A Fault-Injection Attack on Fiat-Shamir Cryptosystems Artemios G. Voyiatzis and Dimitrios N. Serpanos Computer Systems Laboratory Department of Electrical and Computer Engineering University of Patras GR-26504 Rion Patras Greece {bogart,serpanos}@ee.upatras.gr Abstract Fault-injection attacks and cryptanalysis is a realistic threat for systems implementing cryptographic algorithms. We revisit the fault-injection attacks on the Fiat-Shamir au- thentication scheme, a popular authentication scheme for service providers like pay per view television, video distri- bution and cellular phones. We present a new and effective attack on cryptosystems that implement the Fiat-Shamir identification scheme. The attack is successful against all system configurations in con- trast to the original Bellcore attack, which has been proven incomplete (easy to defend against). 1. Introduction System parameters measured during execution of crypto- graphic algorithms can be exploited by an attacker in order to discover the secret keys used [11] [6]. The field of im- plementation or side-channel cryptanalysis has drawn sig- nificant attention by the security community. In contrast to classical (mathematical) cryptanalysis, implementation cryptanalysis targets implementations of cryptographic al- gorithms. Side-channels, not covered by the mathematical model of the algorithms, transmit to the environment infor- mation for the secret keys employed in a cryptographic op- eration. Appropriate analysis of this information can be uti- lized to extract the whole keys and thus render insecure the specific cryptographic system. Implementation cryptanalysis can be categorized in two classes of attacks: passive and active. In passive attacks, the side-channel is a measurable parameter of the implemen- tation. Examples include algorithm execution time [11], power consumption [12], and EM radiation [10], [1]. In all cases, the attacker does not physically alter the system un- der attack, but only collects information from it with exter- nal measurement equipment. In active (or fault-injection) attacks, the attacker injects hardware faults, such as flip bits in memory, which lead to undetectable erroneous output. Appropriate use of erroneous output by the attacker can lead to full disclosure of system secret keys. Fault injection in cryptographic devices can be realized, for example, by op- eration in extreme conditions, as described in [2],[9]. The applicability and practicality of implementa- tion cryptanalysis and, especially, for the active attacks has been questioned [13]. Research results demonstrated that such attacks are indeed feasible [4]. Furthermore, the ap- plicability of timing attacks (a case of passive attacks) was expanded to Internet systems and, more specifically, to se- cure web servers utilizing the OpenSSL cryptographic li- brary [8]. Thus, implementation cryptanalysis is a real threat, and appropriate countermeasures must be em- ployed to protect cryptosystems. Most popular cryptographic algorithms have been shown vulnerable to fault-injection attacks: RSA us- ing Chinese Remainder Theorem, RSA using Montogom- mery arithmetic, the Schnorr identification scheme and the Fiat-Shamir authentication scheme [6]; DES and other sym- metric key cryptosystems [5]; and lately AES [9]. We revisit the active attack on the Fiat-Shamir authen- tication scheme. In [14], we proved that the Bellcore at- tack [6] is incomplete, since it is based on an assumption that does not always hold. Thus, we proved that there ex- ist Fiat-Shamir systems configuration which defend the at- tack and resist such kind of cryptanalysis. In this paper, we propose a new fault-injection attack, which is provably valid against all Fiat-Shamir configura- tions, under stronger assumptions. This new attack is not only successful but efficient and realistic for typical envi- ronments, such as smart cards. The paper is organized as follows. Section 2 describes briefly the Fiat-Shamir identification scheme, the Bellcore attack and its fault insertion model, and the proposed de- Proceedings of the 24th International Conference on Distributed Computing Systems Workshops (ICDCSW’04) 0-7695-2087-1/04 $20.00 © 2004 IEEE