The chain and sum primitive and its applications to MACs and stream ciphers

Abstract

We present a new scheme called universal block chaining with sum (or chain & sum primitive (C&S) for short), and show its application to the problem of combined encryption and authentication of data. The primitive is a weak CBC-type encryption along with a summing step, and can be used as a front end to stream ciphers to encrypt pages or blocks of data (e.g., in an encrypted file system or in a video stream). Under standard assumptions, the resulting encryption scheme provably acts as a random permutation on the blocks, and has message integrity features of standard CBC encryption. The primitive also yields a very fast message authentication code (MAC), which is a multivariate polynomial evaluation hash. The multivariate feature and the summing aspect are novel parts of the design. Our tests show that the chain & sum primitive adds approximately 20 percent overhead to the fastest stream ciphers.