Comment by aftbit

>Maybe this is silly to you. But framing the problem as “come up with a function that looks like it adds entropy but is reversible” was a lot easier for me to think about than “preserve bijectivity between these two sets.

IMO that's half of "real work" - figuring out how to think about a problem. Insights are funny that way - stuff that is obvious to one person is game-changing to others.