4.1 billion people around the world communicate over GSM. Besides communication, more and more additional services - like payment functionality - are being deployed on top of GSM. It has been over 20 years since GSM was designed, and in that time several security problems have been found, both in the protocols and in the - originally secret - cryptography. 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