A fault attack against the FOX cipher family

Since its first introduction, differential fault analysis has proved to be one of the most effective techniques to break a cipher implementation. In this paper, we apply a fault attack to a generic implementation of the recently introduced FOX family of symmetric block ciphers (also known as Idea Nxt). We show the steps needed to mount an effective attack against FOX-64. Although the basic characteristics of this cipher are similar to those of AES, FOX uses a non-invertible key schedule which makes it necessary to use a different attack plan. We also estimate the number of faulty ciphertexts required to reveal the secret key. Our results can be easily extended to other variations of the cipher that use longer inputs and keys.