 +50,7 @@ func (m *clientHelloMsg) equal(i interface{}) bool {
 		bytes.Equal(m.supportedPoints, m1.supportedPoints) &&
 		m.ticketSupported == m1.ticketSupported &&
 		bytes.Equal(m.sessionTicket, m1.sessionTicket) &&
-		eqSignatureAndHashes(m.signatureAndHashes, m1.signatureAndHashes) &&
+		eqSignatureAlgorithms(m.supportedSignatureAlgorithms, m1.supportedSignatureAlgorithms) &&
 		m.secureRenegotiationSupported == m1.secureRenegotiationSupported &&
 		bytes.Equal(m.secureRenegotiation, m1.secureRenegotiation) &&
 		eqStrings(m.alpnProtocols, m1.alpnProtocols)
@@ -87,8 +87,8 @@ func (m *clientHelloMsg) marshal() []byte {
 		extensionsLength += len(m.sessionTicket)
 		numExtensions++
 	}
-	if len(m.signatureAndHashes) > 0 {
-		extensionsLength += 2 + 2*len(m.signatureAndHashes)
+	if len(m.supportedSignatureAlgorithms) > 0 {
+		extensionsLength += 2 + 2*len(m.supportedSignatureAlgorithms)
 		numExtensions++
 	}
 	if m.secureRenegotiationSupported {
@@ -234,11 +234,11 @@ func (m *clientHelloMsg) marshal() []byte {
 		copy(z, m.sessionTicket)
 		z = z[len(m.sessionTicket):]
 	}
-	if len(m.signatureAndHashes) > 0 {
+	if len(m.supportedSignatureAlgorithms) > 0 {
 		// https://tools.ietf.org/html/rfc5246#section-7.4.1.4.1
 		z[0] = byte(extensionSignatureAlgorithms >> 8)
 		z[1] = byte(extensionSignatureAlgorithms)
-		l := 2 + 2*len(m.signatureAndHashes)
+		l := 2 + 2*len(m.supportedSignatureAlgorithms)
 		z[2] = byte(l >> 8)
 		z[3] = byte(l)
 		z = z[4:]
@@ -247,9 +247,9 @@ func (m *clientHelloMsg) marshal() []byte {
 		z[0] = byte(l >> 8)
 		z[1] = byte(l)
 		z = z[2:]
-		for _, sigAndHash := range m.signatureAndHashes {
-			z[0] = sigAndHash.hash
-			z[1] = sigAndHash.signature
+		for _, sigAlgo := range m.supportedSignatureAlgorithms {
+			z[0] = byte(sigAlgo >> 8)
+			z[1] = byte(sigAlgo)
 			z = z[2:]
 		}
 	}
@@ -344,7 +344,7 @@ func (m *clientHelloMsg) unmarshal(data []byte) bool {
 	m.ocspStapling = false
 	m.ticketSupported = false
 	m.sessionTicket = nil
-	m.signatureAndHashes = nil
+	m.supportedSignatureAlgorithms = nil
 	m.alpnProtocols = nil
 	m.scts = false
 
@@ -455,10 +455,9 @@ func (m *clientHelloMsg) unmarshal(data []byte) bool {
 			}
 			n := l / 2
 			d := data[2:]
-			m.signatureAndHashes = make([]signatureAndHash, n)
-			for i := range m.signatureAndHashes {
-				m.signatureAndHashes[i].hash = d[0]
-				m.signatureAndHashes[i].signature = d[1]
+			m.supportedSignatureAlgorithms = make([]SignatureScheme, n)
+			for i := range m.supportedSignatureAlgorithms {
+				m.supportedSignatureAlgorithms[i] = SignatureScheme(d[0])<<8 | SignatureScheme(d[1])
 				d = d[2:]
 			}
 		case extensionRenegotiationInfo:
@@ -1203,9 +1202,9 @@ type certificateRequestMsg struct {
 	// 1.2.
 	hasSignatureAndHash bool
 
-	certificateTypes       []byte
-	signatureAndHashes     []signatureAndHash
-	certificateAuthorities [][]byte
+	certificateTypes             []byte
+	supportedSignatureAlgorithms []SignatureScheme
+	certificateAuthorities       [][]byte
 }
 
 func (m *certificateRequestMsg) equal(i interface{}) bool {
@@ -1217,7 +1216,7 @@ func (m *certificateRequestMsg) equal(i interface{}) bool {
 	return bytes.Equal(m.raw, m1.raw) &&
 		bytes.Equal(m.certificateTypes, m1.certificateTypes) &&
 		eqByteSlices(m.certificateAuthorities, m1.certificateAuthorities) &&
-		eqSignatureAndHashes(m.signatureAndHashes, m1.signatureAndHashes)
+		eqSignatureAlgorithms(m.supportedSignatureAlgorithms, m1.supportedSignatureAlgorithms)
 }
 
 func (m *certificateRequestMsg) marshal() (x []byte) {
@@ -1234,7 +1233,7 @@ func (m *certificateRequestMsg) marshal() (x []byte) {
 	length += casLength
 
 	if m.hasSignatureAndHash {
-		length += 2 + 2*len(m.signatureAndHashes)
+		length += 2 + 2*len(m.supportedSignatureAlgorithms)
 	}
 
 	x = make([]byte, 4+length)
@@ -1249,13 +1248,13 @@ func (m *certificateRequestMsg) marshal() (x []byte) {
 	y := x[5+len(m.certificateTypes):]
 
 	if m.hasSignatureAndHash {
-		n := len(m.signatureAndHashes) * 2
+		n := len(m.supportedSignatureAlgorithms) * 2
 		y[0] = uint8(n >> 8)
 		y[1] = uint8(n)
 		y = y[2:]
-		for _, sigAndHash := range m.signatureAndHashes {
-			y[0] = sigAndHash.hash
-			y[1] = sigAndHash.signature
+		for _, sigAlgo := range m.supportedSignatureAlgorithms {
+			y[0] = uint8(sigAlgo >> 8)
+			y[1] = uint8(sigAlgo)
 			y = y[2:]
 		}
 	}
@@ -1312,11 +1311,10 @@ func (m *certificateRequestMsg) unmarshal(data []byte) bool {
 		if len(data) < int(sigAndHashLen) {
 			return false
 		}
-		numSigAndHash := sigAndHashLen / 2
-		m.signatureAndHashes = make([]signatureAndHash, numSigAndHash)
-		for i := range m.signatureAndHashes {
-			m.signatureAndHashes[i].hash = data[0]
-			m.signatureAndHashes[i].signature = data[1]
+		numSigAlgos := sigAndHashLen / 2
+		m.supportedSignatureAlgorithms = make([]SignatureScheme, numSigAlgos)
+		for i := range m.supportedSignatureAlgorithms {
+			m.supportedSignatureAlgorithms[i] = SignatureScheme(data[0])<<8 | SignatureScheme(data[1])
 			data = data[2:]
 		}
 	}
@@ -1355,7 +1353,7 @@ func (m *certificateRequestMsg) unmarshal(data []byte) bool {
 type certificateVerifyMsg struct {
 	raw                 []byte
 	hasSignatureAndHash bool
-	signatureAndHash    signatureAndHash
+	signatureAlgorithm  SignatureScheme
 	signature           []byte
 }
 
@@ -1367,8 +1365,7 @@ func (m *certificateVerifyMsg) equal(i interface{}) bool {
 
 	return bytes.Equal(m.raw, m1.raw) &&
 		m.hasSignatureAndHash == m1.hasSignatureAndHash &&
-		m.signatureAndHash.hash == m1.signatureAndHash.hash &&
-		m.signatureAndHash.signature == m1.signatureAndHash.signature &&
+		m.signatureAlgorithm == m1.signatureAlgorithm &&
 		bytes.Equal(m.signature, m1.signature)
 }
 
@@ -1390,8 +1387,8 @@ func (m *certificateVerifyMsg) marshal() (x []byte) {
 	x[3] = uint8(length)
 	y := x[4:]
 	if m.hasSignatureAndHash {
-		y[0] = m.signatureAndHash.hash
-		y[1] = m.signatureAndHash.signature
+		y[0] = uint8(m.signatureAlgorithm >> 8)
+		y[1] = uint8(m.signatureAlgorithm)
 		y = y[2:]
 	}
 	y[0] = uint8(siglength >> 8)
@@ -1417,8 +1414,7 @@ func (m *certificateVerifyMsg) unmarshal(data []byte) bool {
 
 	data = data[4:]
 	if m.hasSignatureAndHash {
-		m.signatureAndHash.hash = data[0]
-		m.signatureAndHash.signature = data[1]
+		m.signatureAlgorithm = SignatureScheme(data[0])<<8 | SignatureScheme(data[1])
 		data = data[2:]
 	}
 
@@ -1554,13 +1550,12 @@ func eqByteSlices(x, y [][]byte) bool {
 	return true
 }
 
-func eqSignatureAndHashes(x, y []signatureAndHash) bool {
+func eqSignatureAlgorithms(x, y []SignatureScheme) bool {
 	if len(x) != len(y) {
 		return false
 	}
 	for i, v := range x {
-		v2 := y[i]
-		if v.hash != v2.hash || v.signature != v2.signature {
+		if v != y[i] {
 			return false
 		}
 	}

src/crypto/tls/handshake_messages_test.go

@@ -145,7 +145,7 @@ func (*clientHelloMsg) Generate(rand *rand.Rand, size int) reflect.Value {
 		}
 	}
 	if rand.Intn(10) > 5 {
-		m.signatureAndHashes = supportedSignatureAlgorithms
+		m.supportedSignatureAlgorithms = supportedSignatureAlgorithms
 	}
 	m.alpnProtocols = make([]string, rand.Intn(5))
 	for i := range m.alpnProtocols {

src/crypto/tls/handshake_server.go

@@ -418,7 +418,7 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 		}
 		if c.vers >= VersionTLS12 {
 			certReq.hasSignatureAndHash = true
-			certReq.signatureAndHashes = supportedSignatureAlgorithms
+			certReq.supportedSignatureAlgorithms = supportedSignatureAlgorithms
 		}
 
 		// An empty list of certificateAuthorities signals to
@@ -519,27 +519,30 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 		}
 
 		// Determine the signature type.
-		var signatureAndHash signatureAndHash
+		var signatureAlgorithm SignatureScheme
+		var sigType uint8
 		if certVerify.hasSignatureAndHash {
-			signatureAndHash = certVerify.signatureAndHash
-			if !isSupportedSignatureAndHash(signatureAndHash, supportedSignatureAlgorithms) {
+			signatureAlgorithm = certVerify.signatureAlgorithm
+			if !isSupportedSignatureAlgorithm(signatureAlgorithm, supportedSignatureAlgorithms) {
 				return errors.New("tls: unsupported hash function for client certificate")
 			}
+			sigType = signatureFromSignatureScheme(signatureAlgorithm)
 		} 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.
+			// algorithm was possible. Leave signatureAlgorithm
+			// unset.
 			switch pub.(type) {
 			case *ecdsa.PublicKey:
-				signatureAndHash.signature = signatureECDSA
+				sigType = signatureECDSA
 			case *rsa.PublicKey:
-				signatureAndHash.signature = signatureRSA
+				sigType = signatureRSA
 			}
 		}
 
 		switch key := pub.(type) {
 		case *ecdsa.PublicKey:
-			if signatureAndHash.signature != signatureECDSA {
+			if sigType != signatureECDSA {
 				err = errors.New("tls: bad signature type for client's ECDSA certificate")
 				break
 			}
@@ -552,20 +555,20 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 				break
 			}
 			var digest []byte
-			if digest, _, err = hs.finishedHash.hashForClientCertificate(signatureAndHash, hs.masterSecret); err != nil {
+			if digest, _, err = hs.finishedHash.hashForClientCertificate(sigType, signatureAlgorithm, 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 {
+			if sigType != 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 {
+			if digest, hashFunc, err = hs.finishedHash.hashForClientCertificate(sigType, signatureAlgorithm, hs.masterSecret); err != nil {
 				break
 			}
 			err = rsa.VerifyPKCS1v15(key, hashFunc, digest, certVerify.signature)
@@ -818,17 +821,12 @@ func (hs *serverHandshakeState) clientHelloInfo() *ClientHelloInfo {
 		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,
+		SignatureSchemes:  hs.clientHello.supportedSignatureAlgorithms,
 		SupportedProtos:   hs.clientHello.alpnProtocols,
 		SupportedVersions: supportedVersions,
 		Conn:              hs.c.conn,

src/crypto/tls/key_agreement.go

@@ -110,14 +110,14 @@ func md5SHA1Hash(slices [][]byte) []byte {
 }
 
 // hashForServerKeyExchange hashes the given slices and returns their digest
-// and the identifier of the hash function used. The sigAndHash argument is
-// only used for >= TLS 1.2 and precisely identifies the hash function to use.
-func hashForServerKeyExchange(sigAndHash signatureAndHash, version uint16, slices ...[]byte) ([]byte, crypto.Hash, error) {
+// and the identifier of the hash function used. The signatureAlgorithm argument
+// is only used for >= TLS 1.2 and identifies the hash function to use.
+func hashForServerKeyExchange(sigType uint8, signatureAlgorithm SignatureScheme, version uint16, slices ...[]byte) ([]byte, crypto.Hash, error) {
 	if version >= VersionTLS12 {
-		if !isSupportedSignatureAndHash(sigAndHash, supportedSignatureAlgorithms) {
+		if !isSupportedSignatureAlgorithm(signatureAlgorithm, supportedSignatureAlgorithms) {
 			return nil, crypto.Hash(0), errors.New("tls: unsupported hash function used by peer")
 		}
-		hashFunc, err := lookupTLSHash(sigAndHash.hash)
+		hashFunc, err := lookupTLSHash(signatureAlgorithm)
 		if err != nil {
 			return nil, crypto.Hash(0), err
 		}
@@ -128,7 +128,7 @@ func hashForServerKeyExchange(sigAndHash signatureAndHash, version uint16, slice
 		digest := h.Sum(nil)
 		return digest, hashFunc, nil
 	}
-	if sigAndHash.signature == signatureECDSA {
+	if sigType == signatureECDSA {
 		return sha1Hash(slices), crypto.SHA1, nil
 	}
 	return md5SHA1Hash(slices), crypto.MD5SHA1, nil
@@ -137,20 +137,27 @@ func hashForServerKeyExchange(sigAndHash signatureAndHash, version uint16, slice
 // pickTLS12HashForSignature returns a TLS 1.2 hash identifier for signing a
 // ServerKeyExchange given the signature type being used and the client's
 // advertised list of supported signature and hash combinations.
-func pickTLS12HashForSignature(sigType uint8, clientList []signatureAndHash) (uint8, error) {
+func pickTLS12HashForSignature(sigType uint8, clientList []SignatureScheme) (SignatureScheme, error) {
 	if len(clientList) == 0 {
 		// If the client didn't specify any signature_algorithms
 		// extension then we can assume that it supports SHA1. See
 		// http://tools.ietf.org/html/rfc5246#section-7.4.1.4.1
-		return hashSHA1, nil
+		switch sigType {
+		case signatureRSA:
+			return PKCS1WithSHA1, nil
+		case signatureECDSA:
+			return ECDSAWithSHA1, nil
+		default:
+			return 0, errors.New("tls: unknown signature algorithm")
+		}
 	}
 
-	for _, sigAndHash := range clientList {
-		if sigAndHash.signature != sigType {
+	for _, sigAlg := range clientList {
+		if signatureFromSignatureScheme(sigAlg) != sigType {
 			continue
 		}
-		if isSupportedSignatureAndHash(sigAndHash, supportedSignatureAlgorithms) {
-			return sigAndHash.hash, nil
+		if isSupportedSignatureAlgorithm(sigAlg, supportedSignatureAlgorithms) {
+			return sigAlg, nil
 		}
 	}
 
@@ -240,16 +247,17 @@ NextCandidate:
 	serverECDHParams[3] = byte(len(ecdhePublic))
 	copy(serverECDHParams[4:], ecdhePublic)
 
-	sigAndHash := signatureAndHash{signature: ka.sigType}
+	var signatureAlgorithm SignatureScheme
 
 	if ka.version >= VersionTLS12 {
 		var err error
-		if sigAndHash.hash, err = pickTLS12HashForSignature(ka.sigType, clientHello.signatureAndHashes); err != nil {
+		signatureAlgorithm, err = pickTLS12HashForSignature(ka.sigType, clientHello.supportedSignatureAlgorithms)
+		if err != nil {
 			return nil, err
 		}
 	}
 
-	digest, hashFunc, err := hashForServerKeyExchange(sigAndHash, ka.version, clientHello.random, hello.random, serverECDHParams)
+	digest, hashFunc, err := hashForServerKeyExchange(ka.sigType, signatureAlgorithm, ka.version, clientHello.random, hello.random, serverECDHParams)
 	if err != nil {
 		return nil, err
 	}
@@ -287,8 +295,8 @@ NextCandidate:
 	copy(skx.key, serverECDHParams)
 	k := skx.key[len(serverECDHParams):]
 	if ka.version >= VersionTLS12 {
-		k[0] = sigAndHash.hash
-		k[1] = sigAndHash.signature
+		k[0] = byte(signatureAlgorithm >> 8)
+		k[1] = byte(signatureAlgorithm)
 		k = k[2:]
 	}
 	k[0] = byte(len(sig) >> 8)
@@ -368,11 +376,11 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 		}
 	}
 
-	sigAndHash := signatureAndHash{signature: ka.sigType}
+	var signatureAlgorithm SignatureScheme
 	if ka.version >= VersionTLS12 {
 		// handle SignatureAndHashAlgorithm
-		sigAndHash = signatureAndHash{hash: sig[0], signature: sig[1]}
-		if sigAndHash.signature != ka.sigType {
+		signatureAlgorithm = SignatureScheme(sig[0])<<8 | SignatureScheme(sig[1])
+		if signatureFromSignatureScheme(signatureAlgorithm) != ka.sigType {
 			return errServerKeyExchange
 		}
 		sig = sig[2:]
@@ -386,7 +394,7 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 	}
 	sig = sig[2:]
 
-	digest, hashFunc, err := hashForServerKeyExchange(sigAndHash, ka.version, clientHello.random, serverHello.random, serverECDHParams)
+	digest, hashFunc, err := hashForServerKeyExchange(ka.sigType, signatureAlgorithm, ka.version, clientHello.random, serverHello.random, serverECDHParams)
 	if err != nil {
 		return err
 	}

src/crypto/tls/prf.go

@@ -12,6 +12,7 @@ import (
 	"crypto/sha256"
 	"crypto/sha512"
 	"errors"
+	"fmt"
 	"hash"
 )
 
@@ -180,17 +181,17 @@ func keysFromMasterSecret(version uint16, suite *cipherSuite, masterSecret, clie
 }
 
 // lookupTLSHash looks up the corresponding crypto.Hash for a given
-// TLS hash identifier.
-func lookupTLSHash(hash uint8) (crypto.Hash, error) {
-	switch hash {
-	case hashSHA1:
+// hash from a TLS SignatureScheme.
+func lookupTLSHash(signatureAlgorithm SignatureScheme) (crypto.Hash, error) {
+	switch signatureAlgorithm {
+	case PKCS1WithSHA1, ECDSAWithSHA1:
 		return crypto.SHA1, nil
-	case hashSHA256:
+	case PKCS1WithSHA256, PSSWithSHA256, ECDSAWithP256AndSHA256:
 		return crypto.SHA256, nil
-	case hashSHA384:
+	case PKCS1WithSHA384, PSSWithSHA384, ECDSAWithP384AndSHA384:
 		return crypto.SHA384, nil
 	default:
-		return 0, errors.New("tls: unsupported hash algorithm")
+		return 0, fmt.Errorf("tls: unsupported signature algorithm: %#04x", signatureAlgorithm)
 	}
 }
 
@@ -310,31 +311,26 @@ func (h finishedHash) serverSum(masterSecret []byte) []byte {
 	return out
 }
 
-// selectClientCertSignatureAlgorithm returns a signatureAndHash to sign a
+// selectClientCertSignatureAlgorithm returns a SignatureScheme to sign a
 // client's CertificateVerify with, or an error if none can be found.
-func (h finishedHash) selectClientCertSignatureAlgorithm(serverList []signatureAndHash, sigType uint8) (signatureAndHash, error) {
-	if h.version < VersionTLS12 {
-		// Nothing to negotiate before TLS 1.2.
-		return signatureAndHash{signature: sigType}, nil
-	}
-
+func (h finishedHash) selectClientCertSignatureAlgorithm(serverList []SignatureScheme, sigType uint8) (SignatureScheme, error) {
 	for _, v := range serverList {
-		if v.signature == sigType && isSupportedSignatureAndHash(v, supportedSignatureAlgorithms) {
+		if signatureFromSignatureScheme(v) == sigType && isSupportedSignatureAlgorithm(v, supportedSignatureAlgorithms) {
 			return v, nil
 		}
 	}
-	return signatureAndHash{}, errors.New("tls: no supported signature algorithm found for signing client certificate")
+	return 0, errors.New("tls: no supported signature algorithm found for signing client certificate")
 }
 
 // hashForClientCertificate returns a digest, hash function, and TLS 1.2 hash
 // id suitable for signing by a TLS client certificate.
-func (h finishedHash) hashForClientCertificate(signatureAndHash signatureAndHash, masterSecret []byte) ([]byte, crypto.Hash, error) {
+func (h finishedHash) hashForClientCertificate(sigType uint8, signatureAlgorithm SignatureScheme, masterSecret []byte) ([]byte, crypto.Hash, error) {
 	if (h.version == VersionSSL30 || h.version >= VersionTLS12) && h.buffer == nil {
 		panic("a handshake hash for a client-certificate was requested after discarding the handshake buffer")
 	}
 
 	if h.version == VersionSSL30 {
-		if signatureAndHash.signature != signatureRSA {
+		if sigType != signatureRSA {
 			return nil, 0, errors.New("tls: unsupported signature type for client certificate")
 		}
 
@@ -345,7 +341,7 @@ func (h finishedHash) hashForClientCertificate(signatureAndHash signatureAndHash
 		return finishedSum30(md5Hash, sha1Hash, masterSecret, nil), crypto.MD5SHA1, nil
 	}
 	if h.version >= VersionTLS12 {
-		hashAlg, err := lookupTLSHash(signatureAndHash.hash)
+		hashAlg, err := lookupTLSHash(signatureAlgorithm)
 		if err != nil {
 			return nil, 0, err
 		}
@@ -354,7 +350,7 @@ func (h finishedHash) hashForClientCertificate(signatureAndHash signatureAndHash
 		return hash.Sum(nil), hashAlg, nil
 	}
 
-	if signatureAndHash.signature == signatureECDSA {
+	if sigType == signatureECDSA {
 		return h.server.Sum(nil), crypto.SHA1, nil
 	}