A novel stream cipher with dynamic structure

Abstract

Stream ciphers are commonly used to provide confidentiality for a wide range of frame based applications such as mobile devices or embedded systems. For these applications, stream ciphers are preferred for encryption due to the simplicity of their implementation, efficiency and high throughput. However, practical attacks have been discovered on well-known stream ciphers. Many stream ciphers are designed to resist these attacks. The majority of these ciphers has a fixed structure, which is an advantage that their security against the known attacks can be proved. However, the fixed building structure of these ciphers also provides opportunities for potential new attacks. There have been a few approaches that have tried to add dynamicity to the structure of these ciphers to improve their security level. The idea behind this is that when the structures of ciphers are unknown to attackers, they are more resistant to attacks. However, these ciphers are not widely discussed among researchers. Moreover, the existing research concerning stream ciphers with dynamic structures has focused mostly on dynamic polynomial switching in the Linear Feedback Shift Registers. This study proposes a novel dynamic structure stream cipher algorithm based on using a dynamic structure and some parameters to increase the complexity of the cipher, consequently, improving its security level. In the proposed cipher, the dynamic parameters are the number of registers, the length of the registers, the clocking system, the initialization procedure, the confusion and diffusion method, and the output function. These parameters are specified using bits from the secret key. The components of the proposed algorithm were tested using the NIST test suite in order to readjust them to improve the results. The NIST test suite was conducted for several keys. Afterward, fine tuning was conducted to enhance the statistical properties of the generated keystream. Then, performance and security analyses were conducted to evaluate the proposed algorithm. Furthermore, a comparison between the proposed algorithm and the seven winners of the eSTREAM competition in terms of the statistical properties was performed. The results showed that the proposed algorithm outperforms the compared ciphers in the linear complexity test. In addition to this, a performance comparison between the proposed algorithm and Mickey 128 was carried out. The result showed that the proposed algorithm is faster than Mickey 128, despite the fact that it has more dynamic parameters than Mickey 128. Therefore, the proposed algorithm is very promising for practical use.