Method generateServerKeyExchange

gmtls/key_agreement.go:173–270

(config *Config, signCert, cipherCert *Certificate,
	clientHello *clientHelloMsg, hello *serverHelloMsg)

}

func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, signCert, cipherCert *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 := signCert.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)