Stairway To Rainbow

Abstract

A cryptanalytic time-memory trade-off is a technique introduced by M. Hellman in 1980 to perform brute-force attacks. It consists of a time-consuming precomputation phase performed and stored once and for all, which is then used to reduce the computation time of brute-force attacks. A variant, known as rainbow tables, introduced by Oechslin in 2003 is used by most of today’s off-the-shelf password-guessing tools. Precomputation of such tables is highly inefficient however, because much of the values computed during this task are eventually discarded. This paper revisits rainbow tables precomputation, challenging what has so far been regarded as an immutable foundation. The key idea consists in recycling values discarded during the precomputation phase, and adapting the brute force phase to make use of these recycled values. For a given memory and probability of success, the stepped rainbow tables thus created significantly reduce the workload induced by both the precomputation phase and the attack phase. The speedup obtained by using such tables is provided, and backed up by practical experiments.