On the Importance of Protecting Δ in SFLASH against Side Channel Attacks

Katsuyuki OKEYA  Tsuyoshi TAKAGI  Camille VUILLAUME  

IEICE TRANSACTIONS on Fundamentals of Electronics, Communications and Computer Sciences   Vol.E88-A   No.1   pp.123-131
Publication Date: 2005/01/01
Online ISSN: 
DOI: 10.1093/ietfec/e88-a.1.123
Print ISSN: 0916-8508
Type of Manuscript: Special Section PAPER (Special Section on Cryptography and Information Security)
Category: Tamper-Resistance
asymmetric signature,  SFLASH,  side channel attacks,  SHA-1,  C*,  C*--,  hidden monomial problem,  

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SFLASH was chosen as one of the final selection of the NESSIE project in 2003. It is one of the most efficient digital signature scheme and is suitable for implementation on memory-constrained devices such as smartcards. Side channel attacks (SCA) are a serious threat to memory-constrained devices. If the implementation on them is careless, the secret key may be revealed. In this paper, we experimentally analyze the effectiveness of a side channel attack on SFLASH. There are two different secret keys for SFLASH, namely the proper secret key (s,t) and the random seed Δ used for the hash function SHA-1. Whereas many papers discussed the security of (s,t), little is known about that of Δ. Steinwandt et al. proposed a theoretical DPA for finding Δ by observing the XOR operations. We propose another DPA on Δ using the addition operation modulo 232, and present an experimental result of the DPA. After obtaining the secret key Δ, the underlying problem of SFLASH can be reduced to the C* problem broken by Patarin. From our simulation, about 1408 pairs of messages and signatures are needed to break SFLASH. Consequently, SHA-1 must be carefully implemented in order to resist SCA on SFLASH.