When you attempt to restore an iPhone, the device sends a unique request (including its ECID and a random "nonce") to Apple’s servers. Apple only returns a valid signature for the versions they currently support. An "unsigned IPSW" is any firmware version for which Apple no longer provides these real-time signatures.
For legitimate research, use dedicated test devices, never your daily driver, and be aware of local laws. iphone install unsigned ipsw
If you need a formal academic paper, I recommend searching IEEE Xplore, arXiv, or the USENIX Security proceedings for topics like: When you attempt to restore an iPhone, the
If you venture into forums like Reddit’s r/jailbreak or use command-line tools to force an unsigned install, you face significant risks: use dedicated test devices