International Association
for Cryptologic Research

Pseudo-random generators are deterministic algorithms that take in input a random secret seed and output a flow of random-looking numbers. The Knapsack generator, presented by Rueppel and Massey in 1985 is one of the many attempt at designing a pseudo-random generator that is cryptographically secure. It is based on the subset-sum problem, a variant of the Knapsack optimization problem, which is considered computationally hard.

In 2011 Simon Knellwolf et Willi Meier found a way to go around this hard problem and exhibited a weakness of this generator. In addition to be able to distinguish the outputs from the uniform distribution, they designed an algorithm that retrieves a large portion of the secret. We present here an alternate version of the attack, with similar costs, that works on the same range of parameters but retrieves a larger portion of the secret.