A New Algorithm for the Unbalanced Meet-in-the-Middle Problem

A collision search for a pair of n-bit unbalanced functions (one is R times more expensive than the other) is an instance of the meet-in-the-middle problem, solved with the familiar standard algorithm that follows the tradeoff TM = N , where T and M are time and memory complexities and N = 2. By combining two ideas, unbalanced interleaving and van Oorschot-Wiener parallel collision search, we construct an alternative algorithm that follows T M = RN , where M ≤ R. Among others, the algorithm solves the well-known open problem: how to reduce the memory of unbalanced collision search.