High-speed architectures for binary-tree based stream ciphers: Leviathan case study

Abstract Real-time applications such as streaming media and voice require encryption algorithms that do not propagate errors and support fast encryption on small blocks. Since IP packets are delivered out-of-order in routed networks it is difficult to synchronize the source and the destination, therefore requiring encryption algorithms to support out-of-order generation of key stream. In this paper we investigate high-speed parallel and pipeline architectures for Leviathan, a binary tree based synchronous stream cipher that does not propagate errors and supports ordered and out-of-order key stream generation. The parallel architecture bounds the worst-case variance in the time between consecutive key words and the pipeline architecture generates key words at a uniform rate. In order to eliminate the storage overhead associated with straightforward preorder traversal, we developed a novel traversal scheme that intertwines inorder traversal with preorder traversal. We implemented pipeline architecture using Xilinx FPGAs to support 1 Gbps key stream generation rate.