Researchers demonstrated how to crack GSM A5/1 Stream Cipher using a general-purpose graphics processing unit computer with 3 NVIDIA GeForce GTX690 cards. Researchers have exploited two security weaknesses to compute a look-up table using commodity hardware in 55 days. Once calculated the rainbow table, that has a side of 984GB, they are able to determine the secret key used to encrypt communications in just nine seconds. The practical results again show that GSM should no longer use it and GSM.”]
Source: http://securityaffairs.co/wordpress/52666/hacking/gsm-crypto-hacking.html