International Association
for Cryptologic Research

We present a construction, called Kirby, for building a variable-input-length pseudorandom function (VIL-PRF) from a $b$-bit permutation. For this construction we prove a tight bound of $b/2$ bits of security on the PRF distinguishing advantage in the random permutation model and in the multi-user setting. Similar to full-state keyed sponge/duplex, it supports full-state absorbing and additionally supports full-state squeezing, where the latter can at most squeeze $b-c$ bits per permutation call for a security level of $c$ bits. This advantage is especially relevant on constrained platforms when using a permutation with small width $b$. For instance, for $b=256$ at equal security strength the squeezing rate of Kirby is twice that of keyed sponge/duplex. We define a simple mode on top of Kirby that turns it into a deck function with parallel expansion. This deck function is suited for lightweight applications in the sense that it has a low memory footprint. Moreover, for short inputs it can be used for low-latency stream encryption: the time between the availability of the input and the keystream is only a single permutation call. Another feature that sets Kirby apart from other constructions is that leakage of an intermediate state does not allow recovering the key or $\textit{earlier states}$.