Global System for Mobile communications (GSM system) has become the most popular standard for digital cellular communication for day to day communication in the world. Cellular phones have become a ubiquitous means of communications with over 6 billion users worldwide in 2013, of which 80% are GSM subscribers. It has been over 20 years since GSM was designed, and during this time several security problems have been found. However practical exploits of these weaknesses are complicated because of all the signal processing involved and have not been seen much outside of their use by law enforcement agencies.