A Biased Fault Attack on the Time Redundancy Countermeasure for AES Sikhar Patranabis, Abhishek Chakraborty, Phuong Ha Nguyen and Debdeep Mukhopadhyay Department of Computer Science and Engineering IIT Kharagpur, India sikharpatranabis@gmail.com, abhishek.chakraborty@cse.iitkgp.ernet.in, phuongha@gmail.com, debdeep@cse.iitkgp.ernet.in Abstract. In this paper we propose the first practical fault attack on the time redundancy countermeasure for AES using a biased fault model. We develop a scheme to show the effectiveness of a biased fault model in the analysis of the time redundancy countermeasure. Our attack requires only faulty ciphertexts and does not assume strong adversarial powers. We successfully demonstrate our attack on simulated data and 128-bit time redundant AES implemented on Xilinx Spartan-3A FPGA. Keywords: Cryptanalysis, Time Redundancy, Biased Faults, AES 1 Introduction Implementation attacks on secure embedded systems come in different flavors. One of these is the Side-Channel Analysis (SCA) such as Differential Power Analysis [8]. The other popular variety is the active Fault Analysis (FA) involving injection of faults into cryptographic systems and analysis under different fault models [2]. Attacks such as the Differential Fault Intensity Analysis (DFIA) [4] have in fact combined DPA with fault injection principles to obtain biased fault models. The advantage of a biased fault model lies in the ability of the adversary to derive an intermediate key-dependent state variable under several key hypotheses. The correct key hypothesis produces small changes to the faulty state while incorrect ones infer big, random changes. This work attacks the time redundancy countermeasure using a biased fault model. The model is not as strict as some proposed earlier, such as stuck-at-zero or stuck-at-one faults [3]. The time redundancy technique is as an effective coun- termeasure, in which an encryption is followed by a redundant encryption, and in the event of a mismatch, the faulty ciphertext is either suppressed or replaced by a random ciphertext. Literature proposes time redundancy as a classical fault tolerance technique [11], [10] with the assumption of a uniform unbiased fault distribution. For a time redundant AES, in order to obtain the faulty cipher- text, the adversary must introduce exactly the same fault in both the actual