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/elliptic"
 	"crypto/md5"
 	"crypto/rsa"
 	"crypto/sha1"
 	"crypto/x509"
 	"errors"
 	"io"
 	"math/big"

 	"golang_org/x/crypto/curve25519"
 )

 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
 	}

 	ciphertext := ckx.ciphertext
 	if version != VersionSSL30 {
 		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).
 func hashForServerKeyExchange(sigType uint8, hashFunc crypto.Hash, version uint16, slices ...[]byte) ([]byte, error) {
 	if version >= VersionTLS12 {
 		h := hashFunc.New()
 		for _, slice := range slices {
 			h.Write(slice)
 		}
 		digest := h.Sum(nil)
 		return digest, nil
 	}
 	if sigType == signatureECDSA {
 		return sha1Hash(slices), nil
 	}
 	return md5SHA1Hash(slices), nil
 }

 func curveForCurveID(id CurveID) (elliptic.Curve, bool) {
 	switch id {
 	case CurveP256:
 		return elliptic.P256(), true
 	case CurveP384:
 		return elliptic.P384(), true
 	case CurveP521:
 		return elliptic.P521(), true
 	default:
 		return nil, false
 	}

 }

 // 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
 // either be ECDSA or RSA.
 type ecdheKeyAgreement struct {
 	version    uint16
 	isRSA      bool
 	privateKey []byte
 	curveid    CurveID

 	// publicKey is used to store the peer's public value when X25519 is
 	// being used.
 	publicKey []byte
 	// x and y are used to store the peer's public value when one of the
 	// NIST curves is being used.
 	x, y *big.Int
 }

 func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
 	preferredCurves := config.curvePreferences()

 NextCandidate:
 	for _, candidate := range preferredCurves {
 		for _, c := range clientHello.supportedCurves {
 			if candidate == c {
 				ka.curveid = c
 				break NextCandidate
 			}
 		}
 	}

 	if ka.curveid == 0 {
 		return nil, errors.New("tls: no supported elliptic curves offered")
 	}

 	var ecdhePublic []byte

 	if ka.curveid == X25519 {
 		var scalar, public [32]byte
 		if _, err := io.ReadFull(config.rand(), scalar[:]); err != nil {
 			return nil, err
 		}

 		curve25519.ScalarBaseMult(&public, &scalar)
 		ka.privateKey = scalar[:]
 		ecdhePublic = public[:]
 	} else {
 		curve, ok := curveForCurveID(ka.curveid)
 		if !ok {
 			return nil, errors.New("tls: preferredCurves includes unsupported curve")
 		}

 		var x, y *big.Int
 		var err error
 		ka.privateKey, x, y, err = elliptic.GenerateKey(curve, config.rand())
 		if err != nil {
 			return nil, err
 		}
 		ecdhePublic = elliptic.Marshal(curve, x, y)
 	}

 	// https://tools.ietf.org/html/rfc4492#section-5.4
 	serverECDHParams := make([]byte, 1+2+1+len(ecdhePublic))
 	serverECDHParams[0] = 3 // named curve
 	serverECDHParams[1] = byte(ka.curveid >> 8)
 	serverECDHParams[2] = byte(ka.curveid)
 	serverECDHParams[3] = byte(len(ecdhePublic))
 	copy(serverECDHParams[4:], ecdhePublic)

 	priv, ok := cert.PrivateKey.(crypto.Signer)
 	if !ok {
 		return nil, errors.New("tls: certificate private key does not implement crypto.Signer")
 	}

 	signatureAlgorithm, sigType, hashFunc, err := pickSignatureAlgorithm(priv.Public(), clientHello.supportedSignatureAlgorithms, supportedSignatureAlgorithms(), ka.version)
 	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")
 	}

 	digest, err := hashForServerKeyExchange(sigType, hashFunc, ka.version, clientHello.random, hello.random, serverECDHParams)
 	if err != nil {
 		return nil, err
 	}

 	signOpts := crypto.SignerOpts(hashFunc)
 	if sigType == signatureRSAPSS {
 		signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: hashFunc}
 	}
 	sig, err := priv.Sign(config.rand(), digest, 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(serverECDHParams)+sigAndHashLen+2+len(sig))
 	copy(skx.key, serverECDHParams)
 	k := skx.key[len(serverECDHParams):]
 	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
 	}

 	if ka.curveid == X25519 {
 		if len(ckx.ciphertext) != 1+32 {
 			return nil, errClientKeyExchange
 		}

 		var theirPublic, sharedKey, scalar [32]byte
 		copy(theirPublic[:], ckx.ciphertext[1:])
 		copy(scalar[:], ka.privateKey)
 		curve25519.ScalarMult(&sharedKey, &scalar, &theirPublic)
 		return sharedKey[:], nil
 	}

 	curve, ok := curveForCurveID(ka.curveid)
 	if !ok {
 		panic("internal error")
 	}
 	x, y := elliptic.Unmarshal(curve, ckx.ciphertext[1:]) // Unmarshal also checks whether the given point is on the curve
 	if x == nil {
 		return nil, errClientKeyExchange
 	}
 	x, _ = curve.ScalarMult(x, y, ka.privateKey)
 	preMasterSecret := make([]byte, (curve.Params().BitSize+7)>>3)
 	xBytes := x.Bytes()
 	copy(preMasterSecret[len(preMasterSecret)-len(xBytes):], xBytes)

 	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")
 	}
 	ka.curveid = CurveID(skx.key[1])<<8 | CurveID(skx.key[2])

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

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

 	if ka.curveid == X25519 {
 		if len(publicKey) != 32 {
 			return errors.New("tls: bad X25519 public value")
 		}
 		ka.publicKey = publicKey
 	} else {
 		curve, ok := curveForCurveID(ka.curveid)
 		if !ok {
 			return errors.New("tls: server selected unsupported curve")
 		}
 		ka.x, ka.y = elliptic.Unmarshal(curve, publicKey) // Unmarshal also checks whether the given point is on the curve
 		if ka.x == nil {
 			return errServerKeyExchange
 		}
 	}

 	var signatureAlgorithm SignatureScheme
 	if ka.version >= VersionTLS12 {
 		// handle SignatureAndHashAlgorithm
 		signatureAlgorithm = SignatureScheme(sig[0])<<8 | SignatureScheme(sig[1])
 		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 == 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:]

 	digest, err := hashForServerKeyExchange(sigType, hashFunc, ka.version, clientHello.random, serverHello.random, serverECDHParams)
 	if err != nil {
 		return err
 	}
 	return verifyHandshakeSignature(sigType, cert.PublicKey, hashFunc, digest, sig)
 }

 func (ka *ecdheKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, cert *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {
 	if ka.curveid == 0 {
 		return nil, nil, errors.New("tls: missing ServerKeyExchange message")
 	}

 	var serialized, preMasterSecret []byte

 	if ka.curveid == X25519 {
 		var ourPublic, theirPublic, sharedKey, scalar [32]byte

 		if _, err := io.ReadFull(config.rand(), scalar[:]); err != nil {
 			return nil, nil, err
 		}

 		copy(theirPublic[:], ka.publicKey)
 		curve25519.ScalarBaseMult(&ourPublic, &scalar)
 		curve25519.ScalarMult(&sharedKey, &scalar, &theirPublic)
 		serialized = ourPublic[:]
 		preMasterSecret = sharedKey[:]
 	} else {
 		curve, ok := curveForCurveID(ka.curveid)
 		if !ok {
 			panic("internal error")
 		}
 		priv, mx, my, err := elliptic.GenerateKey(curve, config.rand())
 		if err != nil {
 			return nil, nil, err
 		}
 		x, _ := curve.ScalarMult(ka.x, ka.y, priv)
 		preMasterSecret = make([]byte, (curve.Params().BitSize+7)>>3)
 		xBytes := x.Bytes()
 		copy(preMasterSecret[len(preMasterSecret)-len(xBytes):], xBytes)

 		serialized = elliptic.Marshal(curve, mx, my)
 	}

 	ckx := new(clientKeyExchangeMsg)
 	ckx.ciphertext = make([]byte, 1+len(serialized))
 	ckx.ciphertext[0] = byte(len(serialized))
 	copy(ckx.ciphertext[1:], serialized)

 	return preMasterSecret, 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/elliptic"
	"crypto/md5"
	"crypto/rsa"
	"crypto/sha1"
	"crypto/x509"
	"errors"
	"io"
	"math/big"

	"golang_org/x/crypto/curve25519"
	)

	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
	}

	ciphertext := ckx.ciphertext
	if version != VersionSSL30 {
	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).
	func hashForServerKeyExchange(sigType uint8, hashFunc crypto.Hash, version uint16, slices ...[]byte) ([]byte, error) {
	if version >= VersionTLS12 {
	h := hashFunc.New()
	for _, slice := range slices {
	h.Write(slice)
	}
	digest := h.Sum(nil)
	return digest, nil
	}
	if sigType == signatureECDSA {
	return sha1Hash(slices), nil
	}
	return md5SHA1Hash(slices), nil
	}

	func curveForCurveID(id CurveID) (elliptic.Curve, bool) {
	switch id {
	case CurveP256:
	return elliptic.P256(), true
	case CurveP384:
	return elliptic.P384(), true
	case CurveP521:
	return elliptic.P521(), true
	default:
	return nil, false
	}

	}

	// 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
	// either be ECDSA or RSA.
	type ecdheKeyAgreement struct {
	version uint16
	isRSA bool
	privateKey []byte
	curveid CurveID

	// publicKey is used to store the peer's public value when X25519 is
	// being used.
	publicKey []byte
	// x and y are used to store the peer's public value when one of the
	// NIST curves is being used.
	x, y *big.Int
	}

	func (ka ecdheKeyAgreement) generateServerKeyExchange(config Config, cert Certificate, clientHello clientHelloMsg, hello serverHelloMsg) (serverKeyExchangeMsg, error) {
	preferredCurves := config.curvePreferences()

	NextCandidate:
	for _, candidate := range preferredCurves {
	for _, c := range clientHello.supportedCurves {
	if candidate == c {
	ka.curveid = c
	break NextCandidate
	}
	}
	}

	if ka.curveid == 0 {
	return nil, errors.New("tls: no supported elliptic curves offered")
	}

	var ecdhePublic []byte

	if ka.curveid == X25519 {
	var scalar, public [32]byte
	if _, err := io.ReadFull(config.rand(), scalar[:]); err != nil {
	return nil, err
	}

	curve25519.ScalarBaseMult(&public, &scalar)
	ka.privateKey = scalar[:]
	ecdhePublic = public[:]
	} else {
	curve, ok := curveForCurveID(ka.curveid)
	if !ok {
	return nil, errors.New("tls: preferredCurves includes unsupported curve")
	}

	var x, y *big.Int
	var err error
	ka.privateKey, x, y, err = elliptic.GenerateKey(curve, config.rand())
	if err != nil {
	return nil, err
	}
	ecdhePublic = elliptic.Marshal(curve, x, y)
	}

	// https://tools.ietf.org/html/rfc4492#section-5.4
	serverECDHParams := make([]byte, 1+2+1+len(ecdhePublic))
	serverECDHParams[0] = 3 // named curve
	serverECDHParams[1] = byte(ka.curveid >> 8)
	serverECDHParams[2] = byte(ka.curveid)
	serverECDHParams[3] = byte(len(ecdhePublic))
	copy(serverECDHParams[4:], ecdhePublic)

	priv, ok := cert.PrivateKey.(crypto.Signer)
	if !ok {
	return nil, errors.New("tls: certificate private key does not implement crypto.Signer")
	}

	signatureAlgorithm, sigType, hashFunc, err := pickSignatureAlgorithm(priv.Public(), clientHello.supportedSignatureAlgorithms, supportedSignatureAlgorithms(), ka.version)
	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")
	}

	digest, err := hashForServerKeyExchange(sigType, hashFunc, ka.version, clientHello.random, hello.random, serverECDHParams)
	if err != nil {
	return nil, err
	}

	signOpts := crypto.SignerOpts(hashFunc)
	if sigType == signatureRSAPSS {
	signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: hashFunc}
	}
	sig, err := priv.Sign(config.rand(), digest, 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(serverECDHParams)+sigAndHashLen+2+len(sig))
	copy(skx.key, serverECDHParams)
	k := skx.key[len(serverECDHParams):]
	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
	}

	if ka.curveid == X25519 {
	if len(ckx.ciphertext) != 1+32 {
	return nil, errClientKeyExchange
	}

	var theirPublic, sharedKey, scalar [32]byte
	copy(theirPublic[:], ckx.ciphertext[1:])
	copy(scalar[:], ka.privateKey)
	curve25519.ScalarMult(&sharedKey, &scalar, &theirPublic)
	return sharedKey[:], nil
	}

	curve, ok := curveForCurveID(ka.curveid)
	if !ok {
	panic("internal error")
	}
	x, y := elliptic.Unmarshal(curve, ckx.ciphertext[1:]) // Unmarshal also checks whether the given point is on the curve
	if x == nil {
	return nil, errClientKeyExchange
	}
	x, _ = curve.ScalarMult(x, y, ka.privateKey)
	preMasterSecret := make([]byte, (curve.Params().BitSize+7)>>3)
	xBytes := x.Bytes()
	copy(preMasterSecret[len(preMasterSecret)-len(xBytes):], xBytes)

	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")
	}
	ka.curveid = CurveID(skx.key[1])<<8 \| CurveID(skx.key[2])

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

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

	if ka.curveid == X25519 {
	if len(publicKey) != 32 {
	return errors.New("tls: bad X25519 public value")
	}
	ka.publicKey = publicKey
	} else {
	curve, ok := curveForCurveID(ka.curveid)
	if !ok {
	return errors.New("tls: server selected unsupported curve")
	}
	ka.x, ka.y = elliptic.Unmarshal(curve, publicKey) // Unmarshal also checks whether the given point is on the curve
	if ka.x == nil {
	return errServerKeyExchange
	}
	}

	var signatureAlgorithm SignatureScheme
	if ka.version >= VersionTLS12 {
	// handle SignatureAndHashAlgorithm
	signatureAlgorithm = SignatureScheme(sig[0])<<8 \| SignatureScheme(sig[1])
	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 == 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:]

	digest, err := hashForServerKeyExchange(sigType, hashFunc, ka.version, clientHello.random, serverHello.random, serverECDHParams)
	if err != nil {
	return err
	}
	return verifyHandshakeSignature(sigType, cert.PublicKey, hashFunc, digest, sig)
	}

	func (ka ecdheKeyAgreement) generateClientKeyExchange(config Config, clientHello clientHelloMsg, cert x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {
	if ka.curveid == 0 {
	return nil, nil, errors.New("tls: missing ServerKeyExchange message")
	}

	var serialized, preMasterSecret []byte

	if ka.curveid == X25519 {
	var ourPublic, theirPublic, sharedKey, scalar [32]byte

	if _, err := io.ReadFull(config.rand(), scalar[:]); err != nil {
	return nil, nil, err
	}

	copy(theirPublic[:], ka.publicKey)
	curve25519.ScalarBaseMult(&ourPublic, &scalar)
	curve25519.ScalarMult(&sharedKey, &scalar, &theirPublic)
	serialized = ourPublic[:]
	preMasterSecret = sharedKey[:]
	} else {
	curve, ok := curveForCurveID(ka.curveid)
	if !ok {
	panic("internal error")
	}
	priv, mx, my, err := elliptic.GenerateKey(curve, config.rand())
	if err != nil {
	return nil, nil, err
	}
	x, _ := curve.ScalarMult(ka.x, ka.y, priv)
	preMasterSecret = make([]byte, (curve.Params().BitSize+7)>>3)
	xBytes := x.Bytes()
	copy(preMasterSecret[len(preMasterSecret)-len(xBytes):], xBytes)

	serialized = elliptic.Marshal(curve, mx, my)
	}

	ckx := new(clientKeyExchangeMsg)
	ckx.ciphertext = make([]byte, 1+len(serialized))
	ckx.ciphertext[0] = byte(len(serialized))
	copy(ckx.ciphertext[1:], serialized)

	return preMasterSecret, ckx, nil
	}