crypto/tls: consolidate signatures handling in SKE and CV

ServerKeyExchange and CertificateVerify can share the same logic for
picking a signature algorithm (based on the certificate public key and
advertised algorithms), selecting a hash algorithm (depending on TLS
version) and signature verification.

Refactor the code to achieve code reuse, have common error checking
(especially for intersecting supported signature algorithms) and to
prepare for addition of new signature algorithms. Code should be easier
to read since version-dependent logic is concentrated at one place.

Change-Id: I978dec3815d28e33c3cfbc85f0c704b1894c25a3
Reviewed-on: https://go-review.googlesource.com/79735Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/tls/auth.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tls
+
+import (
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/rsa"
+	"encoding/asn1"
+	"errors"
+	"fmt"
+)
+
+// pickSignatureAlgorithm selects a signature algorithm that is compatible with
+// the given public key and the list of algorithms from the peer and this side.
+// The lists of signature algorithms (peerSigAlgs and ourSigAlgs) are ignored
+// for tlsVersion < VersionTLS12.
+//
+// The returned SignatureScheme codepoint is only meaningful for TLS 1.2,
+// previous TLS versions have a fixed hash function.
+func pickSignatureAlgorithm(pubkey crypto.PublicKey, peerSigAlgs, ourSigAlgs []SignatureScheme, tlsVersion uint16) (sigAlg SignatureScheme, sigType uint8, hashFunc crypto.Hash, err error) {
+	if tlsVersion < VersionTLS12 || len(peerSigAlgs) == 0 {
+		// For TLS 1.1 and before, the signature algorithm could not be
+		// negotiated and the hash is fixed based on the signature type.
+		// For TLS 1.2, if the client didn't send signature_algorithms
+		// extension then we can assume that it supports SHA1. See
+		// https://tools.ietf.org/html/rfc5246#section-7.4.1.4.1
+		switch pubkey.(type) {
+		case *rsa.PublicKey:
+			if tlsVersion < VersionTLS12 {
+				return 0, signatureRSA, crypto.MD5SHA1, nil
+			} else {
+				return PKCS1WithSHA1, signatureRSA, crypto.SHA1, nil
+			}
+		case *ecdsa.PublicKey:
+			return ECDSAWithSHA1, signatureECDSA, crypto.SHA1, nil
+		default:
+			return 0, 0, 0, fmt.Errorf("tls: unsupported public key: %T", pubkey)
+		}
+	}
+	for _, sigAlg := range peerSigAlgs {
+		if !isSupportedSignatureAlgorithm(sigAlg, ourSigAlgs) {
+			continue
+		}
+		hashAlg, err := lookupTLSHash(sigAlg)
+		if err != nil {
+			panic("tls: supported signature algorithm has an unknown hash function")
+		}
+		sigType := signatureFromSignatureScheme(sigAlg)
+		switch pubkey.(type) {
+		case *rsa.PublicKey:
+			if sigType == signatureRSA {
+				return sigAlg, sigType, hashAlg, nil
+			}
+		case *ecdsa.PublicKey:
+			if sigType == signatureECDSA {
+				return sigAlg, sigType, hashAlg, nil
+			}
+		default:
+			return 0, 0, 0, fmt.Errorf("tls: unsupported public key: %T", pubkey)
+		}
+	}
+	return 0, 0, 0, errors.New("tls: peer doesn't support any common signature algorithms")
+}
+
+// verifyHandshakeSignature verifies a signature against pre-hashed handshake
+// contents.
+func verifyHandshakeSignature(sigType uint8, pubkey crypto.PublicKey, hashFunc crypto.Hash, digest, sig []byte) error {
+	switch sigType {
+	case signatureECDSA:
+		pubKey, ok := pubkey.(*ecdsa.PublicKey)
+		if !ok {
+			return errors.New("tls: ECDSA signing requires a ECDSA public key")
+		}
+		ecdsaSig := new(ecdsaSignature)
+		if _, err := asn1.Unmarshal(sig, ecdsaSig); err != nil {
+			return err
+		}
+		if ecdsaSig.R.Sign() <= 0 || ecdsaSig.S.Sign() <= 0 {
+			return errors.New("tls: ECDSA signature contained zero or negative values")
+		}
+		if !ecdsa.Verify(pubKey, digest, ecdsaSig.R, ecdsaSig.S) {
+			return errors.New("tls: ECDSA verification failure")
+		}
+	case signatureRSA:
+		pubKey, ok := pubkey.(*rsa.PublicKey)
+		if !ok {
+			return errors.New("tls: RSA signing requires a RSA public key")
+		}
+		if err := rsa.VerifyPKCS1v15(pubKey, hashFunc, digest, sig); err != nil {
+			return err
+		}
+	default:
+		return errors.New("tls: unknown signature algorithm")
+	}
+	return nil
+}

src/crypto/tls/auth_test.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tls
+
+import (
+	"crypto"
+	"testing"
+)
+
+func TestSignatureSelection(t *testing.T) {
+	rsaCert := &testRSAPrivateKey.PublicKey
+	ecdsaCert := &testECDSAPrivateKey.PublicKey
+	sigsPKCS1WithSHA := []SignatureScheme{PKCS1WithSHA256, PKCS1WithSHA1}
+	sigsECDSAWithSHA := []SignatureScheme{ECDSAWithP256AndSHA256, ECDSAWithSHA1}
+
+	tests := []struct {
+		pubkey      crypto.PublicKey
+		peerSigAlgs []SignatureScheme
+		ourSigAlgs  []SignatureScheme
+		tlsVersion  uint16
+
+		expectedSigAlg  SignatureScheme // or 0 if ignored
+		expectedSigType uint8
+		expectedHash    crypto.Hash
+	}{
+		// Hash is fixed for RSA in TLS 1.1 and before.
+		// https://tools.ietf.org/html/rfc4346#page-44
+		{rsaCert, nil, nil, VersionTLS11, 0, signatureRSA, crypto.MD5SHA1},
+		{rsaCert, nil, nil, VersionTLS10, 0, signatureRSA, crypto.MD5SHA1},
+		{rsaCert, nil, nil, VersionSSL30, 0, signatureRSA, crypto.MD5SHA1},
+
+		// Before TLS 1.2, there is no signature_algorithms extension
+		// nor field in CertificateRequest and digitally-signed and thus
+		// it should be ignored.
+		{rsaCert, sigsPKCS1WithSHA, nil, VersionTLS11, 0, signatureRSA, crypto.MD5SHA1},
+		{rsaCert, sigsPKCS1WithSHA, sigsPKCS1WithSHA, VersionTLS11, 0, signatureRSA, crypto.MD5SHA1},
+		// Use SHA-1 for TLS 1.0 and 1.1 with ECDSA, see https://tools.ietf.org/html/rfc4492#page-20
+		{ecdsaCert, sigsPKCS1WithSHA, sigsPKCS1WithSHA, VersionTLS11, 0, signatureECDSA, crypto.SHA1},
+		{ecdsaCert, sigsPKCS1WithSHA, sigsPKCS1WithSHA, VersionTLS10, 0, signatureECDSA, crypto.SHA1},
+
+		// TLS 1.2 without signature_algorithms extension
+		// https://tools.ietf.org/html/rfc5246#page-47
+		{rsaCert, nil, sigsPKCS1WithSHA, VersionTLS12, PKCS1WithSHA1, signatureRSA, crypto.SHA1},
+		{ecdsaCert, nil, sigsPKCS1WithSHA, VersionTLS12, ECDSAWithSHA1, signatureECDSA, crypto.SHA1},
+
+		{rsaCert, []SignatureScheme{PKCS1WithSHA1}, sigsPKCS1WithSHA, VersionTLS12, PKCS1WithSHA1, signatureRSA, crypto.SHA1},
+		{rsaCert, []SignatureScheme{PKCS1WithSHA256}, sigsPKCS1WithSHA, VersionTLS12, PKCS1WithSHA256, signatureRSA, crypto.SHA256},
+		// "sha_hash" may denote hashes other than SHA-1
+		// https://tools.ietf.org/html/draft-ietf-tls-rfc4492bis-17#page-17
+		{ecdsaCert, []SignatureScheme{ECDSAWithSHA1}, sigsECDSAWithSHA, VersionTLS12, ECDSAWithSHA1, signatureECDSA, crypto.SHA1},
+		{ecdsaCert, []SignatureScheme{ECDSAWithP256AndSHA256}, sigsECDSAWithSHA, VersionTLS12, ECDSAWithP256AndSHA256, signatureECDSA, crypto.SHA256},
+	}
+
+	for testNo, test := range tests {
+		sigAlg, sigType, hashFunc, err := pickSignatureAlgorithm(test.pubkey, test.peerSigAlgs, test.ourSigAlgs, test.tlsVersion)
+		if err != nil {
+			t.Errorf("test[%d]: unexpected error: %v", testNo, err)
+		}
+		if test.expectedSigAlg != 0 && test.expectedSigAlg != sigAlg {
+			t.Errorf("test[%d]: expected signature scheme %#x, got %#x", testNo, test.expectedSigAlg, sigAlg)
+		}
+		if test.expectedSigType != sigType {
+			t.Errorf("test[%d]: expected signature algorithm %#x, got %#x", testNo, test.expectedSigType, sigType)
+		}
+		if test.expectedHash != hashFunc {
+			t.Errorf("test[%d]: expected hash function %#x, got %#x", testNo, test.expectedHash, hashFunc)
+		}
+	}
+
+	badTests := []struct {
+		pubkey      crypto.PublicKey
+		peerSigAlgs []SignatureScheme
+		ourSigAlgs  []SignatureScheme
+		tlsVersion  uint16
+	}{
+		{rsaCert, sigsECDSAWithSHA, sigsPKCS1WithSHA, VersionTLS12},
+		{ecdsaCert, sigsPKCS1WithSHA, sigsPKCS1WithSHA, VersionTLS12},
+		{ecdsaCert, sigsECDSAWithSHA, sigsPKCS1WithSHA, VersionTLS12},
+		{rsaCert, []SignatureScheme{0}, sigsPKCS1WithSHA, VersionTLS12},
+
+		// ECDSA is unspecified for SSL 3.0 in RFC 4492.
+		// TODO a SSL 3.0 client cannot advertise signature_algorithms,
+		// but if an application feeds an ECDSA certificate anyway, it
+		// will be accepted rather than trigger a handshake failure. Ok?
+		//{ecdsaCert, nil, nil, VersionSSL30},
+	}
+
+	for testNo, test := range badTests {
+		sigAlg, sigType, hashFunc, err := pickSignatureAlgorithm(test.pubkey, test.peerSigAlgs, test.ourSigAlgs, test.tlsVersion)
+		if err == nil {
+			t.Errorf("test[%d]: unexpected success, got %#x %#x %#x", testNo, sigAlg, sigType, hashFunc)
+		}
+	}
+}

src/crypto/tls/handshake_client.go

@@ -479,26 +479,16 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 			return fmt.Errorf("tls: client certificate private key of type %T does not implement crypto.Signer", chainToSend.PrivateKey)
 		}
 
-		var signatureType uint8
-		switch key.Public().(type) {
-		case *ecdsa.PublicKey:
-			signatureType = signatureECDSA
-		case *rsa.PublicKey:
-			signatureType = signatureRSA
-		default:
-			c.sendAlert(alertInternalError)
-			return fmt.Errorf("tls: failed to sign handshake with client certificate: unknown client certificate key type: %T", key)
-		}
-
-		// SignatureAndHashAlgorithm was introduced in TLS 1.2.
-		if certVerify.hasSignatureAndHash {
-			certVerify.signatureAlgorithm, err = hs.finishedHash.selectClientCertSignatureAlgorithm(certReq.supportedSignatureAlgorithms, signatureType)
+		signatureAlgorithm, sigType, hashFunc, err := pickSignatureAlgorithm(key.Public(), certReq.supportedSignatureAlgorithms, hs.hello.supportedSignatureAlgorithms, c.vers)
 		if err != nil {
 			c.sendAlert(alertInternalError)
 			return err
 		}
+		// SignatureAndHashAlgorithm was introduced in TLS 1.2.
+		if certVerify.hasSignatureAndHash {
+			certVerify.signatureAlgorithm = signatureAlgorithm
 		}
-		digest, hashFunc, err := hs.finishedHash.hashForClientCertificate(signatureType, certVerify.signatureAlgorithm, hs.masterSecret)
+		digest, err := hs.finishedHash.hashForClientCertificate(sigType, hashFunc, hs.masterSecret)
 		if err != nil {
 			c.sendAlert(alertInternalError)
 			return err

src/crypto/tls/handshake_server.go

@@ -10,7 +10,6 @@ import (
 	"crypto/rsa"
 	"crypto/subtle"
 	"crypto/x509"
-	"encoding/asn1"
 	"errors"
 	"fmt"
 	"io"
@@ -520,59 +519,15 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 		}
 
 		// Determine the signature type.
-		var signatureAlgorithm SignatureScheme
-		var sigType uint8
-		if certVerify.hasSignatureAndHash {
-			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 signatureAlgorithm
-			// unset.
-			switch pub.(type) {
-			case *ecdsa.PublicKey:
-				sigType = signatureECDSA
-			case *rsa.PublicKey:
-				sigType = signatureRSA
-			}
+		_, sigType, hashFunc, err := pickSignatureAlgorithm(pub, []SignatureScheme{certVerify.signatureAlgorithm}, supportedSignatureAlgorithms, c.vers)
+		if err != nil {
+			c.sendAlert(alertIllegalParameter)
+			return err
 		}
 
-		switch key := pub.(type) {
-		case *ecdsa.PublicKey:
-			if sigType != signatureECDSA {
-				err = errors.New("tls: bad signature type for client's ECDSA certificate")
-				break
-			}
-			ecdsaSig := new(ecdsaSignature)
-			if _, err = asn1.Unmarshal(certVerify.signature, ecdsaSig); err != nil {
-				break
-			}
-			if ecdsaSig.R.Sign() <= 0 || ecdsaSig.S.Sign() <= 0 {
-				err = errors.New("tls: ECDSA signature contained zero or negative values")
-				break
-			}
 		var digest []byte
-			if digest, _, err = hs.finishedHash.hashForClientCertificate(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 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(sigType, signatureAlgorithm, hs.masterSecret); err != nil {
-				break
-			}
-			err = rsa.VerifyPKCS1v15(key, hashFunc, digest, certVerify.signature)
+		if digest, err = hs.finishedHash.hashForClientCertificate(sigType, hashFunc, hs.masterSecret); err == nil {
+			err = verifyHandshakeSignature(sigType, pub, hashFunc, digest, certVerify.signature)
 		}
 		if err != nil {
 			c.sendAlert(alertBadCertificate)

src/crypto/tls/key_agreement.go

@@ -6,13 +6,11 @@ package tls
 
 import (
 	"crypto"
-	"crypto/ecdsa"
 	"crypto/elliptic"
 	"crypto/md5"
 	"crypto/rsa"
 	"crypto/sha1"
 	"crypto/x509"
-	"encoding/asn1"
 	"errors"
 	"io"
 	"math/big"
@@ -110,58 +108,21 @@ func md5SHA1Hash(slices [][]byte) []byte {
 }
 
 // hashForServerKeyExchange hashes the given slices and returns their digest
-// 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) {
+// using the given hash function (for >= TLS 1.2) or using a default based on
+// the sigType (for earlier TLS versions).
+func hashForServerKeyExchange(sigType uint8, hashFunc crypto.Hash, version uint16, slices ...[]byte) ([]byte, error) {
 	if version >= VersionTLS12 {
-		if !isSupportedSignatureAlgorithm(signatureAlgorithm, supportedSignatureAlgorithms) {
-			return nil, crypto.Hash(0), errors.New("tls: unsupported hash function used by peer")
-		}
-		hashFunc, err := lookupTLSHash(signatureAlgorithm)
-		if err != nil {
-			return nil, crypto.Hash(0), err
-		}
 		h := hashFunc.New()
 		for _, slice := range slices {
 			h.Write(slice)
 		}
 		digest := h.Sum(nil)
-		return digest, hashFunc, nil
+		return digest, nil
 	}
 	if sigType == signatureECDSA {
-		return sha1Hash(slices), crypto.SHA1, nil
-	}
-	return md5SHA1Hash(slices), crypto.MD5SHA1, nil
-}
-
-// 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 []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
-		// https://tools.ietf.org/html/rfc5246#section-7.4.1.4.1
-		switch sigType {
-		case signatureRSA:
-			return PKCS1WithSHA1, nil
-		case signatureECDSA:
-			return ECDSAWithSHA1, nil
-		default:
-			return 0, errors.New("tls: unknown signature algorithm")
+		return sha1Hash(slices), nil
 	}
-	}
-
-	for _, sigAlg := range clientList {
-		if signatureFromSignatureScheme(sigAlg) != sigType {
-			continue
-		}
-		if isSupportedSignatureAlgorithm(sigAlg, supportedSignatureAlgorithms) {
-			return sigAlg, nil
-		}
-	}
-
-	return 0, errors.New("tls: client doesn't support any common hash functions")
+	return md5SHA1Hash(slices), nil
 }
 
 func curveForCurveID(id CurveID) (elliptic.Curve, bool) {
@@ -247,41 +208,25 @@ NextCandidate:
 	serverECDHParams[3] = byte(len(ecdhePublic))
 	copy(serverECDHParams[4:], ecdhePublic)
 
-	var signatureAlgorithm SignatureScheme
+	priv, ok := cert.PrivateKey.(crypto.Signer)
+	if !ok {
+		return nil, errors.New("tls: certificate private key does not implement crypto.Signer")
+	}
 
-	if ka.version >= VersionTLS12 {
-		var err error
-		signatureAlgorithm, err = pickTLS12HashForSignature(ka.sigType, clientHello.supportedSignatureAlgorithms)
+	signatureAlgorithm, sigType, hashFunc, err := pickSignatureAlgorithm(priv.Public(), clientHello.supportedSignatureAlgorithms, supportedSignatureAlgorithms, ka.version)
 	if err != nil {
 		return nil, err
 	}
+	if sigType != ka.sigType {
+		return nil, errors.New("tls: certificate cannot be used with the selected cipher suite")
 	}
 
-	digest, hashFunc, err := hashForServerKeyExchange(ka.sigType, signatureAlgorithm, ka.version, clientHello.random, hello.random, serverECDHParams)
+	digest, err := hashForServerKeyExchange(sigType, hashFunc, ka.version, clientHello.random, hello.random, serverECDHParams)
 	if err != nil {
 		return nil, err
 	}
 
-	priv, ok := cert.PrivateKey.(crypto.Signer)
-	if !ok {
-		return nil, errors.New("tls: certificate private key does not implement crypto.Signer")
-	}
-	var sig []byte
-	switch ka.sigType {
-	case signatureECDSA:
-		_, ok := priv.Public().(*ecdsa.PublicKey)
-		if !ok {
-			return nil, errors.New("tls: ECDHE ECDSA requires an ECDSA server key")
-		}
-	case signatureRSA:
-		_, ok := priv.Public().(*rsa.PublicKey)
-		if !ok {
-			return nil, errors.New("tls: ECDHE RSA requires a RSA server key")
-		}
-	default:
-		return nil, errors.New("tls: unknown ECDHE signature algorithm")
-	}
-	sig, err = priv.Sign(config.rand(), digest, hashFunc)
+	sig, err := priv.Sign(config.rand(), digest, hashFunc)
 	if err != nil {
 		return nil, errors.New("tls: failed to sign ECDHE parameters: " + err.Error())
 	}
@@ -380,53 +325,30 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 	if ka.version >= VersionTLS12 {
 		// handle SignatureAndHashAlgorithm
 		signatureAlgorithm = SignatureScheme(sig[0])<<8 | SignatureScheme(sig[1])
-		if signatureFromSignatureScheme(signatureAlgorithm) != ka.sigType {
-			return errServerKeyExchange
-		}
 		sig = sig[2:]
 		if len(sig) < 2 {
 			return errServerKeyExchange
 		}
 	}
+	_, sigType, hashFunc, err := pickSignatureAlgorithm(cert.PublicKey, []SignatureScheme{signatureAlgorithm}, clientHello.supportedSignatureAlgorithms, ka.version)
+	if err != nil {
+		return err
+	}
+	if sigType != ka.sigType {
+		return errServerKeyExchange
+	}
+
 	sigLen := int(sig[0])<<8 | int(sig[1])
 	if sigLen+2 != len(sig) {
 		return errServerKeyExchange
 	}
 	sig = sig[2:]
 
-	digest, hashFunc, err := hashForServerKeyExchange(ka.sigType, signatureAlgorithm, ka.version, clientHello.random, serverHello.random, serverECDHParams)
+	digest, err := hashForServerKeyExchange(sigType, hashFunc, ka.version, clientHello.random, serverHello.random, serverECDHParams)
 	if err != nil {
 		return err
 	}
-	switch ka.sigType {
-	case signatureECDSA:
-		pubKey, ok := cert.PublicKey.(*ecdsa.PublicKey)
-		if !ok {
-			return errors.New("tls: ECDHE ECDSA requires a ECDSA server public key")
-		}
-		ecdsaSig := new(ecdsaSignature)
-		if _, err := asn1.Unmarshal(sig, ecdsaSig); err != nil {
-			return err
-		}
-		if ecdsaSig.R.Sign() <= 0 || ecdsaSig.S.Sign() <= 0 {
-			return errors.New("tls: ECDSA signature contained zero or negative values")
-		}
-		if !ecdsa.Verify(pubKey, digest, ecdsaSig.R, ecdsaSig.S) {
-			return errors.New("tls: ECDSA verification failure")
-		}
-	case signatureRSA:
-		pubKey, ok := cert.PublicKey.(*rsa.PublicKey)
-		if !ok {
-			return errors.New("tls: ECDHE RSA requires a RSA server public key")
-		}
-		if err := rsa.VerifyPKCS1v15(pubKey, hashFunc, digest, sig); err != nil {
-			return err
-		}
-	default:
-		return errors.New("tls: unknown ECDHE signature algorithm")
-	}
-
-	return nil
+	return verifyHandshakeSignature(sigType, cert.PublicKey, hashFunc, digest, sig)
 }
 
 func (ka *ecdheKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, cert *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {

src/crypto/tls/prf.go

@@ -309,50 +309,35 @@ func (h finishedHash) serverSum(masterSecret []byte) []byte {
 	return out
 }
 
-// selectClientCertSignatureAlgorithm returns a SignatureScheme to sign a
-// client's CertificateVerify with, or an error if none can be found.
-func (h finishedHash) selectClientCertSignatureAlgorithm(serverList []SignatureScheme, sigType uint8) (SignatureScheme, error) {
-	for _, v := range serverList {
-		if signatureFromSignatureScheme(v) == sigType && isSupportedSignatureAlgorithm(v, supportedSignatureAlgorithms) {
-			return v, nil
-		}
-	}
-	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(sigType uint8, signatureAlgorithm SignatureScheme, masterSecret []byte) ([]byte, crypto.Hash, error) {
+// hashForClientCertificate returns a digest over the handshake messages so far,
+// suitable for signing by a TLS client certificate.
+func (h finishedHash) hashForClientCertificate(sigType uint8, hashAlg crypto.Hash, masterSecret []byte) ([]byte, 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 sigType != signatureRSA {
-			return nil, 0, errors.New("tls: unsupported signature type for client certificate")
+			return nil, errors.New("tls: unsupported signature type for client certificate")
 		}
 
 		md5Hash := md5.New()
 		md5Hash.Write(h.buffer)
 		sha1Hash := sha1.New()
 		sha1Hash.Write(h.buffer)
-		return finishedSum30(md5Hash, sha1Hash, masterSecret, nil), crypto.MD5SHA1, nil
+		return finishedSum30(md5Hash, sha1Hash, masterSecret, nil), nil
 	}
 	if h.version >= VersionTLS12 {
-		hashAlg, err := lookupTLSHash(signatureAlgorithm)
-		if err != nil {
-			return nil, 0, err
-		}
 		hash := hashAlg.New()
 		hash.Write(h.buffer)
-		return hash.Sum(nil), hashAlg, nil
+		return hash.Sum(nil), nil
 	}
 
 	if sigType == signatureECDSA {
-		return h.server.Sum(nil), crypto.SHA1, nil
+		return h.server.Sum(nil), nil
 	}
 
-	return h.Sum(), crypto.MD5SHA1, nil
+	return h.Sum(), nil
 }
 
 // discardHandshakeBuffer is called when there is no more need to