Raise our Conscience against the Killing of RTI Activists

Wednesday, July 7, 2010

Average logical puzzle

You want to send a gift to your friend securely in your box. The box can be fitted wth multiple locks and keys which you possess. But your friend doesn't have any keys for your locks. If you send the key in a open box, it might get copied in the route. How can you send the gift securely?

Answers are expected as comments.


  1. Put the message in the box and lock it with your lock. Send the box to your friend. Let him put another lock on it and send it back to you. Once you receive the box (which now has 2 locks), unlock and remove *your* lock. The box will still have one lock (your friend's). Send it to your friend, who will be able to unlock it and read the message.

    By the way, this is a very old and well known cryptography problem of key distribution over a un-secure channel, the solution to which is commonly known as 'Shamir's three-pass algorithm' : http://en.wikipedia.org/wiki/Shamir_three-pass_algorithm

  2. Its correct shyam. But I was not knowing of the algorithm. Thanks