Granting access with RFID card - c

Question

I've got a problem with my arduino program. I'm using a RFID reader to scan cards (this part works flawlessly). I want the program to grant access to using some buttons (doesn't matter to be honest). The user who uses a verified card can use the functionalities until he scans a card again. I've deleted some unnecessary parts of the code.
#include <SPI.h>
#include <MFRC522.h>
String read_rfid;
String ok_rfid_1="a3f90f7"; //ID of the verified CARD
const int buzzer=8; //Buzzer
const int redLed=7;
const int greenLed=6;
const int yellowLed=5;
byte access=false; //
void setup() {
Serial.begin(9600); // Initialize serial communications with the PC
SPI.begin(); // Init SPI bus
mfrc522.PCD_Init(); // Init MFRC522 card
}
/*
* Helper routine to dump a byte array as hex values to Serial.
*/
void dump_byte_array(byte *buffer, byte bufferSize) {
read_rfid="";
for (byte i = 0; i < bufferSize; i++) {
read_rfid=read_rfid + String(buffer[i], HEX);
}
}
void granted() { } //it lights up the green led
void denied() { } //it lights up the red led
void login() {
if (read_rfid==ok_rfid_1) {
granted();
access=true;
} else {
denied();
}
delay(1000);
}
void logout() {
if (read_rfid==ok_rfid_1 && access==1) {
access=false;
Serial.println("Logout ");
}
}
void loop() {
// Look for new cards
if ( ! mfrc522.PICC_IsNewCardPresent())
return;
// Select one of the cards
if ( ! mfrc522.PICC_ReadCardSerial())
return;
dump_byte_array(mfrc522.uid.uidByte, mfrc522.uid.size);
Serial.println(read_rfid);
login();
while(access==true)
{logout();};
}
The logging in part works well, but it logs out automatically. As I observed the problem might be that if(read_rfid==ok_rfid_1 && access==1) is always true, because the read_rfid does not change. Do you have any ideas to solve my dillema?