GSM is the most widely used standard for digital cellular mobile communication in the world. The use of radio communications for transmission to the mobile subscriber makes GSM Public Land Mobile Network (PLMN) particularly sensitive to misuse of its resources by unauthorized persons using manipulated Mobile Stations, who try to impersonate authorized subscribers and eavesdropping of the various information, which are exchanged on the radio path.

Mobile network is the shared media and any user of the media can intercept the network.  When the media are shared, anyone can listen to or transmit on the media. Thus communication is no longer private. When media are shared, privacy and authentication are lost unless some method is established to regain it. Cryptography provides the means to regain control over privacy and authentication.

A5/x are the encryption algorithms used in order to ensure privacy of conversations on GSM mobile phones. A5/x algorithms are used to secure the information sent over the air interface. The strong version A5/1 is used in most countries. A5/2 is weaker version used in countries on which export restrictions apply. A5/3 encryption algorithm is used for GSM and ECSD and GEA3 encryption algorithm is used for GPRS.