A 6- or 8-pin micro would be a good solution, but may be problematic depending on how tamper proof you need it to be. While most micros have some provision for securing the program memory, I'm not sure what your options are for preventing the EEPROM from being erased. Usually you're trying to prevent data from being read from memory, but want to be able to erase it at will. You have the opposite problem. I think on an ATTiny13, for example, if you set the lock bits to mode 3 and set the EESAVE fuse that might do the trick--but it might still be possible to unlock and erase the EEPROM via high voltage programming if you have a really determined tamperer.