No, the chief protection against the PIN code hacking comes from the retry counter. The FBI doesn't need the crypto keys, it just needs the PIN code. So it needs to brute force about 10,000 PIN codes.
Any mechanism that prevents the application processor from either a) remembering it incremented the count b) corrupts the count or c) patches the logic that handles a retry count of 10, is sufficient to attack the phone.
Somewhere in the application processor, code like this is running:
if (numTries >= MAX_RETRY_ATTEMPTS) {
wipe();
}
or
if (numTries >= MAX_RETRY_ATTEMPTS) {
retryTime = retryTime * 2;
}
Now there are two possibilities. Either there are redundant checks, or there aren't. If there aren't redundant checks, all you need to do is corrupt this code path or memory in a way that prevents it's execution, even if it is to crash the phone and trigger a reboot. Even with 5 minutes between crash reboot cycles, they could try all 10,000 pins in 34 days.
But you could also use more sophisticated attacks if you know where in RAM this state is stored. You couldn't need to de-capp the chip, you could just use local methods to flip the bits. The iPhone doesn't use ECC RAM, so there are a number of techniques you could use.
https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-630.pdf
Any mechanism that prevents the application processor from either a) remembering it incremented the count b) corrupts the count or c) patches the logic that handles a retry count of 10, is sufficient to attack the phone.
Somewhere in the application processor, code like this is running:
if (numTries >= MAX_RETRY_ATTEMPTS) { wipe(); }
or
if (numTries >= MAX_RETRY_ATTEMPTS) { retryTime = retryTime * 2; }
Now there are two possibilities. Either there are redundant checks, or there aren't. If there aren't redundant checks, all you need to do is corrupt this code path or memory in a way that prevents it's execution, even if it is to crash the phone and trigger a reboot. Even with 5 minutes between crash reboot cycles, they could try all 10,000 pins in 34 days.
But you could also use more sophisticated attacks if you know where in RAM this state is stored. You couldn't need to de-capp the chip, you could just use local methods to flip the bits. The iPhone doesn't use ECC RAM, so there are a number of techniques you could use.
https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-630.pdf