Systematic Security Analysis of Stream Encryption With Key Erasure

Abstract

We consider a generalized construction of stream ciphers with forward security. The design framework is modular: it is built from a so-called layer function that updates the key and (optionally) the nonce and generates a new pseudorandom output stream. We analyze the generalized construction for four different instantiations: two possible layer functions that are in turn instantiated with either a block cipher or a pseudorandom function. We prove that each of these instantiations gives a stream cipher that is pseudorandom and forward secure in the multi-user setting with a very tight bound. A comprehensive analysis shows that the two block cipher based instantiations achieve very similar bounds. For the pseudorandom function based instantiations there is no clear winner: either layer can be beneficial over the other one, depending on the choice of parameters. By instantiating the pseudorandom function with a generic construction such as the sum of permutations, we obtain a highly efficient and competitive stream cipher based on an nbit block cipher that is secure beyond the 2 n/2 birthday bound.