Comment on 'Targeted Ciphers for Format-Preserving Encryption' from Selected Areas in Cryptography 2018

National Natural Science Foundation of China, Grant/Award Number: U19B2021, U1736111; National Cryptography Development Fund, Grant/ Award Number: MMJJ20180111; Key Research and Development Programme of Shaanxi, Grant/Award Number: 2020ZDLGY08‐04 Abstract Format‐preserving encryption (FPE) allows encrypting plaintexts while preserving a specific format. In Selected Areas in Cryptography 2018, two targeted ciphers were proposed as new FPE schemes. The second scheme was designed with an algorithm called Mix–Swap–Unmix that is shown to be equivalent to a particular matching exchange process under a specific setting. In this comment paper, we prove that the matching exchange process is invalid. As a result, this equivalence does not exist.