Cryptanalytic weaknesses in modem stream ciphers and recommendations for improving their security levels

In this paper we discuss security problems in some modern stream ciphers. As we observe some times that a designer claims that the algorithm designed is more secure but when it comes to open literature we find a number of problems. We discuss SNOW, scream and rabbit. Some efforts have been made to overcome the problems those were pointed out in these cryptosystems by different cryptanalystst. The stream ciphers are faster and efficient than block ciphers but comparatively less secure. Our emphasis in this paper is to make some compromise on efficiency but to get more security.