About 80 million digital cell phones worldwide implement the Global System for Mobile communications (GSM) protocols. Recently it was announced that COMP128, the cryptographic algorithm that protects the "identity key" in the majority of these phones, was extremely weak, thus allowing GSM phones to be "cloned". In this talk, we will examine how COMP128 is used in the GSM protocol, describe the algorithm itself, and demonstrate how to break it. We will also discuss the implications this result has for the security of of the voice privacy features of GSM.