turkmath.org

In this talk, I open a discussion on how to design secure stream ciphers whose internal state sizes are less than twice of their key sizes, which we call SISS (Small Internal State Stream) ciphers. In general, SISS ciphers in industry have notoriously bad reputation in terms of security, such as A5/1, E0, Hitag2. On the other hand, there are not sufficiently many studies in the literature about how to design and analyze SISS ciphers. The main reason is the criterion that internal states must be at least twice as large as the key sizes due to tradeoff attacks. In this talk, I will address the consequences of this strict criterion. I argue that the tradeoff attacks should have the same security threshold, regardless of key or internal state recovery. I introduce a new strategy to design SISS ciphers by introducing a new construction method for diffusion layers of SPN ciphers with its security proofs and then I introduce a new SISS cipher family having unkeyed state update, which we call DIZY, to embody our construction method.