Minus 0x20000 with Back Extend Erase

From The Apple Wiki

This is the exploit used to unlock all phones with a 4.6 bootloader.




Writing -0x20000 before the firmware starts allows you to write anything you want.

0x3C0000 can't have a valid secpack to allow booting. The explicit addresses 0xA03D0000-0xA03F0000 will always erase. This exploit relied on two things, the secaddrs are copied before the secpack is validated(stupid), and the erase command extends the range to whatever is in the secpack. So erase 0xA03D0000-0xA03F0000, the erase command sees 0xA03C0000 to 0xA03F0000 in the secpack; BOOM secpack erased.