GSM [2G] Authentication Procedure

Authentication procedure is needed to know whether a User/ UE (User Equipment i.e. Mobile)/SIM (subscriber) has a valid access to an operator network. As we know there are there major entities participate in it.

1) User Entity (SIM - UE)

2) Controlling Entities(MSC, VLR, SGSN)

3) Authentication Entity(HLR, Auc)

General Details

Before going into the technical details of a Authentication procedure here is brief idea what actually happens.During Authentication Procedure some messages have been exchanged between User Entity and Authentication Entity via controlling entity. During this message exchange some algorithms are applied on User Entity and Authentication Entity; as a result of these algorithms one or two secret information (KEYs) have been generated; these generated information (KEYS) are  sent by User Entity to controlling entity and Authentication entity to controlling entity. After receiving information from both entities controlling entities compares  them if they are identical then authentication is successful otherwise not a valid subscriber.

Following Image gives the idea how it works with MSC (Controlling Entity)

Following Image gives the idea how it works with SGSN (Controlling Entity)

Now let's see how it works technically.

SIM - Subscriber Identity Module contains following data for Authentication for each subscriber

1) IMSI:- International Mobile Subscriber Identity

2) Secret Key (K 128 Bits):-  This key some times known as shared   

         key as well because this is present in both SIM and HSS.

3)Algorithm  A3:- Authentication algorithm

4)Algorithm  A8:- Ciphering key generator.

5)Algorithm  A5:- Ciphering/deciphering algorithm (Used after 

      authentication to hide data with CK Ciphering key )

HSS/AuC (Home Subscriber Server) contains following things

1) IMSI:- International Mobile Subscriber Identity

2) Secret Key (K  128Bits):-  This key some times known as shared  

      key as well because this is present in both SIM and HSS.

3) Algorithm A3:- Authentication algorithm

4) Algorithm A8:- Ciphering key generator.

5) RAND generator:-  HSS also have a way to generate a 128 Bits random number generator.

GSM Authentication Scheme

When UE attaches then HSS receives MAP_SEND_AUTHENTICATION_INFO request, upon receiving this request HSS computes authentication vectors(AVs) consisting of RAND,SRES,Kc. Following steps to be taken in complete cycle of authentication

Step -1 HSS generates a RAND (Random Number 128 Bits) 

Step -2 This generated RAND and Secret KEY (K) already 

      provisioned at HSS for given User identified by IMSI  shall    

      be passed to Algorithm A3 to generate SRES.

Step -3 RAND and Secret KEY (K) shall be passed to Algorithm A8 

      to generate Kc i.e. Confidentiality Key.

Step -4 This generated information along with RAND shall be send 

      to VLR or SGSN.

Step -5 VLR sends AV(Authentication Vectors) to MSC.

Step -6 MSC Keeps Kc and SRES with it and send RAND to UE over 

      network.

Step -7 Now UE shall generate the same AV using received RAND as 

      done in HSS such as SRES is generated by passing Shared 

      KEY(K) already stored in SIM along with received RAND to A3  

      algorithm.

Step -8 Similarly Kc shall be generated by passing RAND and 

      Shared KEY (K) to A8 algorithm.

Step -9 UE send generated SRES to MSC.

Step -10 On receive of SRES from UE; MSC matches with the SRES 

      received from HSS. If both are identical then User is  

      Authenticated User. Otherwise not.
Step -11 An Authenticated User shall use Kc Ciphering Key during 
      to encrypt/decrypt data using A5 function 

Your Comments /Suggestions and Questions are always welcome. We would try to clear your doubts with best of my knowledge. So feel free to put Questions.