> Maria mails Jan one of her padlocks

I think the idea is that that padlock would get stolen unless it would be sent in a padlocked box.

Or even better: a thief replaces the padlock with one of his own, then later steals the ring.

What you really need is a trusted "padlock authority" that can verify the authenticity of your padlocks...

I don't know, why would someone steal a padlock unless it was attached to a box? Maybe so, though.

The wording of the problem is "anything sent through the mail will be stolen unless it is enclosed in a padlocked box".

Jan constructs an enormous box around the entire country of Kleptopia, and places his own padlock on it from the inside. Then he mails the ring with no additional security measures.

The problem is fatally flawed by not explicitly stating that boxes locked with padlocks are also not stolen, despite not being enclosed in a padlocked box.

Jan takes a normal box and locks it. He declares the space enclosed by the box to be the "outside," and the world to be "inside."

Unfortunately, the mail thieves declare "outside" to be the volume in conformal space on the side of the boundary definition that contains the point at infinity, and "inside" to be the volume that does not, and thus steal the ring.

Those jerks, they're always one step ahead.

Of course the point at infinity had been stolen some time ago, and its whereabouts are currently unknown.

> anything sent through the mail will be stolen unless it is enclosed in a padlocked box

If someone stole the padlocked box then they would necessarily also steal whatever's inside. Therefore (non-empty) padlocked boxes are safe to mail.

Aha!

Then you put a tiny corundum crystal inside a tiny padlocked box, and permanently affix it to the ring. (Or perhaps the ring has a lockable box portion.) Now the ring cannot be stolen without also stealing an object that is inside a padlocked box. Safe to mail.

If not steal, can definitely make a copy of the key :)