Small Arduino code snippet to save/read EPS8266 or ESP32 WIFI password and SSID from/to EEPROM.

read_write_wifi_credentials_to_eprom.ino

#include <EEPROM.h>

int eeprom_size = 512; // depends on microcontroller
int max_ssid_size = 32; 
int max_psk_size = 63;

void setup()
{
  EEPROM.begin(eeprom_size);
  Serial.begin(115200);
  Serial.println("");

  eraseEEPROM();
  storePSK("test_password_test_1231231231233test_password_test_1231231231mj");
  storeSSID("test_netowrk_name1234567890qwert");

  Serial.println("Start:");
  Serial.print(readPSK());
  Serial.print(",");
  Serial.println(readSSID());
}

void loop()
{

}

void storePSK(String router_PSK) {
  if (router_PSK.length() > max_psk_size) {
    Serial.println("PSK too long!");
    return;
  }
  for (int i = 0; i < router_PSK.length(); i++)
  {
    EEPROM.write(max_ssid_size + i, router_PSK[i]);
  }

  EEPROM.commit();
}

void storeSSID(String router_SSID) {
  if (router_SSID.length() > max_ssid_size) {
    Serial.println("SSID too long!");
    return;
  }
  for (int i = 0; i < router_SSID.length(); i++)
  {
    EEPROM.write(0 + i, router_SSID[i]);
  }

  EEPROM.commit();
}

String readPSK() {
  String router_PSK;
  for (int i = 0; i < max_psk_size; i++)
  {
    router_PSK = router_PSK + char(EEPROM.read(max_ssid_size + i));
  }

  return router_PSK;
}

String readSSID() {
  String router_SSID;
  for (int i = 0; i < max_ssid_size; i++)
  {
    router_SSID = router_SSID + char(EEPROM.read(0 + i));
  }

  return router_SSID;
}

void eraseEEPROM() {
  EEPROM.begin(eeprom_size);
  for (int i = 0 ; i < eeprom_size ; i++) {
    EEPROM.write(i, 0);
  }
  delay(200);
  EEPROM.commit();
}