Proceeding of the 3 rd International Conference on Artificial Intelligence and Computer Science (AICS2015), 12 - 13 October 2015, Penang, MALAYSIA. (e-ISBN 978-967-0792-06-4). Organized by http://worldconferences.net 154 MODIFIED A5/1 STREAM CIPHER FOR SECURED GLOBAL SYSTEM FOR MOBILE (GSM) COMMUNICATION Siti Yohana Akmal Mohd Fauzi, Marinah Othman, Farrah Masyitah Mohd Shuib and Kamaruzzaman Seman Faculty of Science & Technology University Sains Islam Malaysia (USIM) Bandar Baru Nilai, 71800 Nilai Negeri Sembilan Malaysia sy.akmal91@gmail.com ABSTRACT A5/1 is well-known as the encryption standard for Global System for Mobile (GSM) communication, one of the most largely used cellular system in the world. Despite its wide usage however, it has numerous security vulnerabilities that leaves it susceptible to attacks. In this paper, a modified A5/1 is described. The proposed design looks at the effect of modifying the combinational function to enhance its random features, thus making it more secure overall. The modified algorithm which was simulated using C++, has been found to give the desired results when its randomness was tested using the NIST (National Institute of Standard and Technology) test suite, following which, further analysis was done by varying its parameters, and the results are then compared against other published work. Keywords: A5/1 stream cipher, NIST test suite, LFSR, GSM communication, information security. 1. Introduction With the advent of technology, comes the challenge of safeguarding the data being both transmitted and received. In information security, there are three vital points that are used as the policy within a company or an organisation that is commonly known as the CIA - confidentiality, integrity and availability - triad. Explained in words, confidentiality refers to the securing of the private information, integrity being the act of both preserving and confining it, while availability is the ready accessibility of an item. While GSM is currently the world leading communication service, with a pool of subscribers making up more than 80% out of the world population, many remain unaware that the security of a discreet information can be compromised via either the voice or text messaging services, a threat which this service faces (Ericsson, 2014). The use of the A5/1 stream cipher as the encryption for over-the-air voice and text messaging is no longer sufficient ever since it was reverse-engineered back in 1999, opening it up to numerous attacks, targeting the leaked design (Gendrulllis, Martin, & Andy, 2008; Sankaliya, Mishra, & Mandloi, 2011; Alireza Jolfaei, 2010; Nohl, 2010; Meyer, 2010; Kalenderi, Pnevmatikatos, Papaefstathiou, & Manifavas, 2012; Jain & Chaudhari, 2013; AlHamdan, Harry, Bartlett, Dawson, Ed, Simpson, Leonie, & Wong, Kenneth Koon-Ho, 2014; Bhal & Dhillon, Zhilmil, 2014; Mahalanobis & Shah, 2014).