Readout Firmware with Bossac

I lost the source of a important project of mine. My arduino Due is still programmed with the last status of this project but it is not finished yet. Is it possible to Readout the Firmware and decomplie it to a state where I can at least make some changes? I already tried to use bossac to readout the firmware and it says that 524288 bytes were read from flash but if I open the .bin file it only consist of 65536 chars of one symbol. Do I need a JTAG debugger to download the firmware?