## Cryptology ePrint Archive: Report 2015/144

Security of the AES with a Secret S-box

Tyge Tiessen and Lars R. Knudsen and Stefan Kölbl and Martin M. Lauridsen

Abstract: How does the security of the AES change when the S-box is replaced by a secret S-box, about which the adversary has no knowledge? Would it be safe to reduce the number of encryption rounds?

In this paper, we demonstrate attacks based on integral cryptanalysis which allows to recover both the secret key and the secret S-box for respectively four, five, and six rounds of the AES. Despite the significantly larger amount of secret information which an adversary needs to recover, the attacks are very efficient with time/data complexities of $2^{17}/2^{16}$, $2^{38}/2^{40}$ and $2^{90}/2^{64}$, respectively.

Another interesting aspect of our attack is that it works both as chosen plaintext and as chosen ciphertext attack. Surprisingly, the chosen ciphertext variant has a significantly lower time complexity in the attacks on four and five round, compared to the respective chosen plaintext attacks.

Category / Keywords: secret-key cryptography / AES, integral cryptanalysis, secret S-box

Original Publication (in the same form): IACR-FSE-2015

Date: received 20 Feb 2015, last revised 2 Mar 2015

Contact author: tyti at dtu dk

Available format(s): PDF | BibTeX Citation