You want to send a valuable object to a friend securely. You have a box which can be fitted with multiple locks, and you have several locks and their corresponding keys. However, your friend does not have any keys to your locks, and if you send a key in an unlocked box, the key could be copied en route.

How can you send the object securely?

The 'official' solution is not the most secure method. It is assumed that the box can be intercepted, else there is no need of a lock.

One must suppose there is a probability of one 'stealing' or 'misdelivering' the box, with or without the object. With the proposed solution, the box is being transferred thrice, thus there is three times the probability of the 'loss' of the valuable object to that of a single transit:

1. valuable object locked and sent with sender's lock
2. valuable object locked and sent with receiver's added lock
3. valuable object locked and sent with receiver's lock with sender's lock removed

Though it does not remove the possibility of one intercepting the secured box, examine the following:

1. box sent without lock to recipient
2. box returned with receiver's own unlocked lock inside the box (recipient would retain the key)
3. valuable object locked in the box with receiver's lock and resent to the recipient

This solution, of course, presupposes an unsecured lock and/or box would hold virtually no value or too small of an interest for an interceptor to intercept.

In addition, shipping is usually based upon weight. In the 'official' solution given, there would be a total transit weight [assuming all locks weigh the same]:
(box x 3) + (valuable object x 3) + (lock x 4)
While in the latter proposed solution, the total transit weight is:
(box x 3) + (valuable object x 1) + (lock x 2)
A "savings" of a transit weight cost of (valuable object x 2) + (lock x 2). 

Exoticorn's solution, sending the valuable object locked with the sender's lock, then, after the recepient has received the box, sending the key separately does seem a better solution. This presupposes the value of a key and its transit weight cost is less than the transit cost of (box x 2) + (lock x 1). In addition, only two transits occur as opposed to three.

Edited on September 26, 2008, 9:09 am

Posted by Dej Mar on 2008-09-26 07:11:52