Apple is notorious for strictly controlling which versions of iOS users can run. Typically, Apple only "signs" the latest version of iOS. If you want to downgrade from iOS 17.5 to iOS 17.3, you generally cannot do so because Apple has stopped signing iOS 17.3.
For the average iPhone user, the acronym "ECID" likely means nothing. It is buried deep within the technical architecture of the device, invisible to the daily user interface. However, for the jailbreak community, security researchers, and power users, the ECID is the single most important identifier for maintaining control over their device. Register Ecid Iphone
Locate your iPhone under the USB tree; the ECID will be listed in the field. 2. Register Your ECID for SHSH Blobs Apple is notorious for strictly controlling which versions