Qubit Optimized Quantum Implementation of SLIM
Hasan Ozgur Cildiroglu, Oguz Yayla
TL;DR
Threats from quantum computing motivate exploring quantum-resistant encryption, especially lightweight block ciphers. The authors design a quantum circuit for SLIM by exploiting the Feistel structure and reversing the KSP process to minimize qubits, achieving $112$ qubits and a quantum cost of $27{,}220$ with depth $4{,}066$. They implement the S-box via the LIGHTER-R reversible synthesis to avoid ancilla, and realize the P-box with zero-cost SWAPs, resulting in a compact, ancilla-free encryption in 32 rounds. Compared with other BC quantum implementations (Simon, Rectangle, LBLOCK, Puffin), SLIM offers one of the lowest qubit counts while maintaining competitive depth and cryptographic strength, making it a practical candidate for quantum-era encryption.
Abstract
The advent of quantum computing has profound implications for current technologies, offering advancements in optimization while posing significant threats to cryptographic algorithms. Public-key cryptosystems relying on prime factorization or discrete logarithms are particularly vulnerable, whereas block ciphers (BCs) remain secure through increased key lengths. In this study, we introduce a novel quantum implementation of SLIM, a lightweight block cipher optimized for 32-bit plaintext and an 80-bit key, based on a Feistel structure. This implementation distinguishes itself from other BC quantum implementations in its class (64-128-bit) by utilizing a minimal number of qubits while maintaining robust cryptographic strength and efficiency. By employing an innovative design that minimizes qubit usage, this work highlights SLIM's potential as a resource-efficient and secure candidate for quantum-resistant encryption protocols.