Extended Meet in the Middle Attacks Using Cryptanalytic Approximations as DMZ
摘要
Traditional meet in the middle attacks divide the cipher into two halves, and partially encrypt one half and decrypt the other to search for a collision in the middle. In this paper we show that cryptanalytic techniques such as linear cryptanalysis, scattered linear cryptanalysis and differential cryptanalysis can be used as a third (free) zone in between, and allow to perform the meet in the middle analysis on fewer rounds, thus reduce the meet in the middle complexity. I.e., our new meet in the middle attacks divide the analyzed ciphers into three zones, where the middle zone is non-probabilistically approximated by some cryptanalytic approximation (e.g., linear approximation), and the meet in the middle analysis is performed on the two external zones only. We call the middle zone a free zone or a DMZ as the attack does not actively attack it. We exemplify our attacks using the FEAL cipher, which has relatively “long” non-probabilistic approximations.