Secret Can Be Public: Low-Memory AEAD Mode for High-Order Masking

Abstract

We propose a new AEAD mode of operation for an efficient countermeasure against side-channel attacks. Our mode achieves the smallest memory with high-order masking, by minimizing the states that are duplicated in masking. An $s$-bit key-dependent state is necessary for achieving $s$-bit security, and the conventional schemes always protect the entire $s$ bits with masking. We reduce the protected state size by introducing an unprotected state in the key-dependent state: we protect only a half and give another half to a side-channel adversary. Ensuring independence between the unprotected and protected states is the key technical challenge since mixing these states reveals the protected state to the adversary. We propose a new mode $\mathsf{HOMA}$ that achieves $s$-bit security using a tweakable block cipher with the $s/2$-bit block size. We also propose a new primitive for instantiating $\mathsf{HOMA}$ with $s=128$ by extending the SKINNY tweakable block cipher to a 64-bit plaintext block, a 128-bit key, and a $(256+3)$-bit tweak. We make hardware performance evaluation by implementing $\mathsf{HOMA}$ with high-order masking for $d \le 5$. For any $d > 0$, $\mathsf{HOMA}$ outperforms the current state-of-the-art $\mathsf{PFB\_Plus}$ by reducing the circuit area larger than that of the entire S-box