[Cryptography] Simple non-invertible function?

[John Kelsey]

The simplest one way function I can think of is

F(x) = E(x,0)

That is, use the input as a block cipher key and encrypt a counter with it.  You could use a stream cipher in a similar way--use the input to your one way function as a key and generate n bits of key stream.  

Neither of these are super lightweight, but they do give you one way functions whose security comes from the underlying crypto primitive.

--John