## Cryptology ePrint Archive: Report 2015/033

On the Security of Fresh Re-keying to Counteract Side-Channel and Fault Attacks

Christoph Dobraunig and Maria Eichlseder and Stefan Mangard and Florian Mendel

Abstract: At AFRICACRYPT 2010 and CARDIS 2011, fresh re-keying schemes to counter side-channel and fault attacks were introduced. The idea behind those schemes is to shift the main burden of side-channel protection to a re-keying function $g$ that is easier to protect than the main block cipher. This function produces new session keys based on the secret master key and random nonces for every block of message that is encrypted. In this paper, we present a generic chosen-plaintext key-recovery attack on both fresh re-keying schemes. The attack is based on two observations: Since session key collisions for the same message are easy to detect, it is possible to recover one session key with a simple time-memory trade-off strategy; and if the re-keying function is easy to invert (such as the suggested multiplication constructions), the attacker can use the session key to recover the master key. The attack has a complexity of about $2 \cdot 2^{n/2}$ (instead of the expected $2^n$) for an $n$-bit key. For the typically employed block cipher AES-128, this would result in a key-recovery attack complexity of only $2^{65}$. If weaker primitives like 80-bit PRESENT are used, even lower attack complexities are possible.

Category / Keywords: secret-key cryptography / side-channel attacks, fresh re-keying, key-recovery attack

Original Publication (in the same form): CARDIS 2014