Well as iPhone Dev Team explained:
- It looks like Apple is about to aggressively combat the â€œreplay attacksâ€ that have until now allowed users to use iTunes to restore to previous firmware versions using saved SHSH blobs.
- Those of you who have been jailbreaking for a while have probably heard us periodically warn you to â€œsave your blobsâ€ for each firmware using either Cydia or TinyUmbrella (or even the â€œcopy from /tmp during restoreâ€ method for advanced users). Saving your blobs for a given firmware on your specific device allows you to restore *that* device to *that* firmware even after Apple has stopped signing it. Thatâ€™s all about to change.
- Starting with the iOS5 beta, the role of the â€œAPTicketâ€ is changing â€” itâ€™s being used much like the â€œBBTicketâ€ has always been used. 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 doesnâ€™t depend merely on your ECID and firmware versionâ€¦it changes every time you restore, based partly on a random number). 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.
- This will only affect restores starting at iOS5 and onward, and Apple will be able to flip that switch off and on at will (by opening or closing the APTicket signing window for that firmware, like they do for the BBTicket). geohotâ€™s limera1n exploit occurs before any of this new checking is done, so tethered jailbreaks will still always be possible. Also, restoring to pre-5.0 firmwares with saved blobs will still be possible (but youâ€™ll soon start to need to use older iTunes versions for that). Note that iTunes ultimately is *not* the component that matters here..itâ€™s the boot sequence on the device starting with the LLB.
Although itâ€™s always been just â€œa matter of timeâ€ before Apple started doing this (theyâ€™ve always done this with the BBTicket), itâ€™s still a significant move on Appleâ€™s part (and it also dovetails with certain technical requirements of their upcoming OTA â€œdeltaâ€ updates).
- Although there may still be ways to combat this, a beta period is really not the time or place to discuss them. Weâ€™re just letting you know what Apple has already done in their exisiting beta releases â€” theyâ€™ve stepped up their game!
- Firmwares earlier than iOS 5 are unaffected so it would be wise to wait before updating your device