The design principles of linear feedback shift register based clock controlled stream ciphers

Abstract

Stream ciphers are one of the most important classes of encryption algorithms usedto ensure security in digital communication. The design of many stream ciphers is based onuse of Linear Feedback Shift Registers (LFSRs), due to their simplicity, speed of implementation in hardware and providing sequences with good statistical properties. Astream cipher can not be considered suitable for cryptographic applications unless itsoutput sequences have large periods, large linear complexities and possess certainrandomness properties. Moreover a stream cipher must provide high resistance against well known cryptanalytic attacks such as time-memory trade-off attacks, divide-conquer attacksand correlation attacks. The use of clock-controlled shift registers in key stream generatorscan be a good alternative for achieving these properties.In this thesis, the design principles of a cryptographically secure LFSR based clock controlled stream ciphers are described and two new stream cipher algorithms ANER-Hand CSDS are presented. In addition, key stream properties of these algorithms and theirresistance with respect to some well known cryptographic attacks are investigated. Fromthe mathematical expressions and simulation results, it is shown that the two algorithms produce key stream sequences with satisfying basic security requirements and provide highresistance against currently known styles of attacks. Within their security powers, theCSDS is also designed to be very fast, especially for software usage. On the other hand, thestream cipher ANER-H can be applicable for both in hardware and software, due to its conceptually simple design.