The need for lightweight (that is, compact, low-power, low-energy) cryptographic hash functions has been repeatedly expressed by application designers, notably for implementing RFID protocols. However not many designs are available, and the SHA-3 Competition probably will not help, as it concerns general-purpose designs and focuses on software performance. This paper thus proposes a novel design philosophy for lightweight hash functions, based on the sponge construction in order to minimize memory requirements. Inspired by the stream cipher Grain and by the block cipher KATAN (among the lightest schemes in their respective categories), we present the hash function family Quark, composed of three instances: u-Quark, d-Quark, and s-Quark. Our hardware evaluation shows that Quark compares well to previous lightweight hash functions. For example, our lightest instance uQuark conjecturally provides at least 64-bit security against all attacks (collisions, multicollisions, distinguishers, et...