My summary of our discussion was pretty terse. :)  dmalcolm has more detailed TODO list notes that include things like the error cases and .rst documentation.

As for how to commit it, i'd make 0001 its own commit as it is a useful refactoring otherwise unrelated to this change.

I'll leave it entirely up to dmalcolm how many commits he wants 0002 onward to be.  No need to be picky.

usedforsecurity vs used_for_security, agreed, used_for_security is better.

dmalcolm was going to make an enum to index the two element array, that'd give meaningful names instead of 0 or 1.  simply using two named variables would also work but it would require the loop in 0003 to be expanded or turned into a small static method for the body (not a bad idea) instead.  i'm fine with either.