Method generate_keys

* @brief Generate encryption and decryption key pair * * @param size size of matrix key (typically \f$\text{size}\le10\f$) * @param limit1 lower limit of range of random elements (default=0) * @param limit2 upper limit of range of random elements (default=10) * @return std::pair , matrix > encryption and decryption * keys as a pair * * @see ::