Protecting White-Box AES with Dual Ciphers

In order to protect AES software running on untrusted platforms, Chow et al. (2002) designed a white-box implementation. However, Billet et al. (2004) showed that the secret key can be extracted with a time complexity of 2(30). In this paper, we present an improved white-box implementation of AES. We use dual ciphers to modify the state and key representations in each round as well as two of the four classical AES operations, SubBytes and MixColumns. We show that, with 61200 possible dual ciphers the complexity of Billet et al. attack is raised to 2(91). Interestingly, our white-box implementation does not require more memory space than that of Chow et at implementation.