Related-Key Differential Attacks on Cobra-S128, Cobra-F64a, and Cobra-F64b

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Data-dependent permutations (DDPs) which are very suitable for cheap hardware implementations have been introduced as a cryptographic primitive. Cobra-S128 and Cobra-F64 (which is a generic name for Cobra-F64a and Cobra-F64b) are 128-bit and 64-bit iterated block ciphers with a 128-bit key size based on such DDPs, respectively. Unlike the predecessor DDP-based ciphers [16,5], Cobra-S128 is a software-oriented cipher and Cobra-F64 is a firmware-suitable cipher. In this paper, we derive several structural properties of Cobra-S128 and Cobra-F64 and then use them to devise key recovery attacks on Cobra-S128 and Cobra-F64. These works are the first known attacks on Cobra-S128 and Cobra-F64.

This research was supported by the MIC(Ministry of Information and Communication), Korea, under the ITRC(Information Technology Research Center) support program supervised by the IITA(Institute of Information Technology Assessment).