Question

Where does A3 algorithm run in GSM architecture?
HLR
SIM
Either HLR or SIM depending on operator implementation
Both HLR and SIM independent of operator implementation

HLR in GSM can use preshared key to generate
TMSI
RAND
Session Key
Equipment Identity

Answer 1

Solution:-

1) Correct option is (b). i.e. SIM

A3 Algorithm:

The A3 algorithm is a type of symmetric algorithm, this is because it uses the same key for both the purposes i.e., encryption and decryption. A3 is also called as a one-way function as a user aware of the input can easily determine the output.

Now,

Location of A3 algorithm in GSM architecture:

The actual location of the A3 algorithm in the GSM architecture is in the SIM (Subscriber Identity Module). The task of storing and implementation of the A3 algorithm is performed in SIM itself.

This SIM is inserted or installed in the GSM module. The shape of which is as small as a small smartcard. The size of the SIM card is being reduced in order to take less volume and also to make it compact.

The SIM is supposed to be one of the safest locations to store the A3 algorithm as it is already protected with its own security PIN. This PIN is required whenever making any kind of access to it. As a result, the algorithm is safe and the entire business logic can be implemented without any hustle.

2) Correct option is (b) i.e. RAND

• RAND generate HLR in GSM can use preshared key.
• Suppose a mobile station is enters a new call (128 bits)
• The Authorisation centre (AC) at the users HLR is the contacted by foreign network.
• The AC generate a random number RAND (128 bits) and a value SRES (32 bits) by employing A3 with Ki to RAND.
• HLR sends (RAND,SREs) encrypted and signed to the foreign network.
• The foreign network send RAND to the mobile station is and asks it to calculate SRES by executing A3 with Ki on RAND.
• Then the mobile station sends calculated SRES back to foreign network.
• If SRES=SRES the mobile station is authentication successfully.