Help with reading and storing data from sim module

I'm using a sim800C module and an Arduino Uno. I want the code to send back an SMS to a number which sends it the password. So, if the sim module receives 'abcd' I want it to send back an SMS, saying 'ok'. Where I'm stuck is actually storing the read SMS. Since I need to check the SMS with a pre-defined password, I need to store the SMS the module received somehow. Please help ? Here's the code so far:

#include <SoftwareSerial.h>
SoftwareSerial sim(10, 11);
int _timeout;
String _buffer;
String number = "+000000000000"; //-> change with your number
String incoming;

void setup() {
  //delay(7000); //delay for 7 seconds to make sure the modules get the signal
  Serial.begin(9600);
  _buffer.reserve(50);
  Serial.println("System Started...");
  sim.begin(9600);
  delay(1000);
  Serial.println("Type s to send an SMS, r to receive an SMS, and c to make a call");
}

void loop() {
  char inChar[5];
  if (Serial.available() > 0)
    switch (Serial.read())
    {
      case 's':
        SendMessage();
        break;
      case 'r':
        RecieveMessage();
        break;
    }
// this is where the sim module spits back the received SMS to the serial monitor. How do I store it ?
  if (sim.available() > 0) {
    Serial.write(sim.read());
  
  }
}

void SendMessage()
{
  //Serial.println ("Sending Message");
  sim.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);
  //Serial.println ("Set SMS Number");
  sim.println("AT+CMGS=\"" + number + "\"\r"); //Mobile phone number to send message
  delay(1000);
  String SMS = "GPS-cordinates";
  sim.println(SMS);
  delay(100);
  sim.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
  _buffer = _readSerial();
}

void RecieveMessage()
{
  Serial.println ("SIM800C Read an SMS");
  delay (1000);
  sim.println("AT+CNMI=2,2,0,0,0"); // AT Command to receive a live SMS
  delay(1000);
  Serial.write ("Unread Message done");
}

String _readSerial() {
  _timeout = 0;
  while  (!sim.available() && _timeout < 12000  )
  {
    delay(13);
    _timeout++;
  }
  if (sim.available()) {
    return sim.readString();
  }
}