> 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).

Very good point. You're right that, since the final step is sha512 anyway, it doesn't matter if the extra user entropy is only a couple characters. My argument is clearly wrong.

Alright, go ahead and remove that extra hashing step.