src/crypto/tls/key_agreement.go - go - Git at Google

 // Copyright 2010 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/md5"
 	"crypto/rsa"
 	"crypto/sha1"
 	"crypto/x509"
 	"errors"
 	"fmt"
 	"io"
 )

 var errClientKeyExchange = errors.New("tls: invalid ClientKeyExchange message")
 var errServerKeyExchange = errors.New("tls: invalid ServerKeyExchange message")

 // rsaKeyAgreement implements the standard TLS key agreement where the client
 // encrypts the pre-master secret to the server's public key.
 type rsaKeyAgreement struct{}

 func (ka rsaKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
 	return nil, nil
 }

 func (ka rsaKeyAgreement) processClientKeyExchange(config *Config, cert *Certificate, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) {
 	if len(ckx.ciphertext) < 2 {
 		return nil, errClientKeyExchange
 	}
 	ciphertextLen := int(ckx.ciphertext[0])<<8 | int(ckx.ciphertext[1])
 	if ciphertextLen != len(ckx.ciphertext)-2 {
 		return nil, errClientKeyExchange
 	}
 	ciphertext := ckx.ciphertext[2:]

 	priv, ok := cert.PrivateKey.(crypto.Decrypter)
 	if !ok {
 		return nil, errors.New("tls: certificate private key does not implement crypto.Decrypter")
 	}
 	// Perform constant time RSA PKCS #1 v1.5 decryption
 	preMasterSecret, err := priv.Decrypt(config.rand(), ciphertext, &rsa.PKCS1v15DecryptOptions{SessionKeyLen: 48})
 	if err != nil {
 		return nil, err
 	}
 	// We don't check the version number in the premaster secret. For one,
 	// by checking it, we would leak information about the validity of the
 	// encrypted pre-master secret. Secondly, it provides only a small
 	// benefit against a downgrade attack and some implementations send the
 	// wrong version anyway. See the discussion at the end of section
 	// 7.4.7.1 of RFC 4346.
 	return preMasterSecret, nil
 }

 func (ka rsaKeyAgreement) processServerKeyExchange(config *Config, clientHello *clientHelloMsg, serverHello *serverHelloMsg, cert *x509.Certificate, skx *serverKeyExchangeMsg) error {
 	return errors.New("tls: unexpected ServerKeyExchange")
 }

 func (ka rsaKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, cert *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {
 	preMasterSecret := make([]byte, 48)
 	preMasterSecret[0] = byte(clientHello.vers >> 8)
 	preMasterSecret[1] = byte(clientHello.vers)
 	_, err := io.ReadFull(config.rand(), preMasterSecret[2:])
 	if err != nil {
 		return nil, nil, err
 	}

 	encrypted, err := rsa.EncryptPKCS1v15(config.rand(), cert.PublicKey.(*rsa.PublicKey), preMasterSecret)
 	if err != nil {
 		return nil, nil, err
 	}
 	ckx := new(clientKeyExchangeMsg)
 	ckx.ciphertext = make([]byte, len(encrypted)+2)
 	ckx.ciphertext[0] = byte(len(encrypted) >> 8)
 	ckx.ciphertext[1] = byte(len(encrypted))
 	copy(ckx.ciphertext[2:], encrypted)
 	return preMasterSecret, ckx, nil
 }

 // sha1Hash calculates a SHA1 hash over the given byte slices.
 func sha1Hash(slices [][]byte) []byte {
 	hsha1 := sha1.New()
 	for _, slice := range slices {
 		hsha1.Write(slice)
 	}
 	return hsha1.Sum(nil)
 }

 // md5SHA1Hash implements TLS 1.0's hybrid hash function which consists of the
 // concatenation of an MD5 and SHA1 hash.
 func md5SHA1Hash(slices [][]byte) []byte {
 	md5sha1 := make([]byte, md5.Size+sha1.Size)
 	hmd5 := md5.New()
 	for _, slice := range slices {
 		hmd5.Write(slice)
 	}
 	copy(md5sha1, hmd5.Sum(nil))
 	copy(md5sha1[md5.Size:], sha1Hash(slices))
 	return md5sha1
 }

 // hashForServerKeyExchange hashes the given slices and returns their digest
 // using the given hash function (for >= TLS 1.2) or using a default based on
 // the sigType (for earlier TLS versions). For Ed25519 signatures, which don't
 // do pre-hashing, it returns the concatenation of the slices.
 func hashForServerKeyExchange(sigType uint8, hashFunc crypto.Hash, version uint16, slices ...[]byte) []byte {
 	if sigType == signatureEd25519 {
 		var signed []byte
 		for _, slice := range slices {
 			signed = append(signed, slice...)
 		}
 		return signed
 	}
 	if version >= VersionTLS12 {
 		h := hashFunc.New()
 		for _, slice := range slices {
 			h.Write(slice)
 		}
 		digest := h.Sum(nil)
 		return digest
 	}
 	if sigType == signatureECDSA {
 		return sha1Hash(slices)
 	}
 	return md5SHA1Hash(slices)
 }

 // ecdheKeyAgreement implements a TLS key agreement where the server
 // generates an ephemeral EC public/private key pair and signs it. The
 // pre-master secret is then calculated using ECDH. The signature may
 // be ECDSA, Ed25519 or RSA.
 type ecdheKeyAgreement struct {
 	version uint16
 	isRSA   bool
 	params  ecdheParameters

 	// ckx and preMasterSecret are generated in processServerKeyExchange
 	// and returned in generateClientKeyExchange.
 	ckx             *clientKeyExchangeMsg
 	preMasterSecret []byte
 }

 func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
 	var curveID CurveID
 	for _, c := range clientHello.supportedCurves {
 		if config.supportsCurve(c) {
 			curveID = c
 			break
 		}
 	}

 	if curveID == 0 {
 		return nil, errors.New("tls: no supported elliptic curves offered")
 	}
 	if _, ok := curveForCurveID(curveID); curveID != X25519 && !ok {
 		return nil, errors.New("tls: CurvePreferences includes unsupported curve")
 	}

 	params, err := generateECDHEParameters(config.rand(), curveID)
 	if err != nil {
 		return nil, err
 	}
 	ka.params = params

 	// See RFC 4492, Section 5.4.
 	ecdhePublic := params.PublicKey()
 	serverECDHEParams := make([]byte, 1+2+1+len(ecdhePublic))
 	serverECDHEParams[0] = 3 // named curve
 	serverECDHEParams[1] = byte(curveID >> 8)
 	serverECDHEParams[2] = byte(curveID)
 	serverECDHEParams[3] = byte(len(ecdhePublic))
 	copy(serverECDHEParams[4:], ecdhePublic)

 	priv, ok := cert.PrivateKey.(crypto.Signer)
 	if !ok {
 		return nil, fmt.Errorf("tls: certificate private key of type %T does not implement crypto.Signer", cert.PrivateKey)
 	}

 	var signatureAlgorithm SignatureScheme
 	var sigType uint8
 	var sigHash crypto.Hash
 	if ka.version >= VersionTLS12 {
 		signatureAlgorithm, err = selectSignatureScheme(ka.version, cert, clientHello.supportedSignatureAlgorithms)
 		if err != nil {
 			return nil, err
 		}
 		sigType, sigHash, err = typeAndHashFromSignatureScheme(signatureAlgorithm)
 		if err != nil {
 			return nil, err
 		}
 	} else {
 		sigType, sigHash, err = legacyTypeAndHashFromPublicKey(priv.Public())
 		if err != nil {
 			return nil, err
 		}
 	}
 	if (sigType == signaturePKCS1v15 || sigType == signatureRSAPSS) != ka.isRSA {
 		return nil, errors.New("tls: certificate cannot be used with the selected cipher suite")
 	}

 	signed := hashForServerKeyExchange(sigType, sigHash, ka.version, clientHello.random, hello.random, serverECDHEParams)

 	signOpts := crypto.SignerOpts(sigHash)
 	if sigType == signatureRSAPSS {
 		signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: sigHash}
 	}
 	sig, err := priv.Sign(config.rand(), signed, signOpts)
 	if err != nil {
 		return nil, errors.New("tls: failed to sign ECDHE parameters: " + err.Error())
 	}

 	skx := new(serverKeyExchangeMsg)
 	sigAndHashLen := 0
 	if ka.version >= VersionTLS12 {
 		sigAndHashLen = 2
 	}
 	skx.key = make([]byte, len(serverECDHEParams)+sigAndHashLen+2+len(sig))
 	copy(skx.key, serverECDHEParams)
 	k := skx.key[len(serverECDHEParams):]
 	if ka.version >= VersionTLS12 {
 		k[0] = byte(signatureAlgorithm >> 8)
 		k[1] = byte(signatureAlgorithm)
 		k = k[2:]
 	}
 	k[0] = byte(len(sig) >> 8)
 	k[1] = byte(len(sig))
 	copy(k[2:], sig)

 	return skx, nil
 }

 func (ka *ecdheKeyAgreement) processClientKeyExchange(config *Config, cert *Certificate, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) {
 	if len(ckx.ciphertext) == 0 || int(ckx.ciphertext[0]) != len(ckx.ciphertext)-1 {
 		return nil, errClientKeyExchange
 	}

 	preMasterSecret := ka.params.SharedKey(ckx.ciphertext[1:])
 	if preMasterSecret == nil {
 		return nil, errClientKeyExchange
 	}

 	return preMasterSecret, nil
 }

 func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHello *clientHelloMsg, serverHello *serverHelloMsg, cert *x509.Certificate, skx *serverKeyExchangeMsg) error {
 	if len(skx.key) < 4 {
 		return errServerKeyExchange
 	}
 	if skx.key[0] != 3 { // named curve
 		return errors.New("tls: server selected unsupported curve")
 	}
 	curveID := CurveID(skx.key[1])<<8 | CurveID(skx.key[2])

 	publicLen := int(skx.key[3])
 	if publicLen+4 > len(skx.key) {
 		return errServerKeyExchange
 	}
 	serverECDHEParams := skx.key[:4+publicLen]
 	publicKey := serverECDHEParams[4:]

 	sig := skx.key[4+publicLen:]
 	if len(sig) < 2 {
 		return errServerKeyExchange
 	}

 	if _, ok := curveForCurveID(curveID); curveID != X25519 && !ok {
 		return errors.New("tls: server selected unsupported curve")
 	}

 	params, err := generateECDHEParameters(config.rand(), curveID)
 	if err != nil {
 		return err
 	}
 	ka.params = params

 	ka.preMasterSecret = params.SharedKey(publicKey)
 	if ka.preMasterSecret == nil {
 		return errServerKeyExchange
 	}

 	ourPublicKey := params.PublicKey()
 	ka.ckx = new(clientKeyExchangeMsg)
 	ka.ckx.ciphertext = make([]byte, 1+len(ourPublicKey))
 	ka.ckx.ciphertext[0] = byte(len(ourPublicKey))
 	copy(ka.ckx.ciphertext[1:], ourPublicKey)

 	var sigType uint8
 	var sigHash crypto.Hash
 	if ka.version >= VersionTLS12 {
 		signatureAlgorithm := SignatureScheme(sig[0])<<8 | SignatureScheme(sig[1])
 		sig = sig[2:]
 		if len(sig) < 2 {
 			return errServerKeyExchange
 		}

 		if !isSupportedSignatureAlgorithm(signatureAlgorithm, clientHello.supportedSignatureAlgorithms) {
 			return errors.New("tls: certificate used with invalid signature algorithm")
 		}
 		sigType, sigHash, err = typeAndHashFromSignatureScheme(signatureAlgorithm)
 		if err != nil {
 			return err
 		}
 	} else {
 		sigType, sigHash, err = legacyTypeAndHashFromPublicKey(cert.PublicKey)
 		if err != nil {
 			return err
 		}
 	}
 	if (sigType == signaturePKCS1v15 || sigType == signatureRSAPSS) != ka.isRSA {
 		return errServerKeyExchange
 	}

 	sigLen := int(sig[0])<<8 | int(sig[1])
 	if sigLen+2 != len(sig) {
 		return errServerKeyExchange
 	}
 	sig = sig[2:]

 	signed := hashForServerKeyExchange(sigType, sigHash, ka.version, clientHello.random, serverHello.random, serverECDHEParams)
 	if err := verifyHandshakeSignature(sigType, cert.PublicKey, sigHash, signed, sig); err != nil {
 		return errors.New("tls: invalid signature by the server certificate: " + err.Error())
 	}
 	return nil
 }

 func (ka *ecdheKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, cert *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {
 	if ka.ckx == nil {
 		return nil, nil, errors.New("tls: missing ServerKeyExchange message")
 	}

 	return ka.preMasterSecret, ka.ckx, nil
 }
	// Copyright 2010 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/md5"
	"crypto/rsa"
	"crypto/sha1"
	"crypto/x509"
	"errors"
	"fmt"
	"io"
	)

	var errClientKeyExchange = errors.New("tls: invalid ClientKeyExchange message")
	var errServerKeyExchange = errors.New("tls: invalid ServerKeyExchange message")

	// rsaKeyAgreement implements the standard TLS key agreement where the client
	// encrypts the pre-master secret to the server's public key.
	type rsaKeyAgreement struct{}

	func (ka rsaKeyAgreement) generateServerKeyExchange(config Config, cert Certificate, clientHello clientHelloMsg, hello serverHelloMsg) (*serverKeyExchangeMsg, error) {
	return nil, nil
	}

	func (ka rsaKeyAgreement) processClientKeyExchange(config Config, cert Certificate, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) {
	if len(ckx.ciphertext) < 2 {
	return nil, errClientKeyExchange
	}
	ciphertextLen := int(ckx.ciphertext[0])<<8 \| int(ckx.ciphertext[1])
	if ciphertextLen != len(ckx.ciphertext)-2 {
	return nil, errClientKeyExchange
	}
	ciphertext := ckx.ciphertext[2:]

	priv, ok := cert.PrivateKey.(crypto.Decrypter)
	if !ok {
	return nil, errors.New("tls: certificate private key does not implement crypto.Decrypter")
	}
	// Perform constant time RSA PKCS #1 v1.5 decryption
	preMasterSecret, err := priv.Decrypt(config.rand(), ciphertext, &rsa.PKCS1v15DecryptOptions{SessionKeyLen: 48})
	if err != nil {
	return nil, err
	}
	// We don't check the version number in the premaster secret. For one,
	// by checking it, we would leak information about the validity of the
	// encrypted pre-master secret. Secondly, it provides only a small
	// benefit against a downgrade attack and some implementations send the
	// wrong version anyway. See the discussion at the end of section
	// 7.4.7.1 of RFC 4346.
	return preMasterSecret, nil
	}

	func (ka rsaKeyAgreement) processServerKeyExchange(config Config, clientHello clientHelloMsg, serverHello serverHelloMsg, cert x509.Certificate, skx *serverKeyExchangeMsg) error {
	return errors.New("tls: unexpected ServerKeyExchange")
	}

	func (ka rsaKeyAgreement) generateClientKeyExchange(config Config, clientHello clientHelloMsg, cert x509.Certificate) ([]byte, clientKeyExchangeMsg, error) {
	preMasterSecret := make([]byte, 48)
	preMasterSecret[0] = byte(clientHello.vers >> 8)
	preMasterSecret[1] = byte(clientHello.vers)
	_, err := io.ReadFull(config.rand(), preMasterSecret[2:])
	if err != nil {
	return nil, nil, err
	}

	encrypted, err := rsa.EncryptPKCS1v15(config.rand(), cert.PublicKey.(*rsa.PublicKey), preMasterSecret)
	if err != nil {
	return nil, nil, err
	}
	ckx := new(clientKeyExchangeMsg)
	ckx.ciphertext = make([]byte, len(encrypted)+2)
	ckx.ciphertext[0] = byte(len(encrypted) >> 8)
	ckx.ciphertext[1] = byte(len(encrypted))
	copy(ckx.ciphertext[2:], encrypted)
	return preMasterSecret, ckx, nil
	}

	// sha1Hash calculates a SHA1 hash over the given byte slices.
	func sha1Hash(slices [][]byte) []byte {
	hsha1 := sha1.New()
	for _, slice := range slices {
	hsha1.Write(slice)
	}
	return hsha1.Sum(nil)
	}

	// md5SHA1Hash implements TLS 1.0's hybrid hash function which consists of the
	// concatenation of an MD5 and SHA1 hash.
	func md5SHA1Hash(slices [][]byte) []byte {
	md5sha1 := make([]byte, md5.Size+sha1.Size)
	hmd5 := md5.New()
	for _, slice := range slices {
	hmd5.Write(slice)
	}
	copy(md5sha1, hmd5.Sum(nil))
	copy(md5sha1[md5.Size:], sha1Hash(slices))
	return md5sha1
	}

	// hashForServerKeyExchange hashes the given slices and returns their digest
	// using the given hash function (for >= TLS 1.2) or using a default based on
	// the sigType (for earlier TLS versions). For Ed25519 signatures, which don't
	// do pre-hashing, it returns the concatenation of the slices.
	func hashForServerKeyExchange(sigType uint8, hashFunc crypto.Hash, version uint16, slices ...[]byte) []byte {
	if sigType == signatureEd25519 {
	var signed []byte
	for _, slice := range slices {
	signed = append(signed, slice...)
	}
	return signed
	}
	if version >= VersionTLS12 {
	h := hashFunc.New()
	for _, slice := range slices {
	h.Write(slice)
	}
	digest := h.Sum(nil)
	return digest
	}
	if sigType == signatureECDSA {
	return sha1Hash(slices)
	}
	return md5SHA1Hash(slices)
	}

	// ecdheKeyAgreement implements a TLS key agreement where the server
	// generates an ephemeral EC public/private key pair and signs it. The
	// pre-master secret is then calculated using ECDH. The signature may
	// be ECDSA, Ed25519 or RSA.
	type ecdheKeyAgreement struct {
	version uint16
	isRSA bool
	params ecdheParameters

	// ckx and preMasterSecret are generated in processServerKeyExchange
	// and returned in generateClientKeyExchange.
	ckx *clientKeyExchangeMsg
	preMasterSecret []byte
	}

	func (ka ecdheKeyAgreement) generateServerKeyExchange(config Config, cert Certificate, clientHello clientHelloMsg, hello serverHelloMsg) (serverKeyExchangeMsg, error) {
	var curveID CurveID
	for _, c := range clientHello.supportedCurves {
	if config.supportsCurve(c) {
	curveID = c
	break
	}
	}

	if curveID == 0 {
	return nil, errors.New("tls: no supported elliptic curves offered")
	}
	if _, ok := curveForCurveID(curveID); curveID != X25519 && !ok {
	return nil, errors.New("tls: CurvePreferences includes unsupported curve")
	}

	params, err := generateECDHEParameters(config.rand(), curveID)
	if err != nil {
	return nil, err
	}
	ka.params = params

	// See RFC 4492, Section 5.4.
	ecdhePublic := params.PublicKey()
	serverECDHEParams := make([]byte, 1+2+1+len(ecdhePublic))
	serverECDHEParams[0] = 3 // named curve
	serverECDHEParams[1] = byte(curveID >> 8)
	serverECDHEParams[2] = byte(curveID)
	serverECDHEParams[3] = byte(len(ecdhePublic))
	copy(serverECDHEParams[4:], ecdhePublic)

	priv, ok := cert.PrivateKey.(crypto.Signer)
	if !ok {
	return nil, fmt.Errorf("tls: certificate private key of type %T does not implement crypto.Signer", cert.PrivateKey)
	}

	var signatureAlgorithm SignatureScheme
	var sigType uint8
	var sigHash crypto.Hash
	if ka.version >= VersionTLS12 {
	signatureAlgorithm, err = selectSignatureScheme(ka.version, cert, clientHello.supportedSignatureAlgorithms)
	if err != nil {
	return nil, err
	}
	sigType, sigHash, err = typeAndHashFromSignatureScheme(signatureAlgorithm)
	if err != nil {
	return nil, err
	}
	} else {
	sigType, sigHash, err = legacyTypeAndHashFromPublicKey(priv.Public())
	if err != nil {
	return nil, err
	}
	}
	if (sigType == signaturePKCS1v15 \|\| sigType == signatureRSAPSS) != ka.isRSA {
	return nil, errors.New("tls: certificate cannot be used with the selected cipher suite")
	}

	signed := hashForServerKeyExchange(sigType, sigHash, ka.version, clientHello.random, hello.random, serverECDHEParams)

	signOpts := crypto.SignerOpts(sigHash)
	if sigType == signatureRSAPSS {
	signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: sigHash}
	}
	sig, err := priv.Sign(config.rand(), signed, signOpts)
	if err != nil {
	return nil, errors.New("tls: failed to sign ECDHE parameters: " + err.Error())
	}

	skx := new(serverKeyExchangeMsg)
	sigAndHashLen := 0
	if ka.version >= VersionTLS12 {
	sigAndHashLen = 2
	}
	skx.key = make([]byte, len(serverECDHEParams)+sigAndHashLen+2+len(sig))
	copy(skx.key, serverECDHEParams)
	k := skx.key[len(serverECDHEParams):]
	if ka.version >= VersionTLS12 {
	k[0] = byte(signatureAlgorithm >> 8)
	k[1] = byte(signatureAlgorithm)
	k = k[2:]
	}
	k[0] = byte(len(sig) >> 8)
	k[1] = byte(len(sig))
	copy(k[2:], sig)

	return skx, nil
	}

	func (ka ecdheKeyAgreement) processClientKeyExchange(config Config, cert Certificate, ckx clientKeyExchangeMsg, version uint16) ([]byte, error) {
	if len(ckx.ciphertext) == 0 \|\| int(ckx.ciphertext[0]) != len(ckx.ciphertext)-1 {
	return nil, errClientKeyExchange
	}

	preMasterSecret := ka.params.SharedKey(ckx.ciphertext[1:])
	if preMasterSecret == nil {
	return nil, errClientKeyExchange
	}

	return preMasterSecret, nil
	}

	func (ka ecdheKeyAgreement) processServerKeyExchange(config Config, clientHello clientHelloMsg, serverHello serverHelloMsg, cert x509.Certificate, skx serverKeyExchangeMsg) error {
	if len(skx.key) < 4 {
	return errServerKeyExchange
	}
	if skx.key[0] != 3 { // named curve
	return errors.New("tls: server selected unsupported curve")
	}
	curveID := CurveID(skx.key[1])<<8 \| CurveID(skx.key[2])

	publicLen := int(skx.key[3])
	if publicLen+4 > len(skx.key) {
	return errServerKeyExchange
	}
	serverECDHEParams := skx.key[:4+publicLen]
	publicKey := serverECDHEParams[4:]

	sig := skx.key[4+publicLen:]
	if len(sig) < 2 {
	return errServerKeyExchange
	}

	if _, ok := curveForCurveID(curveID); curveID != X25519 && !ok {
	return errors.New("tls: server selected unsupported curve")
	}

	params, err := generateECDHEParameters(config.rand(), curveID)
	if err != nil {
	return err
	}
	ka.params = params

	ka.preMasterSecret = params.SharedKey(publicKey)
	if ka.preMasterSecret == nil {
	return errServerKeyExchange
	}

	ourPublicKey := params.PublicKey()
	ka.ckx = new(clientKeyExchangeMsg)
	ka.ckx.ciphertext = make([]byte, 1+len(ourPublicKey))
	ka.ckx.ciphertext[0] = byte(len(ourPublicKey))
	copy(ka.ckx.ciphertext[1:], ourPublicKey)

	var sigType uint8
	var sigHash crypto.Hash
	if ka.version >= VersionTLS12 {
	signatureAlgorithm := SignatureScheme(sig[0])<<8 \| SignatureScheme(sig[1])
	sig = sig[2:]
	if len(sig) < 2 {
	return errServerKeyExchange
	}

	if !isSupportedSignatureAlgorithm(signatureAlgorithm, clientHello.supportedSignatureAlgorithms) {
	return errors.New("tls: certificate used with invalid signature algorithm")
	}
	sigType, sigHash, err = typeAndHashFromSignatureScheme(signatureAlgorithm)
	if err != nil {
	return err
	}
	} else {
	sigType, sigHash, err = legacyTypeAndHashFromPublicKey(cert.PublicKey)
	if err != nil {
	return err
	}
	}
	if (sigType == signaturePKCS1v15 \|\| sigType == signatureRSAPSS) != ka.isRSA {
	return errServerKeyExchange
	}

	sigLen := int(sig[0])<<8 \| int(sig[1])
	if sigLen+2 != len(sig) {
	return errServerKeyExchange
	}
	sig = sig[2:]

	signed := hashForServerKeyExchange(sigType, sigHash, ka.version, clientHello.random, serverHello.random, serverECDHEParams)
	if err := verifyHandshakeSignature(sigType, cert.PublicKey, sigHash, signed, sig); err != nil {
	return errors.New("tls: invalid signature by the server certificate: " + err.Error())
	}
	return nil
	}

	func (ka ecdheKeyAgreement) generateClientKeyExchange(config Config, clientHello clientHelloMsg, cert x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {
	if ka.ckx == nil {
	return nil, nil, errors.New("tls: missing ServerKeyExchange message")
	}

	return ka.preMasterSecret, ka.ckx, nil
	}