Editing Network switching subsystem (section)

===Procedures implemented===
The AuC stores the following data for each IMSI:
* the ''K<sub>i</sub>''
* Algorithm id. (the standard algorithms are called A3 or A8, but an operator may choose a proprietary one).

When the MSC asks the AuC for a new set of triplets for a particular IMSI, the AuC first generates a random number known as ''RAND''. This ''RAND'' is then combined with the ''K<sub>i</sub>'' to produce two numbers as follows:
* The ''K<sub>i</sub>'' and ''RAND'' are fed into the A3 algorithm and the signed response (SRES) is calculated.
* The ''K<sub>i</sub>'' and ''RAND'' are fed into the A8 algorithm and a session key called ''K<sub>c</sub>'' is calculated.
The numbers (''RAND'', SRES, ''K<sub>c</sub>'') form the triplet sent back to the MSC. When a particular IMSI requests access to the GSM core network, the MSC sends the ''RAND'' part of the triplet to the SIM. The SIM then feeds this number and the ''K<sub>i</sub>'' (which is burned onto the SIM) into the A3 algorithm as appropriate and an SRES is calculated and sent back to the MSC. If this SRES matches with the SRES in the triplet (which it should if it is a valid SIM), then the mobile is allowed to attach and proceed with GSM services.

After successful authentication, the MSC sends the encryption key ''K<sub>c</sub>'' to the [[Base Station Subsystem#Base station controller|base station controller]] (BSC) so that all communications can be encrypted and decrypted. Of course, the mobile phone can generate the ''K<sub>c</sub>'' itself by feeding the same RAND supplied during authentication and the ''K<sub>i</sub>'' into the A8 algorithm.

The AuC is usually collocated with the HLR, although this is not necessary. Whilst the procedure is secure for most everyday use, it is by no means hack proof. Therefore, a new set of security methods was designed for 3G phones.

In practice, A3 and A8 algorithms are generally implemented together (known as A3/A8, see [[COMP128]]). An A3/A8 algorithm is implemented in Subscriber Identity Module (SIM) cards and in GSM network Authentication Centers. It is used to authenticate the customer and generate a key for encrypting voice and data traffic, as defined in 3GPP TS 43.020 (03.20 before Rel-4). Development of A3 and A8 algorithms is considered a matter for individual GSM network operators, although example implementations are available. To encrypt Global System for Mobile Communications (GSM) cellular communications A5 algorithm is used.<ref>{{cite book | title = A Comprehensive Guide to 5G Security | publisher = John Wiley & Sons Ltd. | first1 = Shahria | last1 = Shahabuddin | first2 = Sadiqur | last2 = Rahaman | first3 = Faisal | last3 = Rehman | first4 = Ijaz | last4 = Ahmad | first5 = Zaheer | last5 = Khan | year = 2018 | page = 12}}</ref>