Editing SIM card (section)

=== Authentication key (K<sub>i</sub>) ===
The K<sub>i</sub> is a 128-bit value used in authenticating the SIMs on a [[GSM]] mobile network (for USIM network, the K{{sub|i}} is still needed but other parameters are also needed). Each SIM holds a unique K<sub>i</sub> assigned to it by the operator during the personalisation process. The K<sub>i</sub> is also stored in a database (termed [[network switching subsystem#Authentication centre (AuC)|authentication center]] or AuC) on the carrier's network.

The SIM card is designed to prevent someone from getting the K<sub>i</sub> by using the [[(U)SIM interface|smart-card interface]]. Instead, the SIM card provides a function, ''Run GSM Algorithm'', that the phone uses to pass data to the SIM card to be signed with the K<sub>i</sub>. This, by design, makes using the SIM card mandatory unless the K<sub>i</sub> can be extracted from the SIM card, or the carrier is willing to reveal the K<sub>i</sub>. In practice, the GSM cryptographic algorithm for computing a signed response (SRES_1/SRES_2: see steps 3 and 4, below) from the K<sub>i</sub> has certain vulnerabilities<ref name=":4">{{cite news |title= Hackers crack open mobile network |url= https://www.bbc.co.uk/news/technology-13013577 |publisher= bbc.co.uk |access-date= 13 August 2011 |date= 20 April 2011 |archive-date= 12 August 2011 |archive-url= https://web.archive.org/web/20110812103131/http://www.bbc.co.uk/news/technology-13013577 |url-status= live }}</ref> that can allow the extraction of the K<sub>i</sub> from a SIM card and the making of a [[Phone cloning|duplicate SIM card]].

Authentication process:

# When the mobile equipment starts up, it obtains the international mobile subscriber identity (IMSI) from the SIM card, and passes this to the mobile operator, requesting access and authentication. The mobile equipment may have to pass a PIN to the SIM card before the SIM card reveals this information.
# The operator network searches its database for the incoming IMSI and its associated K<sub>i</sub>.
# The operator network then generates a random number (RAND, which is a [[cryptographic nonce|nonce]]) and signs it with the K<sub>i</sub> associated with the IMSI (and stored on the SIM card), computing another number, that is split into the Signed Response 1 (SRES_1, 32 bits) and the encryption key K<sub>c</sub> (64 bits).
# The operator network then sends the RAND to the mobile equipment, which passes it to the SIM card. The SIM card signs it with its K<sub>i</sub>, producing Signed Response 2 (SRES_2) and K<sub>c</sub>, which it gives to the mobile equipment. The mobile equipment passes SRES_2 on to the operator network.
# The operator network then compares its computed SRES_1 with the computed SRES_2 that the mobile equipment returned. If the two numbers match, the SIM is authenticated and the mobile equipment is granted access to the operator's network. K<sub>c</sub> is used to encrypt all further communications between the mobile equipment and the operator.