Power analysis of stream ciphers based onfeedback shift registers

In recent days, many cryptographic devices, such as smart-cards and cell phones, are widely accessible to many people. However, wide access to cryptographic devices makes them vulnerable to side channel analysis (SCA) attack. As such, there is a high demand for research in the field of side channel analysis. Although SCA attacks have been extensively applied to block ciphers, only a limited amount of research is available on the effectiveness of side channel analysis on stream ciphers. In this dissertation, we study SCA attacks on stream ciphers and develop some cryptanalysis methods for applying the attacks effectively on practical realization of stream ciphers. The proposed power analysis attacks were first theoretically applied to stream ciphers with a linear feedback shift register (LFSR) and nonlinear filtering function, a structure referred to as a filter generator. Since typical stream ciphers include multiple LFSRs and/or nonlinear feedback shift registers (NLFSRs), we first consider the extension of the typical power analysis attack to stream ciphers with multiple LFSRs and a nonlinear combining function, known as a combination generator. Then, the attack is extended to stream ciphers based on nonlinear feedback shift registers (NLFSRs) and stream ciphers with multiple NLFSRs and LFSRs. In most papers related to applying side channel analysis attacks to stream ciphers, the authors ignore the effect of noise and inaccurate measurements. This limits the applicability of their methods for real applications. This dissertation has developed side channel analysis attacks on feedback shift register (FSR) based stream ciphers with consideration of inaccurate measurement effects. At first, we have developed the attack for stream ciphers based on an individual LFSR and/or NLFSR, while the power measurements are inaccurate and they do not exactly match the theoretical values. Later, considering inaccurate measurements, we have developed power analysis of stream ciphers with multiple LFSRs and NLFSRs. Finally, we consider combining SCA with some classical attacks on stream ciphers based on mathematical and statistical approaches to recover key or state bits of the stream ciphers. Hence, we have extended the correlation attack, fast correlation attack and algebraic attack, which are mathematical (or classical) attacks, such that they are applicable with side channel analysis. The proposed methods are validated through implementation on a practical cryptographic algorithm, the Grain stream cipher. The practical investigations in this dissertation are done using simulated ASIC circuits. To simulate the behavior of ASIC circuits, we have implemented them using Cadence Virtuoso Spectre Circuit Simulator version 5.10. 41. All the circuits including LFSR, NLFSR and Grain, are prototyped in TSMC 180 nm standard cell CMOS technology. The simulated power consumptions are used to investigate the practical application of the proposed attacks. This dissertation shows power analysis is a powerful technique to attack stream ciphers and recover state bits and/or the key of the stream ciphers. Furthermore, combining classical methods and measured power data can significantly reduce the complexity of an attack of a stream cipher and countermeasure methods should be considered in hardware implementation of stream ciphers, to make them resistant to side channel analysis.