Variable size block ciphers

Abstract

Data blocks of various size can be processed in fast, effective block cipher mechanisms. Variable size confusion layers and variable size diffusion layers combine to form variable size block ciphers. Block size can set (typically byte-by-byte) at design time or dynamically during operation. The embodiment of FIG. 1 consists of horizontal layers alternating between confusion and diffusion. Here, each confusion layer, such as the layer including byte-wide substitution operation (16), uses keyed substitution tables which are initialized prior to operation. Simple and fast diffusion layers, such as the layer including byte-wide exclusive-OR (20), generally diffuse in just one direction. Consequently, multiple diffusion layers are usually required, such as those including byte-wide exclusive-OR operations (28) and (36). This kind of diffusion is unusual in a block cipher because it is extremely weak. However, because it can cover the entire block in a single layer, it is unexpectedly advantageous when combined with confusion layers. The resulting variable size block ciphers are unusually fast and can be made arbitrarily strong. These ciphers can better fit existing applications, reduce overhead, and directly support large and efficient data blocks. Block size can be expanded dynamically, with good overall diffusion, without adding processing layers. These ciphers support better testing and analysis than previous designs, and also support unexpected new techniques for dynamic keying and authentication.