While trying to restore an iOS device through iTunes, you may receive the following error message stating that the custom restore image file (IPSW) is not authorized.
The iPad “” could not be restored. TATSU declined to authorize this image on this device for this user.
iTunes is calling out to the gs.apple.com for signed restore. Since Apple doesn’t sign the older iOS images a few days after being released, this message is normal and you will not be able to restore from an older restore IPSW file.
If you have a previous restore of the device already backed up, you can take backups of SHSH Blobs using iFaith or similar utility for Mac/Windows. If you did not previously take SHSH Blobs from a previous restore backup, you are pretty much out of luck trying to restore or downgrade to a different iOS version.