Cryptanalysis of Full-Round Magpie Block Cipher

Magpie is a lightweight block cipher proposed by Li et al. in Acta Electronica Sinica volumn 45, issue 10. It adopts an substitution-permutation network (SPN) structure with a block size of 64 bits and the key size of 96 bits, respectively. To achieve the consistency of the encryption and decryption, which is both hardware and software friendly, 16 bits of the key are used as control signals to select S-boxes and another 16 bits of the key are used to determine the order of the operations. As the designers claimed, the security might be improved as different keys generate different ciphers. This paper analyzes the security of Magpie, studies the difference propagation of Magpie, and finally finds that the cipher has a set of 2 ⁸⁰ weak keys which makes the full-round encryption weak, and corrects the lower bound of the number of active S-boxes to 10 instead of 25 proposed by the designers. In the weak key model, the security of the cipher is reduced by the claimed 2 ⁸⁰ to only $4\times 2^{16}$ .