Devsman's answer is wrong. It is possible to send Jane the ring by following these steps:
- Mark sends a padlocked box and a message asking Jane a question only they know the answer to, along with these instructions.
- Jane keeps the box and sends the answer of the question to Mark if and only if she got the box. (Mark now knows Jane has a box, but we can't be sure that it's the right box)
- Mark sends the key to the padlocked box to Jane
- Jane uses it to open the padlocked box that she kept.
- Inside, she finds a second key and a message that Mark had written, containing the answer to the question (to confirm it's his box), and a second question
- Jane answers the second question, only if Mark's answer to the first question was right.
- Mark checks the answer, and can be sure that Jane and no one else now has one of his keys. All he has to do is to send the ring in a box locked with that key.
To stop Klep from re-sending the second question as a second try to the first question, they agree that the first question must always have a certain topic (about Jane's life, for example) and the second question must be about a different topic (about Mark's life, for example).
(Kleptomaniac could replace the instructions, but we'll assume that Jane would be smart enough not to answer a personal question through mail for no good reason).
M → J: Question A, Instructions and Box A (containing key B, answer to the question A and question B)
J → M: Answer to the Question A if she got the box
M → J: Key A
J → M: Answer B if answer A is correct
M → J: Box B (containing ring) if answer B is correct
Question A is always about Jane's life
Question B is always about Mark's life
This answer assumes that:
- They have at least two secrets they share (they are lovers)
- Locked boxes cannot be duplicated, like packets on the internet could