Fixed-Key-AES Hashes

Using fixed-key AES as a hash function in this context can be traced back to the work of Bellare et al. BHKR13, who considered fixed-key AES for circuit garbling. Prior to that point, most implementations of garbled circuits used a hash function such as $\mathsf{SHA}256$, modelled as a random oracle. But Bellare et al. showed that using fixed-key AES can be up to $50\times$ faster than using a cryptographic hash function due to hardware support for AES provided by modern processors.

Prior to BHKR13 CPU time was the main bottleneck for protocols based on circuit garbling; after the introduction of fixed-key cipher garbling, network throughput became the dominant factor. For this reason, fixed-key AES has been almost universally adopted in subsequent implementations of garbled circuits.

Several instantiations of hash function based on fix-key AES are proposed inspired by the work of Bellare et al. However, most of them have some security flaws as pointed out by GKWY20. (GKWY20) also proposes a provable secure instantiation satisfies the property called Tweakable Circular Correlation Robustness (TCCR). More discussions about the concrete security of fixed-key AES based hash are introduced in GKWWY20.

The TCCR hash based on fixed-key AES is defined as follows.

$$\mathsf{H}(i,x)=TCCR\_Hash(i,x)=\pi (\pi (x)\oplus i)\oplus \pi (x),$$ where $x$ is a $128$-bit string, $i$ a public $128$-bit $\mathsf{tweak}$, and $\pi (x)=\mathsf{AES}(k,x)$ for any fixed-key $k$.