Those who screw up their iPhones with an unsupported jailbreak—or who have bad luck with their iOS upgrades and want to roll back to an older version—are usually able to make everything right again by downgrading their phone's OS back to the previous version. That practice appears to be on the endangered list, however, thanks to changes within iOS 5. A post at the Dev-Team Blog indicates that Apple is taking steps to inhibit downgrades to previous versions of iOS, in addition to untethered jailbreaks.
iOS 5 Beta Hobbles OS Downgrades
Those who regularly jailbreak their iOS devices are usually familiar with the concept of saving your "blobs." Users can save their SHSH blobs using a tool such as Cydia or TinyUmbrella before upgrading to a new version of the OS so that they can eventually restore that specific device to that firmware via iTunes.
Thanks to some changes that Apple is making to "APTicket" within iOS 5, however, this may no longer be possible. "The LLB and iBoot stages of the boot sequence are being refined to depend on the authenticity of the APTicket, which is uniquely generated at each and every restore (in other words, it doesn’t depend merely on your ECID and firmware version… it changes every time you restore, based partly on a random number)," wrote the team behind many iOS jailbreaks and hacks. "This APTicket authentication will happen at every boot, not just at restore time. Because only Apple has the crypto keys to properly sign the per-restore APTicket, replayed APTickets are useless."