Loop implementation only after getting serial value

Projects Programming
1

I am working with Node A [Matlab+Xbee (S1)] in one side and Node B [Arduino UNO+Xbee(S1)] on another side. I am trying to send one message [message 1] from Node A to Node B, Node B would get the message1, read it and send another message [message 2] to Node A.
According to my code below, Node B stops sending the message after once, but I want it to continuously send Node A the message. If I separate receiving and sending portion, the problem occurs is as temperature sensor is always reading data, even whenever Node B is not getting any data (in serial port), it is sending the temperature reading encrypted with previous stored key. But I want that no data would be sent from Node B unless it gets a data from Node A at first. Then Node B would start sending data iteratively to Node A. What I should change in the code below to achieve this?

Code:

char key  = 'QD98750';

void loop()                     
{
 while (Serial.available ( ) ) {
 char c = Serial.read ();
 readString += c;
 }
 if (readString.length() >0) {
     char ssid[3] ;
     readString.toCharArray (ssid, 3) ;
     aes_decryption(key, ssid);%the result is aes_value
     key = aes_value;
     readString ="" ;
     %temperature sensor reading
     int reading = analogRead(sensorPin);
     float voltage = reading * 5.0;
     voltage /= 1024.0;
     float temperatureC = (voltage - 0.5) * 100 ;
     aes_encryption(key, temperatureC);%the output is aes_value;
     Serial.print (aes_value) ;
     }
}
2 
char key  = 'QD98750';

Which ONE key did you press to get that ONE character between the single quotes?

 if (readString.length() >0) {
     char ssid[3] ;
     readString.toCharArray (ssid, 3) ;

As soon as readString (you really don't need to use the String class for this!) contains one character, extract three as ssid. Do you really think that is going to work?

     aes_decryption(key, ssid);%the result is aes_value

Does this function really expect a single character as the key? Does it really expect a character with an impossible value?

     %temperature sensor reading

Posting code that actually compiles is generally preferred, unless the reason for the post is to understand why the error exists. This one is pretty obvious!

     aes_encryption(key, temperatureC);%the output is aes_value;

Does this function really expect a single character as the key? Does it really expect a character with an impossible value?

Is there some reason that the temperature is super-secret? Is there someone that is snooping on your XBee traffic? Is there some way that someone could benefit from stealing your data?