A Tutorial on White-box AES

White-box cryptography concerns the design and analysis of implementations of cryptographic algorithms engineered to execute on untrusted platforms. Such implementations are said to operate in a white-box attack context. This is an attack model where all details of the implementation are completely visible to an attacker: not only do they see input and output, they see every intermediate computation that happens along the way. The goal of a white-box attacker when targeting an implementation of a cipher is typically to extract the cryptographic key; thus, white-box implementations have been designed to thwart this goal (i.e., to make key extraction difficult/infeasible). The academic study of white-box cryptography was initiated in 2002 in the seminal work of Chow, Eisen, Johnson and van Oorschot (SAC 2002). Here, we review the first white-box AES implementation proposed by Chow et al. and give detailed information on how to construct it. We provide a number of diagrams that summarize the flow of data through the various look-up tables in the implementation, which helps clarify the overall design. We then briefly review the impressive 2004 cryptanalysis by Billet, Gilbert and Ech-Chatbi (SAC 2004). The BGE attack can be used to extract an AES key from Chow et al.’s original white-box AES implementation with a work factor of about 2, and this fact has motivated subsequent work on improved AES implementations.