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The only protection features that mobile phones have to keep thieves that stole a target's device from impersonating that subscriber is the PIN/PUK system that's on by default on most SIM cards.

The gist of this is that only the legitimate subscriber would have set the PIN and only they would know it to unlock the SIM on every phone boot-up. Moreover, if the legitimate subscriber would forget the PIN and enter the wrong code 3 times in a row, then the SIM would just become locked and a PUK code would need to be entered to reset it and unlock it back up.

My question is: how is this implemented phisically on the SIM? The only thing I can think of is that the SIM card only has physical memory cells somewhere on the chip that count how many times the PIN code was introduced and was wrong and another, separate, memory location that counts how many times the PUK code is entered and was wrong. And that the phone itself would always read these memory locations off the SIM and make sure that, after a certain number of times was hit, it would just simply prevent the user from simply bruteforcing the codes by not allowing them to keep on entering when the limit is reached.

But, if that was true, then surely someone would have created custom-made hardware that emulated a mobile phone and which simply ignored these checks and just brute-forced the SIM until it found to right unlock code. Since this does not seem to be the case, I can only imagine that, somehow, there are physical security measures on the SIM itself that prevent such a scenario from happening. What are these measures?

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You have one wrong assumption: the phone does not perform the check. The SIM card is a tiny computer. It does the verification by itself and it is able to lock itself and prevent the phone to read its content after being locked.

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