Each personality owns a bracelet with a combination lock. To prove you're you, you unlock your bracelet. This is basically the password system, but localized, and now you just have to worry about making combination locks tamper-proof.
Unfortunately, physical locks interact very badly with the magic system. (In brief: "Lockedness" is a thing. If you are about average at magic, it's a thing you can move from one thing you're touching that is locked to another thing you are touching that can be locked but isn't.)
If it's worth saying, but not worth its own post, even in Discussion, it goes here.