KLEIN: A New Family of Lightweight Block Ciphers. Resource-efficient cryptographic primitives are essential for realizing both security and efficiency in embedded systems like RFID tags and sensor nodes. Among those primitives, lightweight block cipher plays a major role as a building block for security protocols. In this paper, we describe a new family of lightweight block ciphers named KLEIN, which is designed for resource-constrained devices such as wireless sensors and RFID tags. Compared to related proposals, KLEIN has advantage in the software performance on legacy sensor platforms, while its hardware implementation can be compact as well.