Application of nonlinearity to enhance security of direct-sequence spread-spectrum (DS/SS) systems

• Main Author: Pang, Wen Ni
• Other Authors: Li Kwok Hung
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/167300
• Institution: Nanyang Technological University

Direct-sequence spread-spectrum (DS/SS) systems have been developed to protect transmitted signals. Through the use of a spreading code, the transmitted signal is spread across a larger bandwidth, allowing it to avoid detection and be resistant to jamming. The strength of its security lies in the spreading code used, which should resemble a random sequence. Hence the ideal spreading code would be a pseudorandom noise (PN) code. The current method of generating PN codes is through linear feedback shift registers (LFSRs). This presents issues due to its linearity, which allows hackers to develop the same LFSR and thereby decipher both past and future parts of the PN code. This is done through the Berlekamp-Massey algorithm. In this paper, new methods of generating PN codes were designed with nonlinearity introduced. The methods were evaluated over their simplicity (measured using speed), evenness of bit distribution and length of period. Through some rounds of iteration, a new method consisting of 3 parallel LFSRs (3LFSR) was designed. This method was able to produce a PN code of even bit distribution and long periods that extend beyond the maximum period possible by a single LFSR. Another new method using the SHA256 from the Secure Hash Algorithm (SHA) family was also designed and fared extremely well in terms of speed as compared to the 3LFSR design and the original LFSR. However, it had a much lower period compared to the 3LFSR. The results were obtained through experimentation, and thus only showed the potential of these methods. With a more in-depth study into the mathematical foundations underlying the methods, more properties of these designs could be uncovered, allowing for more deliberate design choices which might have better performance.