github.com/hellobchain/newcryptosm@v0.0.0-20221019060107-edb949a317e9/tls/key_agreement.go

/*
Copyright Suzhou Tongji Fintech Research Institute 2017 All Rights Reserved.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package tls

import (
	"crypto"
	"crypto/elliptic"
	"crypto/md5"
	"crypto/rsa"
	"crypto/sha1"
	"encoding/asn1"
	"errors"
	"github.com/hellobchain/newcryptosm"
	"github.com/hellobchain/newcryptosm/ecdsa"
	"github.com/hellobchain/newcryptosm/sm2"
	"github.com/hellobchain/newcryptosm/sm3"
	"github.com/hellobchain/newcryptosm/x509"
	"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)
}

// sm3Hash calculates a sm3 hash over the given byte slices.
func sm3Hash(slices [][]byte) []byte {
	hsm3 := sm3.New()
	for _, slice := range slices {
		hsm3.Write(slice)
	}
	return hsm3.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
// 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, newcryptosm.Hash, error) {
	if version >= VersionTLS12 {
		if !isSupportedSignatureAndHash(sigAndHash, supportedSignatureAlgorithms) {
			return nil, newcryptosm.Hash(0), errors.New("tls: unsupported hash function used by peer")
		}
		hashFunc, err := lookupTLSHash(sigAndHash.hash)
		if err != nil {
			return nil, newcryptosm.Hash(0), err
		}
		h := hashFunc.New()
		for _, slice := range slices {
			h.Write(slice)
		}
		digest := h.Sum(nil)
		return digest, hashFunc, nil
	}
	if sigAndHash.signature == signatureECDSA {
		return sha1Hash(slices), newcryptosm.SHA1, nil
	}
	if sigAndHash.signature == signatureSM2 {
		return sm3Hash(slices), newcryptosm.SM3, nil
	}
	return md5SHA1Hash(slices), newcryptosm.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 []signatureAndHash) (uint8, 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
	}

	for _, sigAndHash := range clientList {
		if sigAndHash.signature != sigType {
			continue
		}
		if isSupportedSignatureAndHash(sigAndHash, supportedSignatureAlgorithms) {
			return sigAndHash.hash, nil
		}
	}

	return 0, errors.New("tls: client doesn't support any common hash functions")
}

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
	case CureP256SM2:
		return sm2.SM2(), true
	default:
		return nil, false
	}

}

// ecdheRSAKeyAgreement implements a TLS key agreement where the server
// generates a 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
	sigType    uint8
	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 sm2Public []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[:]
		sm2Public = 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
		}
		sm2Public = elliptic.Marshal(curve, x, y) //kG
	}

	// http://tools.ietf.org/html/rfc4492#section-5.4
	serverECDHParams := make([]byte, 1+2+1+len(sm2Public))
	serverECDHParams[0] = 3 // named curve
	serverECDHParams[1] = byte(ka.curveid >> 8)
	serverECDHParams[2] = byte(ka.curveid)
	serverECDHParams[3] = byte(len(sm2Public))
	copy(serverECDHParams[4:], sm2Public)

	sigAndHash := signatureAndHash{signature: ka.sigType}

	if ka.version >= VersionTLS12 {
		var err error
		if sigAndHash.hash, err = pickTLS12HashForSignature(ka.sigType, clientHello.signatureAndHashes); err != nil {
			return nil, err
		}
	}

	digest, hashFunc, err := hashForServerKeyExchange(sigAndHash, 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 signatureSM2:
		_, ok := priv.Public().(*ecdsa.PublicKey)
		if !ok {
			return nil, errors.New("tls: ECDHE SM2 requires an SM2 server key")
		}
	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)
	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] = sigAndHash.hash
		k[1] = sigAndHash.signature
		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:])
	if x == nil {
		return nil, errClientKeyExchange
	}
	if !curve.IsOnCurve(x, y) {
		return nil, errClientKeyExchange
	}
	x, _ = curve.ScalarMult(x, y, ka.privateKey)
	preMasterSecret := make([]byte, (curve.Params().BitSize+7)>>3)
	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)
		if ka.x == nil {
			return errServerKeyExchange
		}
		if !curve.IsOnCurve(ka.x, ka.y) {
			return errServerKeyExchange
		}
	}

	sigAndHash := signatureAndHash{signature: ka.sigType}
	if ka.version >= VersionTLS12 {
		// handle SignatureAndHashAlgorithm
		sigAndHash = signatureAndHash{hash: sig[0], signature: sig[1]}
		if sigAndHash.signature != ka.sigType {
			return errServerKeyExchange
		}
		sig = sig[2:]
		if len(sig) < 2 {
			return errServerKeyExchange
		}
	}
	sigLen := int(sig[0])<<8 | int(sig[1])
	if sigLen+2 != len(sig) {
		return errServerKeyExchange
	}
	sig = sig[2:]

	digest, hashFunc, err := hashForServerKeyExchange(sigAndHash, ka.version, clientHello.random, serverHello.random, serverECDHParams)
	if err != nil {
		return err
	}
	switch ka.sigType {
	case signatureSM2:
		pubKey, ok := cert.PublicKey.(*ecdsa.PublicKey)
		if !ok {
			return errors.New("tls: ECDHE SM2 requires a SM2 server public key")
		}
		sm2Sig := new(sm2Signature)
		if _, err := asn1.Unmarshal(sig, sm2Sig); err != nil {
			return err
		}
		if sm2Sig.R.Sign() <= 0 || sm2Sig.S.Sign() <= 0 {
			return errors.New("tls: SM2 signature contained zero or negative values")
		}
		if !ecdsa.Verify(&ecdsa.PublicKey{
			X:     pubKey.X,
			Y:     pubKey.Y,
			Curve: pubKey.Curve,
		}, digest, sm2Sig.R, sm2Sig.S) {
			return errors.New("tls: SM2 verification failure")
		}
	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, crypto.Hash(hashFunc), digest, sig); err != nil {
			return err
		}
	default:
		return errors.New("tls: unknown ECDHE signature algorithm")
	}

	return nil
}

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
}

// sm2KeyAgreement implements a TLS key agreement where the server
// generates a ephemeral SM2 public/private key pair and signs it. The
// pre-master secret is then calculated using SM2. The signature may
// be SM2.
type sm2KeyAgreement struct {
	version    uint16
	sigType    uint8
	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 *sm2KeyAgreement) 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 sm2Public []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[:]
		sm2Public = 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
		}
		sm2Public = elliptic.Marshal(curve, x, y) //kG
	}

	// http://tools.ietf.org/html/rfc4492#section-5.4
	serverECDHParams := make([]byte, 1+2+1+len(sm2Public))
	serverECDHParams[0] = 3 // named curve
	serverECDHParams[1] = byte(ka.curveid >> 8)
	serverECDHParams[2] = byte(ka.curveid)
	serverECDHParams[3] = byte(len(sm2Public))
	copy(serverECDHParams[4:], sm2Public)

	sigAndHash := signatureAndHash{signature: ka.sigType}

	if ka.version >= VersionTLS12 {
		var err error
		if sigAndHash.hash, err = pickTLS12HashForSignature(ka.sigType, clientHello.signatureAndHashes); err != nil {
			return nil, err
		}
	}

	digest, hashFunc, err := hashForServerKeyExchange(sigAndHash, 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 signatureSM2:
		_, ok := priv.Public().(*ecdsa.PublicKey)
		if !ok {
			return nil, errors.New("tls: ECDHE SM2 requires an SM2 server key")
		}
	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)
	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] = sigAndHash.hash
		k[1] = sigAndHash.signature
		k = k[2:]
	}
	k[0] = byte(len(sig) >> 8)
	k[1] = byte(len(sig))
	copy(k[2:], sig)

	return skx, nil
}

func (ka *sm2KeyAgreement) 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:])
	if x == nil {
		return nil, errClientKeyExchange
	}
	if !curve.IsOnCurve(x, y) {
		return nil, errClientKeyExchange
	}
	x, _ = curve.ScalarMult(x, y, ka.privateKey)
	preMasterSecret := make([]byte, (curve.Params().BitSize+7)>>3)
	xBytes := x.Bytes()
	copy(preMasterSecret[len(preMasterSecret)-len(xBytes):], xBytes)

	return preMasterSecret, nil
}

func (ka *sm2KeyAgreement) 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)
		if ka.x == nil {
			return errServerKeyExchange
		}
		if !curve.IsOnCurve(ka.x, ka.y) {
			return errServerKeyExchange
		}
	}

	sigAndHash := signatureAndHash{signature: ka.sigType}
	if ka.version >= VersionTLS12 {
		// handle SignatureAndHashAlgorithm
		sigAndHash = signatureAndHash{hash: sig[0], signature: sig[1]}
		if sigAndHash.signature != ka.sigType {
			return errServerKeyExchange
		}
		sig = sig[2:]
		if len(sig) < 2 {
			return errServerKeyExchange
		}
	}
	sigLen := int(sig[0])<<8 | int(sig[1])
	if sigLen+2 != len(sig) {
		return errServerKeyExchange
	}
	sig = sig[2:]

	digest, hashFunc, err := hashForServerKeyExchange(sigAndHash, ka.version, clientHello.random, serverHello.random, serverECDHParams)
	if err != nil {
		return err
	}
	switch ka.sigType {
	case signatureSM2:
		pubKey, ok := cert.PublicKey.(*ecdsa.PublicKey)
		if !ok {
			return errors.New("tls: ECDHE SM2 requires a SM2 server public key")
		}
		sm2Sig := new(sm2Signature)
		if _, err := asn1.Unmarshal(sig, sm2Sig); err != nil {
			return err
		}
		if sm2Sig.R.Sign() <= 0 || sm2Sig.S.Sign() <= 0 {
			return errors.New("tls: SM2 signature contained zero or negative values")
		}
		if !ecdsa.Verify(&ecdsa.PublicKey{
			X:     pubKey.X,
			Y:     pubKey.Y,
			Curve: pubKey.Curve,
		}, digest, sm2Sig.R, sm2Sig.S) {
			return errors.New("tls: SM2 verification failure")
		}
	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, crypto.Hash(hashFunc), digest, sig); err != nil {
			return err
		}
	default:
		return errors.New("tls: unknown ECDHE signature algorithm")
	}

	return nil
}

func (ka *sm2KeyAgreement) 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
}