Information Technology Reference
In-Depth Information
Normal mode
When an iPhone is switched on, it is booted to its operating system. This mode is known as
normal mode. Most of the regular activities (calling, texting, and so on) performed on an
iPhone will be run in normal mode.
When an iPhone is turned on, internally, it goes through a
secure boot chain
, as shown in
the following figure. Each step in the boot-up process contains software components that
are cryptographically signed by Apple to ensure integrity.
A secure boot chain of an iPhone in normal mode
The
Boot ROM
, known as the secure ROM, is a
read-only memory
(
ROM
) and is the
first significant code that runs on an iPhone (
http://images.apple.com/ipad/business/docs/
iOS_Security_Feb14.pdf
)
. The Boot ROM code contains the Apple root CA public key,
which is used to verify the signature of the next stage before allowing it to load. When the
iPhone is started, the application processor executes the code from the Boot ROM, which,
in turn, verifies whether the
Low Level Bootloader
(
LLB
) is signed by Apple or not and
loads it accordingly. When LLB finishes its tasks, it verifies and loads the second stage
boot loader (iBoot). iBoot verifies and loads the iOS kernel, which, in turn, verifies and
runs all the user applications as shown in the preceding figure. The secure boot chain en-
sures iOS runs only on validated Apple devices.
