Quantum algorithm for the discrete logarithm problem for matrices over finite group rings

We propose a polynomial time quantum algorithm for solving the discrete logarithm problem in matrices over finite group rings. The hardness of this problem was recently employed in the design of a key-exchange protocol proposed by D. Kahrobaei, C. Koupparis, and V. Shpilrain. Our result implies that the Kahrobaei et al. protocol does not belong to the realm of post-quantum cryptography