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Suspects can refuse to provide phone passcodes to police, court rules::Phone-unlocking case law is “total mess,” may be ripe for Supreme Court review.
Pins are pretty damn secure because after too many wrong guesses the phone will start making you wait a long time between attempts.
OTOH, as a point of reference, Microsoft requires an 8-digit pin on phones that can access comment resources. It was 6 until very recently, though, so that’s probably fine if you’re not a target of corporate espionage.
What makes it secure is the master key in a TPM which is considerably longer than four digits and can only be accessed by for digits in under twenty attempts.
Hypothetically.
Some TPMs are allegedly backdoored by their manufacturers (e.g. Microsoft), and regardless of the original intent of these backdoors they’re now accessible through cracking software to which some law enforcement departments have access.
Then, if a government department really, really wants to get into a phone, the TPM can be cracked with a tunneling electron microscope, but this process is still slow, expensive and requires an expert.
I don’t know about the key length of a TPM. If I had to guess I’d say something like 256 or 512 bits, or even 1024. But I was just addressing the PIN the user might type in to unlock their phone. That’s something the user can control, and it provides plenty of security against naive brute force attacks by people not sophisticated enough to disassemble the phone. I assume that group includes the majority of police departments and any cop whose main work is outside of a lab.
Usually 128 or 256 bit root keys for symmetric keys, almost always 256 bits for ECC for asymmetric keys these days (used to be RSA between 1024 and 2048 bit)
Here in the states, in municipal precincts, we can expect they’ll have some phone cracking software on hand, so if your TPM is backdoored, your PIN isn’t going to matter much. If yours is an early phone (notoriously the iPhone 5, I think) that doesn’t have a TPM, then it might be susceptible to exploits that lift the limits of tries, in which case a four digit PIN can be cracked by a machine using brute force.