[Cryptography] How can you enter a 256-bit key in 12 decimal digits?

[Jerry Leichter]

>> It's possible there's some sort of high-entropy on-device secret. There are
>> also ways of generating these secrets in such a way that attempts to
>> physically tamper with the device will destroy the secret generator, e.g.
>> https://en.wikipedia.org/wiki/Physical_unclonable_function
> the point is though that if they steal the device, then they
> steal the high-entropy on-device secret along with it.  They
> only have to hook up their serial port to the wires that
> the buttons connect to, and try the 10^12 combinations.  They
> never have to try to work out the high-entropy secret.
Since this configuration forces you to go through the hardware, it can easily impose, say, a limit on the total number of failed tries before it wipes the secret.  Or a limit on the rate of tries.  Either way, this makes an attack dependent on either a way to get the secret out of the hardware, or on some hack to prevent the hardware from tracking tries.  (This is, BTW, the iPhone model - and that latter bug was indeed present in earlier iPhones - you could cut the power between the time the phone rejected an unlock code and the time it updated its counter of failed attempts.  I believe that's since been fixed.)
                                                        -- Jerry