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.

A Supplementary Plots

Fig. 9.

Differential power traces $\overline{E}(b_{l,\mathsf {SB}(0)})$ - $\overline{E}(b'_{l,\mathsf {SB}(0)})$ on the 8-bit ATXMEGA128D4 platform for different active state columns.

Fig. 10.

Differential power traces $\overline{E}(b_{i,\mathsf {AK}(1)})$ - $\overline{E}(b'_{i,\mathsf {AK}(1)})$ for $0 \le i \le 3$ on the 32-bit STM32F303 platform with a single inactive byte in the first column.