Many of us know that in the past we were reminded so regularly to save the SHSH blobs of currently installed iOS on the device using the TinyUmbrella for example. The main purpose for this was so that the user can later downgrade to this version after upgrading if intend to do so. In other words, if the SHSH blobs are not saved for the iOS version in question, you can’t downgrade to that later on from an upgraded iOS version! Now coming to the main story it turns out that Apple has blocked the SHSH Blobs saving feature for the iOS for now. Currently it applies only to the iOS 5 and the versions onwards. What it means is that if a new version of the iOS 5 comes in the future say iOS 5.1 then you might not be able to downgrade to iOS 5 if you intend to do. Since you will not have saved SHSH blobs for this version. This issue has already been confirmed by the iPhone Dev Team member Musclenerd. Here is a post from the iPhone Dev-Team blog:
It looks like Apple is about to aggressively combat the “replay attacks” 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 “save your blobs” for each firmware using either Cydia or TinyUmbrella (or even the “copy from /tmp during restore” 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. That’s all about to change.
Starting with the iOS5 beta, the role of the “APTicket” is changing — it’s being used much like the “BBTicket” 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 doesn’t depend merely on your ECID and firmware version…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). geohot’s limera1n exploit occurs before any of this new checking is done, so tethered jailbreaks will still always be possible for devices where limera1n applies. Also, restoring to pre-5.0 firmwares with saved blobs will still be possible (but you’ll soon start to need to use older iTunes versions for that). Note that iTunes ultimately is *not* the component that matters here..it’s the boot sequence on the device starting with the LLB.
You can read the whole post here. Of course, this issue has been already identified by jailbreak community and according to the above quoted posted there may be other ways to combat this issue but for now the users will not be able to save the SHSH blobs until there is some workaround. Please note that if you have SHSH blobs saved for an earlier iOS 4.x version, you can still downgrade to that. This issue applies only to the iOS 5 and versions beyond this.
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