|24 March 2023
The SEP is a coprocessor running SEPOS that was introduced starting with the A7 system on a chip, which gets updated on each iOS update, and often changes between iOS updates. The firmware file for said OS in called the
sep-firmware.(boardconfig without ap part).RELEASE(or factory).im4p.
The problem is that only signed SEP firmware is able to be loaded on devices that require it. When a firmware gets unsigned, the SEP also gets unsigned, meaning it cannot be installed on a device.
To make matters worse, the SEP changes from update to update, and said changes often cause incompatibilities with previous versions of iOS. Said incompatibility can range from the device not restoring at all, to the device restoring but Touch ID not functioning, to activation issues. It's worth noting that dualboots tend to have less stringent SEP compatibility, due to the installation method.