Sharif Digital Repository

Stream ciphers are cryptographic schemes being widely used in communications as a means of providing security of data. Hence, scrutinizing their level of security against different cryptanalytic methods has obviously turned to be of great importance. Randomness of the keystream is an important requirement for stream ciphers. The cipher should be designed in such a way that the keystream is free of any statistical biases that would let attackers distinguish it from a random sequence. A distinguishing attack on a stream cipher is one that determines whether a given stream of symbols is a truly random sequence or generated by a specific cipher with an unknown key. In this thesis, we focus on...