It's mathematically impossible to make a device completely safe from someone who has complete physical control over it. You can encrypt this and that all you like, but it's literally only a matter of time before someone applies enough computing power and breaks said rights-management. Boot loaders can be heavily obfuscated against reverse engineering, but since the device has to actually boot and work at some point, there's a key to the proverbial lock in that haystack somewhere. I hope I'm making sense, coffee hasn't kicked in yet.
You are wrong.
First encryption is not used, it is digital signature (rsa).
Second to be safe the first signature check should be done on the CPU ROM using internal RAM (no MITM).
In this case if the signature algorithm is mathematically safe, you're device is broken if :
- if somebody publish private key
- if there bug in the ROM
There can be others bugs/holes in upper layer of software (bootloader, kernel, ...), they can be patched by an update.