Setting up cryptographic key between Arduino and React Native app - react-native

Question

I am new to Arduino and working on a project to pass cryptographic keys between Arduino (ESP8266) and a React Native app. On the ESP8266 side I am using arduino crypto library and for React Native I am using react-native-crypto-js. The encryption doesn't seem to do the right thing and returns garbage. So, as a first step I was trying to pass the key from Arduino into the mobile app (using the bluetooth HC-05). The following code was used in the arduino side:
void getConnected() {
// wait till someone try to connect to the user
if (btSerial.available() > 0) {
String message = btSerial.readString();
if(message == "Hello") {
Serial.println("user trying to connect");
byte key[32];
device.getKey(key);
Serial.println((char*)key);
btSerial.write((char*)key);
}
}
}
The key is generated using RNG.rand(key, sizeof(key)) and I also printed the generated bytes separately and it is something like:
137 224 186 115 0 0 0 0 172 228 254 63 131 53 32 64 208 218 255 63 13 0 0 0 172 228 254 63 48 70 32 64
As you see above, since there are 0s in the bytes, the code in the React Native app is only getting the first 4 bytes, and the remaining is ignored as it is thinking the 5th byte to be the end of string.
The code used in the App is as below:
async setup(deviceId) {
console.log('connecting');
await BluetoothSerial.connect(deviceId);
console.log('connected');
await BluetoothSerial.write('Hello');
setTimeout(async () => {
let key = await BluetoothSerial.readFromDevice();
console.log(key);
}, 3000);
// const input = await BluetoothSerial.readFromDevice();
return true;
}
I would really appreciate some pointers. Please help.
Thanks