The situation is actually a bit tricky. If they met over the internet, there's no way to guarantee the absence of a Man-in-the-Middle who has been allowing the

The situation is actually a bit tricky. If they met over the internet, there's no way to guarantee the absence of a Man-in-the-Middle who has been allowing the romance to proceed without interference, but who tampers with the messages which give the address to which the ring must be sent (and who fakes the message which says the box has been received.) The only way to avoid this possibility (of a man in the middle attack) is to be "introduced" by a third party you already trust. See e.g. https://en.wikipedia.org/wiki/Needham–Schroeder_protocol https://en.wikipedia.org/wiki/Needham–Schroeder_protocol In the absence of a trusted third party, you could be talking to anybody, through any number of lurking intermediaries. If you've got a trusted third party then the problem, as posed, is moot.