On modes of operation

Abstract

In this paper we study the modes of operation in which a cryptosystem, and in particular DES, can be used. This study shows that attempts to complicate the modes of operation weaken (in many cases) the resultant modes. We conclude that operation modes should be designed around the underlying cryptosystem without any attempt to use intermediate data as feedback, or to mix the feedback into an intermediate round. Thus, in particular, triple-DES used in CBC mode is more secure than a single-DES used in triple-CBC mode. Alternatively, if several encryptions are applied to each block, the best choice is to concatenate them to one long encryption, and build the mode of operation around it.