Cryptanalysis of the full 8.5-round REESSE3+ block cipher

Abstract

This paper describes the first independent cryptanalysis of the full 8.5-round REESSE3+ block cipher, a large-block variant of the IDEA cipher. We show that large classes of weak keys exist in REESSE3+, just like in IDEA, under differential and linear attacks. Moreover, doubling the number of rounds is not enough to avoid weak keys. The existence of weak keys jeopardizes the use of REESSE3+ as a building block in the construction of other cryptographic primitives such as hash functions in modes such as Davies-Meyer’s. We also describe square and impossible differential attacks on reduced-round versions.