Özet:
Stream ciphers are one of the most important classes of encryption algorithms used to ensure security in digital communication. The design of many stream ciphers is based on use of Linear Feedback Shift Registers (LFSRs), due to their simplicity, speed of implementation in hardware and providing sequences with good statistical properties. However, this efficient component is not sufficient when we consider security. The designer should use many nonlinear functions and mechanisms to make the system more resistant against cryptanalysis. A stream cipher should have high period, high linear complexity, good statistical properties and be resistant against most recent successful attacks such as algebraic attacks, correlation attacks, time/memory trade-off attacks, and divide and conquer attacks. In this thesis, a new stream cipher design is proposed. SAFE is designed to be resistant against algebraic and correlation attacks. In the design phase, the objective was to design a stream cipher with good randomness, high period and linear complexity and resistance against many attacks. The innovation in this thesis is the proposal of nonlinear feedback shift registers instead of linear feedback shift registers to provide resistance against correlation and algebraic attacks. In addition, another innovation is the use of a new irregular decimation algorithm, EBSGvariant, for increasing the security of the cipher. Keystream properties of the cipher and its resistance with respect to some well known cryptographic attacks are investigated. From the mathematical expressions and simulation results, it is shown that the cipher produces keystream sequences with satisfying basic security requirements and provides high resistance against well known attack types. Finally, we can say that SAFE can be appropriate for both software and hardware applications due to its simple design.
