GeekGrade

Geekgrade Downgrade is currently the only way to downgrade idevices without SHSH blobs. It only works on devices with the S5L8930 and S5L8920 chip. It is a tethered downgrade.

Exploits used

Geekgrade Downgrade uses many exploits together to create a tethered downgrade without SHSH blobs.They happen in this order:

1.Limera1n- the bootrom exploit that allows pwned dfu mode. Pwned dfu mode puts the device in a state where custom firmware files can be falsely checked as legit when itunes restores to a custom firmware

2.Custom firmware- This only works because iTunes originally only asked the idevice if the firmware was legitimitley from apple. Apple firmware has specific img3 hashes, and itunes asks the idevice if it will accept them. Well, pwned dfu just tells it yes

3.iTunes 11.0-All itunes versions below 11.1 can be exploited with pwned dfu to restore to a custom firmware. This is because itunes previously only asked the idevice if the hashes were correct. All versions above 11.0.5 will recalculate the hash and check it again before restoring.

4.Limera1n-everytime the device is boot tethered, the exploit bypasses SHSH blobs verification, allowing it to boot.