Complete Practical Side-Channel-Assisted Reverse Engineering of AES-Like Ciphers

Abstract

Public knowledge about the structure of a cryptographic system is a standard assumption in the literature and algorithms are expected to guarantee security in a setting where only the encryption key is kept secret. Nevertheless, undisclosed proprietary cryptographic algorithms still find widespread use in applications both in the civil and military domains. Even though side-channel-based reverse engineering attacks that recover the hidden components of custom cryptosystems have been demonstrated for a wide range of constructions, the complete and practical reverse engineering of AES-128-like ciphers remains unattempted. In this work, we close this gap and propose the first practical reverse engineering of AES-128-like custom ciphers, i.e., algorithms that deploy undisclosed SubBytes, ShiftRows and MixColumns functions. By performing a side-channel-assisted differential power analysis, we show that the amount of traces required to fully recover the undisclosed components are relatively small, hence the possibility of a side-channel attack remains as a practical threat. The results apply to both 8-bit and 32-bit architectures and were validated on two common microcontroller platforms.

SCARE of Secret Ciphers under Rough Leakage Model

“…However, there are deviations in our experiment which indicates that the method of [21] and [25] could not be applicable. For example, the frst and second stage of S-box reconstruction in [21] could be afected and attackers would obtain a complete but wrong S-box tree in stage 3.…”

“…It takes about 2 ms to obtain the candidates truth tables and the verifcation time of m! candidate tables when m = 8 is about 990 ms. Caforio et al didn't discuss the time complexity in [25], while the number of required traces to recover the secret Sbox is ten times more than this paper.…”

“…In [21] and [25], S-box is reconstructed based on mathematic analysis. Both methods utilize the relationship between the value of S-box and the HW of the MixColumns intermediate results to flter out wrong candidate S-box output.…”