What does it matter if it looks random? It's being fed into sha512 in `from_master_secret` and then after that the seed is gone forever. Since you can't reverse sha512, you can't know what was fed in and there's nothing to reveal that it's only 3 chars (`abc` in your example).

I might be mis-understanding something here though?

Another way to think about it: in password storage, does anybody hash their salt before using it? I don't think so.