For Boris and Natasha: Boris places the ring into the box and locks it with the padlock. He sends the locked box to Natasha and it reaches her unharmed. Natash

For Boris and Natasha: Boris places the ring into the box and locks it with the padlock. He sends the locked box to Natasha and it reaches her unharmed. Natasha receives the box and places her own padlock on it without removing Boris' padlock, so it now has two locks on it. She sends the doubly-locked box back to Boris and it reaches him unharmed. Boris then removes his own padlock, so that the box now only has Natasha's lock and then mails it back to Natasha. Natasha receives the locked box unharmed and removes her own padlock, thus receiving the ring.