Apple has been getting more and more detailed about documenting the end-to-end security in iOS in the last few years as part of its broader focus on security, encryption and privacy.
As part of that, they have been publishing security white papers detailing both hardware and software security in their mobile OS.
They recently published the latest version of this in a document with the scintillating title of iOS Security. [PDF]
It’s well worth a read and always fascinating to me to see both the lengths they go to to appropriately hide the complexity of the technological solution from the user (who shouldn’t need to worry about it) and also the benefits they get from designing both the hardware and software.