Arduino to PC secure bluetooth connection(cryptography) - security

Question

I'm using an Arduino Uno to build a smoke detection system, which works. Since I have to do an IoT project, I have to establish a secure connection with a device (I thought with my smartphone, or my PC), and I'm using a Bluetooth module HC-05 to do it. The idea is:
Send the smoke sensor data to Arduino IDE, encrypt them and display the encrypted data to the serial (and it works)
Connect Arduino to my smartphone using HC-05 and the app "Makerslab BT Demo" (already done)
Decrypt the value of the sensor when I press "1" on the app and display it;
Decrypt the value of the sensor when there's danger and display a "danger message".
(that's what I have to do now).
That's my code on Arduino IDE:
#include <AES.h>
#include <AESLib.h>
#include <AES_config.h>
#include <xbase64.h>
#include <SoftwareSerial.h>
SoftwareSerial BT(1,0);
#define VCC2 5
int smokeA0 = A0;
int buzzer = 11;
AES aes;
byte cipher[400];
char b64[400];
float sensorValue;
//char a;
void do_encrypt(String msg, String key_str, String iv_str){
byte iv[16];
memcpy(iv,(byte*)iv_str.c_str(),16);
int blen=base64_encode(b64,(char*)msg.c_str(),msg.length());
aes.calc_size_n_pad(blen);
int len=aes.get_size(); //zero padding
byte plain_p[len];
for(int i=0;i<blen;++i) plain_p[i]=b64[i];
for(int i=blen;i<len;++i) plain_p[i]='\0';
// l'AES-128-CBC encryption
int blocks = len/16;
aes.set_key((byte *)key_str.c_str(), 16);
aes.cbc_encrypt(plain_p, cipher, blocks, iv);
// use base64 encoder to encode the encrypted data:
base64_encode(b64,(char *)cipher,len);
Serial.println(String((char *)b64));
}
void setup() {
pinMode(buzzer, OUTPUT);
pinMode(smokeA0, INPUT);
pinMode(VCC2, OUTPUT);
digitalWrite(VCC2, HIGH);
BT.begin(9600);
BT.println(F("Hi! Press "1" to know the sensor value."));
Serial.begin(115200);
Serial.println(F("gas sensor is warming up!"));
delay(2000); //allow the sensor to warm up
noTone(buzzer);
}
void loop() {
String key_str="aaaaaaaaaaaaaaaa"; //16 bytes
String iv_str="aaaaaaaaaaaaaaaa"; //16 bytes
sensorValue = analogRead(smokeA0);
String msg = String(sensorValue, 3);
do_encrypt(msg,key_str,iv_str);
/* if(BT.available()){
a=(BT.read());
if(a=='1'){
Serial.print(F("Air quality: "));
}
}*/
if(sensorValue > 300){
Serial.print(F(" | Danger!"));
BT.print(F(" | Danger!"));
tone(buzzer,1000,2000);
}
else {
noTone(buzzer);
}
Serial.println(F(""));
delay(2000);
}
I'm not sure to use the right security protocol (AES), but encryption works. How I can decrypt data?