Improved cascade stream ciphers using feedback

Most stream ciphers in the literature have been designed based on linear feedback shift registers, but most of these have been attacked. This paper examines the security of a family of stream ciphers called cascade stream ciphers (CSCs) and proposes two improved versions. A cascade stream cipher consists of a number of small RC4 cells that are cascaded to generate the keystream. The CSCs possess several good properties such as simplicity, scalability, and efficiency in hardware. In the initial version of the CSC, the output of the last cell was directly used as the keystream. In this paper, we propose a backward state transition (BST) attack that exploits this structure and is able to break the cipher. To defeat the BST attack, two modifications of the CSC structure are proposed. In the modified CSC, outputs from two cells are combined, and the output of the last cell is used in a feedback loop. The improved structure is immune to the BST attack and has a cycle distribution closer to that of a random permutation