This is because firmware restores never had to be approved by Apple, but starting from i OS 3.0 onwards Apple implemented a technology called You see, when you restore your i Phone, i Pod Touch or i Pad through i Tunes your device phones home and ensures that it is allowed to be restored to a specific firmware version.In order to enforce this limitation SHSH blobs were introduced, which is a hash signature system.Unlike SHSH blobs which would submit the same one already stored on the server, a brand new APTicket is generated using a random string.In the future APTickets will completely replace SHSH blobs, this will happen once the i Phone 4S stops being supported.Meaning you had to have saved your SHSH blobs for the firmware version you are trying to downgrade to, or downgrading will not work.Note #2: We will be updating this page with the latest how to downgrade guides when new firmware versions are released, so make sure you come back soon!There is unfortunately some restrictions that need to be kept in mind though.
Additionally, you will also learn the best way to save your SHSH blobs which will allow you to downgrade your i Phone, i Pad, i Pod Touch and Apple TV firmware version in the future.
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These blobs are essentially gatekeepers and control whether or not a firmware is restorable.
Whenever you upgrade or downgrade your i Phone, i Pod Touch or i Pad, new SHSH blobs have to be generated because one of the parts to the SHSH formula is the firmware version.