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Hello everyone! Thanks to all who shared their responses.
We received 2 correct responses to this puzzle! The best answer was from Sudhanva Yenigalla of Grade 7, Kokilaben Dhirubhai Ambani Reliance Foundation School, Jamnagar, who said-
First I will lock the medal in the box and send it to Vishnu. Then I will ask him to put another lock on the box and send me again. I will unlock my lock and send it back with his lock still on it. He can then unlock the box and get the medal.
Ishan Virda of Grade 7, SN Kansagra School, Rajkot also got the correct idea and gave a correct answer- just a bit more complicated answer than Sudhanva's. We share Ishan's answer later in this post. Students from Gujarat are cracking our challenges!! Come on other states, give them some competition! :-)
GenWise mentor, Navin Kabra, clarifies
The answer is that I can put the gold medal in a box, lock the box, send the locked box to Vishnu (without the key). Vishnu puts an additional lock on the box and sends the double locked box to me. When the box comes back to me, I use my key to remove my lock. Then I send the box (still locked with Vishnu's lock) back to Vishnu. Vishnu has the key to the lock and opens the box.
This problem has parallels in cryptography: your browser needs to solve problems like this when it is contacting a bank's server, and it needs to make sure that the password is not stolen by a dishonest courier in the middle (i.e. your internet service provider, etc).
Cybersecurity expert, Rohit Srivastwa points out that in problems like this, and in modern cyber security, you never send the keys/passwords over. You only send locked boxes. That is why the admonition, you should never give your OTP or your password to anyone over the phone.
Rohit says that another small variation of the method is to lock the box as shown in the picture below. Both people have their keys and they use always use their own key to open the box.
Ishan's answer below- well done Ishan!
You can use a chain of locked boxes to securely send the gold medal to Vishnu. Here's how:
1. Get two chain-locked boxes. Put the gold medal in the smaller box and lock it with one of the chains.
2. Place the small locked box inside a larger one and lock the larger box with the second chain.
3. Send the chained boxes to Vishnu.
4. Vishnu receives the chained boxes. He adds his own lock to the larger box, making it a triple-locked chain.
5. Vishnu sends the triple-locked chain back to you.
6. You remove your lock, leaving Vishnu's lock intact.
7. Send the double-locked chain back to Vishnu.
8. Vishnu removes his lock, and he can now open the smaller box containing the gold medal. This way, the gold medal remains secure throughout the process, even though the courier may be dishonest.
Anusha Kumar of Grade 6, Gems Genesis International School gave a creative answer- she suggested that an alumnium lock be used and Vishnu use gallium at his end to dissolve the lock!!
The Puzzle
I am in Pune, and I need to send an expensive Gifted World gold medal to Vishnu in Bangalore. The only method of sending things from Pune to Bangalore (or back) is a dishonest courier. The courier company steals anything that is not sent in a locked box. If I try to send the gold medal in an unlocked box, it will get stolen. Both Vishnu and I can get boxes with chain locks (see photo). We can get boxes of various sizes, and we can put boxes inside boxes if necessary. But if I send Vishnu a locked box, there is no way for me to send him they key to the lock, except by using the same dishonest courier (who will steal the key if it is in an unlocked box). How can I get the gold medal to Vishnu?
You can submit your answer here.