Generalization and Extension of XEX^* Mode

Abstract

This paper describes an extension of XEX^* mode, which is a method to convert a block cipher into a tagged tweakable block cipher, a notion introduced by Rogaway in 2004 as an extension of the tweakable block cipher by Liskov et al. Our extension attaches an additional encryption function to the original XEX^*, which has some limitation but is slightly faster than the encryption implemented by XEX^*. We prove our scheme's security in a general form, where the offset function, a key component of our construction, is not restricted to the one used by XEX^*. We also provide some applications of our result, in particular to OCB 2.0, an authenticated encryption based on XEX^*.