Karsten Nohl and Sylvain Munaut used cheap Motorola handsets running a replacement firmware based on open source code to intercept data coming from a network base station. They were able to locate the unique ID for any phone using this base, breaking the encryption keys with a rainbow table lookup. Nohl published a method for cracking open GSM’s A5/1 encryption design in 2009 using a lookup table in near real time. The crack does lower the bar from being a hardware problem to one of software expertise, which will cause alarm in the GSM engineering community.”]
Source: https://www.csoonline.com/article/2126441/researchers-hack-gsm-mobile-calls-using–9-handsets.html