…
  // ...

  // retrieve the simple key characteristic
  BLECharacteristic simpleKeyCharacteristic = peripheral.characteristic("ffe1");

  // subscribe to the simple key characteristic
  Serial.println("Subscribing to simple key characteristic ...");
  if (!simpleKeyCharacteristic) {
    Serial.println("no simple key characteristic found!");
    peripheral.disconnect();
    return;
  } else if (!simpleKeyCharacteristic.canSubscribe()) {
    Serial.println("simple key characteristic is not subscribable!");
    peripheral.disconnect();
    return;
  } else if (!simpleKeyCharacteristic.subscribe()) {
    Serial.println("subscription failed!");
    peripheral.disconnect();
    return;
  }

  // ...

  simpleKeyCharacteristic.unsubscribe();
…