I don't recall if this problem was ever discussed on math-fun: --- The Russian Postal Service Problem A man in the west of Russia wants to send a small but expensive gift (a ring) to his lady love who lives in the east. Unfortunately, the Russian postal service is corrupt: they will open every letter they get and remove any valuables from them before sending them on. The man could send the ring in a locked box (the postal service is underfunded and cannot afford tools to break into locked boxes), but his partner would have no way of opening the box. If the postal service believe he will send a key separately, they will hold onto the box until the key arrives. He could use a combination lock and telephone the combination to her, but the KGB monitor all phone lines and will tell their mates in the postal service everything he says. He can still talk to her; he just can't rely on secrecy. So, the postal service will deliver the parcel intact only if it is locked and if they know that no key will ever come their way. Delivering the ring in person is not an option. How does he safely send the ring to his partner?