In fact, in this case the algorithm is only intended for numbers with 2^4096 bits. See section 5 of the paper: In this section we present the main integer mu

In fact, in this case the algorithm is only intended for numbers with 2^4096 bits. See section 5 of the paper: In this section we present the main integer multiplication algorithm. We actually give a family of algorithms, parameterised by a dimension parameter d > 2. Let n_0 := 2^(d^12) > 2^4096, and suppose that we wish to multiply integers with n bits. For n < n_0, we may use any convenient base-case multiplication algorithm, such as the classical O(n^2) algorithm.