Paper 2020/640

Grover on SPECK: Quantum Resource Estimates

Kyungbae Jang, Seungjoo Choi, Hyeokdong Kwon, and Hwajeong Seo

Abstract

Grover search algorithm reduces the security level of symmetric key cryptography with $n$-bit secret key to $O(2^{n/2})$. In order to evaluate the Grover search algorithm, the target block cipher should be implemented in quantum circuits. Recently, many research works evaluated required quantum resources of AES block ciphers by optimizing the expensive substitute layer. However, only few works devoted to ARX-based lightweight block ciphers, which are active research area. In this paper, we present optimized implementations of SPECK 32/64 and SPECK 64/128 block ciphers for quantum computers. To the best of our knowledge, this is the first implementation of SPECK in quantum circuits. Primitive operations, including addition, rotation, and exclusive-or, for SPECK block cipher are finely optimized to achieve the optimal quantum circuit, in terms of qubits, Toffoli gate, CNOT gate, and X gate. The proposed method can be applied to other ARX-based lightweight block ciphers, such as LEA, HIGHT and CHAM block ciphers.