Also known as: Pin Unblocking Key, PUK1, PUK2
The PUK (Pin Unblocking Key) is used to unblock a SIM. As a security feature to avoid brute force attacks on SIMs, a SIM is blocked after three incorrect password entries in a row. Once it reaches this state, an eight-digit PUK is needed to unblock the SIM and allow it to be used on a GSM network again, although it can always be used for emergency calls, even in a blocked state.
A PUK is stored on the SIM and is known by the network operator. Although often included with documentation supplied when subscribing to a network, the PUK is generally unknown to the end user - some networks even take the step of monetising PUK codes by offering access to them as a dedicated service. If a PUK code is entered 10 times incorrectly, the SIM must be returned to the operator for reactivation.
PUK1 and PUK2 are used to unblock PIN1 and PIN2 respectively. As PIN1 is the primary means of access to a handset, blocking PIN1 will block the entire handset until PUK1 is entered. Conversely, as PIN2 only manages restriction of a small set of features, the blocking of PIN2 will still allow normal operation of all handset functions apart from those that normally need a valid PIN2, which remain locked out until the PUK2 is entered.