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Firmware Umbrella

When you try to restore an older firmware on the devices listed below, Apple won’t let you do that. Once you ‘upgrade’ they don’t let you downgrade. The way they stop you is basically by a simple response that iTunes receives when you try to restore the firmware of your choosing. See, the firmware is now signed for the below devices. It is signed with a unique id (ECID) that only your device has. Apple takes the firmware version files and combines them with your ECID and generates a hash that ONLY APPLE can generate. iTunes packages up this valuable information and sends it to your device. Your device checks the information and verifies the signature (making sure it really came from Apple because it CANNOT be forged as the encryption is very high). If the signature matches then the restore process can continue. If the signature does not match, the device raises an error and the restore process stops. Apple stops signing older firmwares once a new one is out.

FirmwareUmbrella sends the same exact request that iTunes sends Apple when requesting the signatures for your device to be restored. It saves SHSH signatures for your device received from the Apple server and let to downgrade OS indefinitely.

TinyTSS

TinyTSS is a small java app that acts as your very own signature service. When iTunes verifies the firmware restore for the devices listed below, it checks if you are allowed to restore to the version you are requesting. With TinyTSS + the shsh blob file(s) you obtained with Umbrella, you will be able to restore to the version of those shsh files forever!

TinyUmbrella
Firmware Umbrella + TinyTSS

TinyUmbrella Supports

iPhone 4
iPhone 3GS, 3G
iPod Touch 3G, 2G
iPad1, 2