Paper 2011/423

Linear Cryptanalysis of PRINTcipher --- Trails and Samples Everywhere

Martin Ågren and Thomas Johansson

Abstract

PRINTcipher is a recent lightweight block cipher designed by Knudsen et al. Some noteworthy characteristics are a burnt-in key, a key-dependent permutation layer and identical round keys. Independent work on PRINTcipher has identified weak key classes that allow for a key recovery --- the obvious countermeasure is to avoid these weak keys at the cost of a small loss of key entropy. This paper identifies several larger classes of weak keys. We show how to distinguish classes of keys and give a $28$-round linear attack applicable to half the keys. We show that there are several similar attacks, each focusing on a specific class of keys. We also observe how some specific properties of PRINTcipher allow us to collect several samples from each plaintext--ciphertext pair. We use this property to construct an attack on $29$-round PRINTcipher applicable to a fraction $2^{-5}$ of the keys.

Note: Substantially revised.