SHOT: Lightweight block cipher for legacy-compatible secure storage in bounded-lifetime IoT deployments

Many deployed internet of things (IoT) devices used for medical sensor logging, industrial telemetry, and smart metering require block-cipher-based random-access encrypted storage on 8-bit microcontrollers. Conventional 128-bit block ciphers impose significant flash and random-access memory (RAM) overhead on 8-bit platforms, introducing up to 100% padding overhead for short sensor records, while modern authenticated encryption with associated data (AEAD) schemes built from sponge permutations do not support constant-time random-access decryption due to their sequential structure. This work introduces SHOT (Substitution-rotation Hybrid Optimized Transform), an 80-bit block cipher tailored for 8-bit devices and co-designed with the storage format. SHOT integrates a 5-bit nonlinear substitution layer (S-box) and a linear layer inspired by the Ascon permutation into a 32-round Feistel network with a 160-bit key, enabling standard cipher block chaining (CBC) and counter (CTR) modes. Mixed-integer linear programming (MILP) analysis indicates estimated security margins of 17 and 20 rounds for differential and linear cryptanalysis, respectively, within our search limits. A fully unrolled SHOT assembly implementation on an AVR-class microcontroller achieves 384.7 cycles per byte and an energy cost of 330.6 nJ per bit, while a compact field-programmable gate array (FPGA) design sustains 251 Mbps using about 1,013 lookup tables. In a continuous glucose monitoring scenario, SHOT reduces padding overhead for per-record encryption and extends flash logging capacity on the same hardware partition.