I. Introduction
Stream ciphers are considered the most widely used and practical primitives for use in secure communication. They are largely adopted by industry in the area of mobile communications [1]. For instance, a stream cipher named A5/1 is utilized to guarantee the confidentiality of phone conversations in the Global System for Mobile communications (GSM). In addition, a part of the IEEE 802.11 wireless networking standard is a security protocol called Wired Equivalent Privacy (WEP). WEP uses the stream cipher RC4 to ensure confidentiality. Another example is E0 stream cipher that is used by Bluetooth, which is an industry standard for short distance wireless networks [2]. Nevertheless, feasible attacks have been discovered on all three encryption schemes. Hence, there is no standard model for stream ciphers at present, though the same are needed for a lot of applications. To deal with the lack of standards for secure stream ciphers that can be utilised by industry, a number of standardization efforts were made by the cryptographic community towards this end. The first one was the New European Schemes for Signature, Integrity and Encryption (NESSIE) project that began in 2000 and ended in 2004. All the stream cipher proposals sent to NESSIE were discarded mainly because of the discovery of cryptanalytic attacks [3]. After that, Japan initiated another standardization effort named the Cryptographic Research and Evaluation Committee (Cryptrec) [4]. Cryptrec highly recommended a number of stream ciphers, among them: 128-bit RC4, MUGI and MULTI-S01 [5]. But, these ciphers were also subsequently found to be vulnerable to cryptanalytic attacks [6]-[8].