Cryptanalysis of the Ajtai-Dwork Cryptosystem

Recently, Ajtai discovered a fascinating connection between the worst-case complexity and the average-case complexity of some wellknown lattice problems. Later, Ajtai and Dwork proposed a cryptosystem inspired by Ajtai’s work, provably secure if a particular lattice problem is difficult in the worst-case. We present a heuristic attack (to recover the private key) against this celebrated cryptosystem. Experiments with this attack suggest that in order to be secure, implementations of the Ajtai-Dwork cryptosystem would require very large keys, making it impractical in a real-life environment. We also adopt a theoretical point of view: we show that there is a converse to the Ajtai-Dwork security result, by reducing the question of distinguishing encryptions of one from encryptions of zero to approximating some lattice problems. In particular, this settles the open question regarding the NP-hardness of the Ajtai-Dwork cryptosystem: from a recent result of Goldreich and Goldwasser, our result shows that breaking the Ajtai-Dwork cryptosystem is not NP-hard, assuming the polynomial-time hierarchy does not collapse.