On the tropical discrete logarithm problem and security of a protocol based on tropical semidirect product

Tropical linear algebra has been recently put forward by Grigoriev and Shpilrain as a promising platform for implementation of protocols of DiffieHellman and Stickel type. Based on the CSR expansion of tropical matrix powers, we suggest a simple algorithm for the following tropical discrete logarithm problem: "Given that A = V circle times F-circle times t for a unique t and matrices A, V, F of appropriate dimensions, find this t." We then use this algorithm to suggest a simple attack on a protocol based on the tropical semidirect product. The algorithm and the attack are guaranteed to work in some important special cases and are shown to be efficient in our numerical experiments.