The Exclusive Chip Identification (ECID) is a unique 64-bit identifier assigned to every iPhone processor. Unlike a serial number, the ECID is hardcoded into the silicon and cannot be changed. For power users, "registering" an ECID usually refers to submitting it to a service like TSS Saver to archive SHSH blobs, which allow you to downgrade or restore your firmware even after Apple stops signing it. How to Find Your iPhone ECID
The Moral of the Story
Common Issues and Troubleshooting
This is the most confusing part for new users. register ecid iphone new
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Method A: The Easiest Way (No Computer)