Given positive integers a 1 , . . . , an, t, the fixed weight subset sum problem is to find a subset of the a i that sum to t, where the subset has a prescribed number of elements. It is this problem that underlies the security of modern knapsack cryptosystems, and solving the problem results directly in a message attack. We present new exponential algorithms that do not rely on lattices, and hence will be applicable when lattice basis reduction algorithms fail. These algorithms rely on a generalization of the notion of splitting system given by Stinson [18]. In particular, if the problem has length n and weight ℓ then for constant k a power of two less than n we apply a k-set birthday algorithm to the splitting system of the problem. This randomized algorithm has time and space complexity that satisfies T • S log k = O( n ℓ ) (where the constant depends uniformly on k). In addition to using space efficiently, the algorithm is highly parallelizable.