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Comment by MBlume | Hacker News Reader

How much of a point is there to device encryption anyway? I have a four-digit PIN -- if someone has access to my phone aren't they just going to try 10,000 PINs and crack the phone pretty much instantly?

EDIT: according to x0x0 things are better than I naively imagined =)

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Depends on whether it’s tarpitted or not (i.e. do subsequent attempts to unlock the device have a progressively longer cool down time before another unlock request can be made). Also, Apple for example will wipe the device after, I believe, 10 failed attempts? It’s possible to defeat 4-digit keys in 10 attempts but it’s not likely.

Right, but that's relying on the OS to lock the attacker out, and you can do that without encryption -- the point of encryption is that even if someone bypasses the operating system and reads the disk directly, they can't read your files. If someone has a full image of your phone's flash storage, they can try PINs as fast as they want.

That could potentially be done at the hardware level, which would make it much harder to bypass.

I'm not using the right words, but it isn't that easy.

On modern phones, even if you have an image of the phone's storage, the code you enter is merged (wrong word) with a large random number stored in an externally inaccessible chip in the phone (different for each phone, obviously). This large random number is then used as a seed to encrypt the drive. So it's not anything like just extracting the disk image then brute force attempting the 10000 valid codes from 0000-9999.

edit: here is a better explanation

   Apple was a front runner in implementing some sort of encryption long before 
   others even thought about it. Apple uses a 256-bit device-unique secret key 
   (UID) stored in the phone’s hardware, where it’s hard to extract from. Apple 
   says this UID is unique to the device and it doesn’t store these values. It 
   also says that no software or hardware can read this key. This UID is mixed 
   with the passcode to generate a new key (passcode key) to secure data on the 
   device. Unlike Android, this UID key can’t be extracted from the device, so 
   all the brute forcing attempts have to be done on the device itself. [1]

So I would guess it's reasonably secure to non-governmental adversaries. If you're worried about governments, the US now supports torture (the only person punished for torture during the war on terror was a whistleblower), so the NSA is free to torture your code out of you.

[1] http://resources.infosecinstitute.com/understanding-disk-enc...

Aha, that's really reassuring, thank you =)

I'd imagine that a sufficiently sophisticated attacker would be able to extract that random number somehow.

You can set your device encryption and screen unlock code separately already. Android also uses scrypt as a PBKDF.

How do you do this on stock Android?

Well one right way is to have a passphrase on boot, then switch over to PIN/swipe/whatever. Seems sorta obvious...

But, if Google just mandated TPMs, then a 4-digit PIN is fine as the hardware can perform a lockout/cooloff/self-destruct.

TPMs aren't super-hardened, but against losing your phone in a taxi, or random local cops trying to crack it, it's probably good enough.

Is this now possible? Last time I checked Android required that your boot passphrase is used as your screen unlock method. It's the main reason I haven't switched over to encrypted on my N5.

It's possible if you're rooted: https://play.google.com/store/apps/details?id=com.kibab.andr...

The pin unlocks the key in the TPM. If you brute force the pin (I hope) the TPM will wipe the key.

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