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Unlocking technology[edit]
A handset can be unlocked by
entering a code provided by the network operator. Alternative mechanisms include software running on the handset or a computer attached to the handset, hardware devices that connect to the handset or
over-the-air by the carrier. Usually the unlock process is permanent. The code required to remove all locks from a phone is referred to as the
master code,
network code key, or
multilock code. There can also be multiple levels of locks placed on the phone by networks, which block the use of other networks' SIM cards. These are usually referred to as "Network" (NCK) or "Service Provider" (SPCK) locks and can be removed using the corresponding unlock codes,
which are unique to each phone depending on its IMEI.[6]
Typically, a locked handset will display a message if a restricted SIM is used, requesting the unlock code. On recent phone models running Android software, the phone will display a message saying “SIM network unlock PIN” or “Enter Network Lock Control Key” if network locked. Windows phones will display the message, “This SIM card can only be used on specific networks. Contact your customer service center for the unlock code”. Other handsets may display different messages such as "Enter special code" or "Enter unlocking code," or in some cases the handset will simply display a message stating that it is locked. Once a valid code is entered, the handset will display "Network unlocked” or “Network unlock successful”.
The unlock code is verified by the handset and is generated by the manufacturer, typically by an algorithm such as a one way
hash or
trapdoor function. Sometimes big telecom providers change the original factory unlock codes as an extra layer of security against unlocking services. For various big brands such as Samsung and Motorola there is no algorithm but just a random code generator where the unlock codes are programmed in the phone itself and then saved in a big database managed by the manufacturer. For the other brands where t
he unlock codes are still based on algorithms those are based on the IMEI number and the
MCC code and have been
reverse-engineered, stolen or leaked. Some handsets can be unlocked using software that generates an unlock code from an IMEI number and country and operator details using the algorithm specific to the handset. Other manufacturers have taken a more cautious approach, and embed a
random number in the handset's
firmwarethat is retained by the network on whose behalf the lock was applied and the manufacturer. These handsets can still be unlocked by online services that have access to either inside people with the manufacturer or with the telecom networks, or they need to be connected to the computer with a cable where specific software will bypass the security and SIM-unlock the phone. Sometimes this is done by advanced calculations to bypass the security the official way and other times using exploits or overwriting parts of the firmware where the lock status is kept, and often even recover a phone that is
bricked or completely damaged in the software sense.
https://en.wikipedia.org/wiki/SIM_lock