Enhancing the Related-Key Security of PIPO Through New Key Schedules

In this paper, we present new key schedules for the PIPO block cipher that enhance its security in the related-key setting. While PIPO has demonstrated noteworthy resistance against attacks in the single-key setting, its security in the related-key setting is very vulnerable owing to its simple key schedule. Given the lightweight property of PIPO, we tweak the key schedule algorithm of PIPO by applying computation only within a single register or from one register to another in key states. By adopting our new key schedules, the tweaked version of PIPO achieves better resistance to related-key attacks and demonstrates competitive implementation results in an 8-bit AVR environment. We expect that this paper will contribute to a better understanding of the PIPO block cipher.