Invariant Attacks a New Tool for Cryptanalysis Toolset

Abstract

Security of block ciphers has always remained the focus of crypto research in order to establish the degree of confidence we can have on one or an entire family of block ciphers. For long, linear and differential cryptanalytic attacks provided the basis for most of the attacks against block ciphers, however, no major cipher in its full form could be successfully broken. Evolving technological landscape calls for lightweight block ciphers with simpler algorithms, key schedules, and round constants to achieve security as well as economy of size, energy, and cost in modern communication systems. This security compromise creates vulnerability of these lightweight ciphers towards modern attacks e.g., invariant attacks, interpolation attacks, boomerang attacks and many more. Newly introduced Invariant Attacks try to map a single round of an SPN cipher in the form of a polynomial under a weak key setting using a step-by-step approach. Such polynomial must be invariant (unchanging) to the linear and non-linear components over multiple rounds of the underlying cipher. These attacks have successfully been applied to break lightweight SPNs like Midori64, Scream, iScream, Print and more. Until now, individual ciphers were attacked using various forms of invariant attacks e.g., nonlinear invariant attacks, invariant subspace attacks, generalized nonlinear invariant attacks and invariant hopping attacks exploiting numerous vulnerabilities. This thesis will focus on invariant attacks against various ciphers, exploited vulnerabilities and present a cryptanalytic toolset that can be utilized to safeguard a cipher against the invariant attacks family. The toolset will provide a set of properties that if satisfied by all linear and non-linear components of a cipher in general and S-Box component in particular, will provide safety against these attacks.