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
}
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)
}
// http://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)
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 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
}