This video shows how to make an iOS device regenerate an APNonce using a generator and shsh2 files.
You need to have a jailbreak with task_for_pid_0 enabled (or a workaround like host_get_special_port) to be able to patch the kernel with nonceEnabler.
After applying the kernelpatch you can write a generator to nvram, which will be used to generate a APNonce on next reboot.
Tutorial how to (down/up)grade using prometheus method:
https://www.youtube.com/watch?v=BIMx2Y13Ukc
futurerestore: (downloadlink can be
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