AbstractIn lightweight cryptographic primitives, non‐linear feedback shift registers (NFSR) are widely used nowadays. At ASIACRYPT 2010, conditional differential cryptanalysis was proposed to analyze NFSR‐based cryptosystems. To get better result, we propose an improved version of this attack. We apply this new method to analyze the security of lightweight hash functions QUARK family of ciphers, including the recently proposed C‐QUARK. Using the new method, for an advantage of 0.68, our results for U‐QUARK, D‐QUARK, S‐QUARK, and subsequently C‐QUARK are, respectively, 121(22)/131(213)/153(218),143(22)/154(212)/159(222),216 (22)/236(218)/248(224),421(22)/436(214)/445(220), whereas a(b) means that with data complexity b, we can attack a rounds of the cipher. All these results are first cryptanalytic results known thus far for QUARK family of ciphers and have been achieved by experiment in practical time. Copyright © 2014 John Wiley & Sons, Ltd.