A chosen-plaintext linear attack on DES

Abstract

In this paper we consider a chosen-plaintext variant of the linear attack on DES introduced by Matsui. By choosing plaintexts in a clever way one can reduce the number of plaintexts required in a succes-sful linear attack. This reduces the amount of plaintexts to find key bits to a factor of more than four compared to Matsui's attack. To estimate the probabilities of success in the attack we did extensive experiments on DES reduced to 8 and 12 rounds. We believe that the results in this paper contain the fastest attack on the DES reported so far in the open literature. As an example, one attack needs about 242 chosen texts, finds 12 bits of key information and succeeds with a probability of about 86%. An additional 12 key bits can be found by similar methods. For compa-rison, Matsui's attack on the DES needs about 244 known texts, finds 13 bits of the key and succeeds with a probability of 78%. Of independent interest is a new approach searching for \pseudo-keys", which are secret key bits added an unknown but fixed value. These bits can be used to find the secret key bits at a later stage in the analysis.