The payload of the RPKG is compressed. Nokia utilized various
: This is an archive format used specifically by EKA2L1 to package the contents of the device's Z: drive (the read-only system drive). nokia 5320 image rom rpkg
When you download a stock firmware for the Nokia 5320, you are downloading an Image ROM package . Without a valid image, your phone is a brick. The payload of the RPKG is compressed