Security systems are used to secure conversations and signaling data from interception as well as to prevent cellular telephone fraud.Electronic Serial Number (ESN), is transmitted "in the clear" in analog systems. cellular telephone fraud can be done by "cloning" another cellular phone and placing calls with it after recieving this ESN . In Advanced Mobile Phone System (AMPS) and the Total Access Communication System (TACS)a radio hobbyist can simply intercept cellular telephone conversations with a police scanner.
GSM is the most secure mobile communication standard currently available. The reason for this is that it is a digital system utilizing a speech coding algorithm, slow frequency hopping,Gaussian Minimum Shift Keying (GMSK) digital modulation, and Time Division Multiple Access (TDMA) time slot architecture.It is very difficult to intercept and reconstruct this signal. The subscriberâ„¢s anonymity is ensured through the use of temporary identification numbers.

Cryptography
a.Symmetric Algorithms
They are algorithms in which the encryption and decryption use the same key. symmetric algorithms are functionally described as follows:

C=Ex(P)
P=Dx©
P=Dx(Ex(P))
where,
p:plaintext
c: ciphrtext,
Ex( ):encryption with key x
Dx( ):decryption with key x

A good example is the Data Encryption Standard (DES).Symmetric encryption algorithms may be further divided into block ciphers and stream ciphers.

GSM Security Features
It consists the following aspects:
1.subscriber identity authentication:
International Mobile Subscriber Identity (IMSI) along with individual subscriber authentication key (Ki)makes up the sensitive identification credentials.These are never transmitted over the radio channel, but a challenge-response mechanism is used to perform authentication.
2. subscriber identity confidentiality
This is done with the Temporary Mobile Subscriber Identity (TMSI). it is sent to the mobile station after the authentication and encryption procedures have taken place.To which the mobile station responds .
3. signaling data confidentiality
The user's SIM contains the ciphering key generating algorithm which is used to produce the 64-bit ciphering key (Kc). The ciphering key may be changed at regular intervals as required by network design and security considerations.
4. user data confidentiality.

The security mechanisms of GSM are implemented in three different system elements; the Subscriber Identity Module (SIM), the GSM handset or MS, and the GSM network.
SIM contains:
a) the IMSI
b)the individual subscriber authentication key (Ki)
c) the ciphering key generating algorithm (A8)
d) the authentication algorithm (A3)
e) Personal Identification Number (PIN)
GSM handset contains:
a) ciphering algorithm (A5)

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