Secure and Efficient Diffusion Layers for Block Ciphers

Abstract

Modern block ciphers are designed to meet confusion and diffusion criteria. Substitution and permutation layers are used in the round function for this purpose. In this paper, we present a number of choices for diffusion layer by using circular shift and XOR operations. These two operations are most efficient for software implementations. We test all possible combinations of circular shift and XOR operations for 16-bit and 32-bit words. We search for optimal number of circular shifts to provide the maximum value of branch number. We also search for secure diffusion layers with efficient inversion for SPN based block ciphers. We categorize the diffusion layers according to the value of their branch number. Finally, we suggest a list of secure and efficient diffusion layers for new block cipher proposals. We also compare the security of a model block cipher using one of these diffusion layers with other lightweight block ciphers.

Keywords

• Block Cipher
• Branch Number
• Diffusion Layer
• Permutation