deriveSymKey method 
    
    
    
  Implementation
  @override
Future<String> deriveSymKey(String privKeyA, String pubKeyB) async {
  final Uint8List sharedKey1 = x25519.X25519(
    hex.decode(privKeyA),
    hex.decode(pubKeyB),
  );
  Uint8List out = Uint8List(KEY_LENGTH);
  final HKDFKeyDerivator hkdf = HKDFKeyDerivator(SHA256Digest());
  final HkdfParameters params = HkdfParameters(sharedKey1, KEY_LENGTH);
  hkdf.init(params);
  // final pc.KeyParameter keyParam = hkdf.extract(null, sharedKey1);
  hkdf.deriveKey(null, 0, out, 0);
  return hex.encode(out);
}