Introduction

Every day millions of people use cellular phones over radio links. With the increasing features, the mobile phone is gradually becoming a handheld computer. In the early 1980’s, when most of the mobile telephone system was analog, the inefficiency in managing the growing demands in a cost-effective manner led to the opening of the door for digital technology (Huynh & Nguyen, 2003). According to Margrave (n.d), “With the older analog-based cellular telephone systems such as the Advanced Mobile Phone System (AMPS) and the Total Access Communication System (TACS)”, cellular fraud is extensive. It’s very simple for a radio hobbyist to tune in and hear cellular telephone conversations since without encryption, the voice and user data of the subscriber is sent to the network (Peng, 2000). Margrave (n.d) states that apart from this, cellular fraud can be committed by using complex equipment to receive the Electronic Serial Number so as to clone another mobile phone and place calls with that. To counteract the aforementioned cellular fraud and to make mobile phone traffic secure to a certain extent, GSM (Global System for Mobile communication or Group Special Mobile) is one of the many solutions now out there. According to GSM-tutorials, formed in 1982, GSM is a worldwide accepted standard for digital cellular communication. GSM operates in the 900MHz, 1800MHz, or 1900Mhz frequency bands by “digitizing and compressing data and then sending it down a channel with two other streams of user data, each in its own time slot.” GSM provides a secure and confidential method of communication.

Security provided by GSM

The limitation of security in cellular communication is a result of the fact that all cellular communication is sent over the air, which then gives rise to threats from eavesdroppers with suitable receivers. Keeping this in account, security controls were integrated into GSM to make the system as secure as public switched telephone networks. The security functions are:

1. Anonymity: It implies that it is not simple and easy to track the user of the system. According to Srinivas (2001), when a new GSM subscriber switches on his/her phone for the first time, its International Mobile Subscriber Identity (IMSI), i.e. real identity is used and a Temporary Mobile Subscriber Identity (TMSI) is issued to the subscriber, which from that time forward is always used. Use of this TMSI, prevents the recognition of a GSM user by the potential eavesdropper.

2. Authentication: It checks the identity of the holder of the smart card and then decides whether the mobile station is allowed on a particular network. The authentication by the network is done by a response and challenge method. A random 128-bit number (RAND) is generated by the network and sent to the mobile. The mobile uses this RAND as an input and through A3 algorithm using a secret key Ki (128 bits) assigned to that mobile, encrypts the RAND and sends the signed response (SRES-32 bits) back. Network performs the same SRES process and compares its value with the response it has received from the mobile so as to check whether the mobile really has the secret key (Margrave, n.d). Authentication becomes successful when the two values of SRES matches which enables the subscriber to join the network. Since every time a new random number is generated, eavesdroppers don’t get any relevant information by listening to the channel. (Srinivas, 2001)

3. User Data and Signalling Protection:
Srinivas (2001) states that to protect both user data and signalling, GSM uses a cipher key. After the authentication of the user, the A8 ciphering key generating algorithm (stored in the SIM card) is used. Taking the RAND and Ki as inputs, it results in the ciphering key Kc which is sent through. To encipher or decipher the data, this Kc (54 bits) is used with the A5 ciphering algorithm. This algorithm is contained within the hardware of the mobile phone so as to encrypt and decrypt the data while roaming.