It is an internal function so the input validation should have already happened.
The hard coded offsets look like they are part of how the crypto works.

The gap between the speed CPUs can consume data and the rate at which main memory can supply it has been growing wider since main memory was invested. The idea is that fast memory is too expensive to have all you need, so put a bunch of cheaper slower memory over that way so you are not going to disk all the time. This is called the memory hierarchy and is getting deeper while the gap is getting worse. For example look at SSDs and spinning disk. Now we buy SSDs for their speed but if we have a lot of data around still have a disk or tape store off somewhere (even if only for backup).