Weak Tweak-Keys for the CRAFT Block Cipher

Abstract

CRAFT is a lightweight tweakable Substitution-Permutation-Network (SPN) block cipher optimized for efficient protection of its implementations against Differential Fault Analysis (DFA) attacks. In this paper, we present an equivalent description of CRAFT up to a simple mapping on the plaintext, ciphertext and round tweakeys. We show that the new representation, for a sub-class of keys, leads to a new structure which is a Feistel network, with non-linear operation and key addition only on half the state. Consequently, it reveals a class of weak keys for which CRAFT is less resistant against differential and linear cryptanalyses. As a result, we present one weak-key single-tweak differential attack on 23 rounds (with time complexity of 294 encryptions and data complexity of 2⁷⁴ chosen plaintext/tweak/ciphertext tuples and works for 2¹¹² weak keys) and one weak-key related-tweak attack on 26 rounds of the cipher (with time complexity of 2¹⁰⁵ encryptions and data complexity 2⁷³ chosen plaintext/tweak/ciphertext tuples and works for 2¹⁰⁸ weak keys). Note that these attacks do not break the security claim of the CRAFT block cipher.