Even though it is not known whether one-way functions exist, there are several candidate functions used in practice for cryptographic applications that are efficiently computable but are conjectured to be hard to invert (such as modular exponentiation, the Rabin function etc.) Moreover, many of these functions are relatively simple to compute in some sense.
Are there any simple candidates for strongly pseudorandom permutations? By a simple candidate, I mean one with small circuit complexity (or maybe small complexity under some different complexity measure). For example, are there any candidates for PRPs that can be computed by $NC^1$ circuits? (I am looking for PRPs which work with arbitrarily long inputs, as opposed to block ciphers operating on fixed-length strings.)