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Somone I know had entered the PIN 3 times wrong. And she didn't have the PUK either. I resolved the issue (here we talk about German Vodafon, prepaid CallYa) by telling her to remove the SIM Card and login to the phone alone, now open the Vodafon CallYa App and find the PUK under Meine SIM.

After thinking about this procedure, I am now wondering, where does the PUK come from? Is the PUK really just a shared secret which is saved on the SIM Card? I don't remember having entered the PUK in the App.

Perhaps somebody could briefly clearify this for me, which technology is used in the case of SIM PUKs?

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    SIM cards largely use proprietary technology which isn’t widely published, so it’s going to be very difficult to find any details. Yes, the PUK (or some derivative of it) is stored on the SIM card. It can additionally be kept by the service provider as backup to help out in case the customers forgets the PUK. How exactly the card stores the PUK will depend on the manufacturer and model.
    – Ja1024
    Commented Aug 16 at 4:50

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The PUK (Personal Unblocking Key) isn't stored on the SIM card but is linked to it and stored by your mobile network provider. When you enter the wrong PIN too many times(or let's say three times...), the PUK is needed to unlock the SIM. You can retrieve the PUK through the Vodafone CallYa App because the app accesses Vodafone’s system, where the PUK is securely stored.

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    How does the SIM card know what's the correct PUK then? The unlock and setting of new PIN is a local operation on the SIM card requiring no cell coverage.
    – vidarlo
    Commented Aug 17 at 7:13
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    – Community Bot
    Commented Aug 17 at 9:40
  • Resetting the PIN needs to be possible to be done offline. Because during that the SIM card is not working. So I don't think the SIM card verifies the PUK online against a providers Database of PUKs for all devices in his network.
    – user1704369
    Commented Aug 17 at 20:37

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