Cryptanalysis of a System based on Twisted Dihedral Group Algebras

Abstract: Several cryptographic protocols constructed based on less-known algorithmic problems, such as those in non-commutative groups, group rings, semigroups, etc., which claim quantum security, have been broken through classical reduction methods within their specific proposed platforms. A rigorous examination of the complexity of these algorithmic problems is therefore an important topic of research. In this paper, we present a cryptanalysis of a public key exchange system based on a decomposition-type problem in the so-called twisted group algebras of the dihedral group $D_{2n}$ over a finite field $\fq$. Our method of analysis relies on an algebraic reduction of the original problem to a set of equations over $\fq$ involving circulant matrices, and a subsequent solution to these equations. Our attack runs in polynomial time and succeeds with probability at least $90$ percent for the parameter values provided by the authors. We also show that the underlying algorithmic problem, while based on a non-commutative structure, may be formulated as a commutative semigroup action problem.