On the security ofPiccolo lightweight block cipher against related-key impossible differentials

International audiencetextit{Piccolo} is a new lightweight block cipher proposed at CHES 2011~\cite{piccolo}. It ciphers plaintext blocks of length 64 bits under keys of lengths 80 or 128 bits. It is based on a modified Feistel structure with a variable number of rounds. In this paper, we present two related key impossible differential attacks against 14 rounds of Piccolo-80 and 21 rounds of Piccolo-128 without the whitening layers. The attack against Piccolo-80 has a time and data complexity of 2^68.19 whereas the time/data complexity of the attack against Piccolo-128 is 2^117.77. Keywords: Lightweight block ciphers, Piccolo, related-key impossible differential attack