[Cryptography] Simple non-invertible function?

[Sandy Harris]

I wrote:

> The world's simplest block cipher is just IDEA multiplication of
> plaintext and key, and that could be made non-invertible with an XOR.
> The small block size (16 bits) does not appear to matter in my
> application and a transform that mixes those blocks could easily be
> added just in case it does matter.

My current code uses idea_multiply(x, key) xor x which I think
is non-invertible. Then there's a GCM hash mixing step so I do
not think the 16-bit blocks are a problem.

Comment?