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# Locks and keys

## Question

You want to send a valuable object to a friend.

You have a box to contain the object.

The box has a locking ring which is more than large enough to have a lock attached.

You and your friend have several locks with keys.

But your friend does not have the key to any lock that you have, and vice versa.

How do you do it?

Note that you cannot send a key in an unlocked box, since it might be copied.

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What makes you think it is unsolved? Also, I don't see how your solution would work. If you make a "lock chain" out of your locks (all of which your friend doesn't have the keys for), then it reall

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1.get your friend to smash the box

2.use her lock to send it to her and she use yours to send it back

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This thread (Oct 11 2007, 02:23 PM) is still unsolved ??? Just chanced upon it today.

The solution is simple. you just form a chain using locks and the locking ring. If you open any lock, the chain is broken and the box opens

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This thread (Oct 11 2007, 02:23 PM) is still unsolved ??? Just chanced upon it today.

The solution is simple. you just form a chain using locks and the locking ring. If you open any lock, the chain is broken and the box opens

What makes you think it is unsolved?

Also, I don't see how your solution would work. If you make a "lock chain" out of your locks (all of which your friend doesn't have the keys for), then it really doesn't help. Also, if it's a "chain", isn't it possible to open up the box (slightly) and grab whatever is inside?

Edited by Jackie Chan
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Your friend sends you a lock, without its respective key. You then put the lock on the box with the valuable object and send it back. Now he can open it without a problem.

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Your friend sends you a lock, without its respective key. You then put the lock on the box with the valuable object and send it back. Now he can open it without a problem.

The first mailing is not secure. A thief could steal your lock.

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give her/him the key and lock the box and give the box to them at the same time as the key

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It is possible to send the numbers of the cuts to your friend and have a locksmith make the key.

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You want to send a valuable object to a friend.

You have a box to contain the object.

The box has a locking ring which is more than large enough to have a lock attached.

You and your friend have several locks with keys.

But your friend does not have the key to any lock that you have, and vice versa.

How do you do it?

Note that you cannot send a key in an unlocked box, since it might be copied.

you send your locked box to your friend,they put on another lock and send it back to you, you unlock it and send it to friend who unlocks it.

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The first mailing is not secure. A thief could steal your lock.

I think Ben Law's solution is the best. Send the empty box to your friend. He puts his lock in one of the two locking rings, locks and sends back. The box is still open and empty. After receiving, you put the important thing inside the box and lock it with your lock using the second locking ring and the friends lock as a chain. Then send back to your friend. He can open the chain as one of the locks is his.

Advantages of this system compared to the others :-

a) the important material is not in transit more than once. No one would like to see the important material in transit many times.

b) at all times the locks are locked and no body can make duplicate keys.

c) the distance of the locking rings can be adjusted to minimise the slack so that no one can slip out the content.

d) once this system is established you and your friend can send and receive things without delay but safely.

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I think Ben Law's solution is the best. Send the empty box to your friend. He puts his lock in one of the two locking rings, locks and sends back. The box is still open and empty. After receiving, you put the important thing inside the box and lock it with your lock using the second locking ring and the friends lock as a chain. Then send back to your friend. He can open the chain as one of the locks is his.

Advantages of this system compared to the others :-

a) the important material is not in transit more than once. No one would like to see the important material in transit many times.

b) at all times the locks are locked and no body can make duplicate keys.

c) the distance of the locking rings can be adjusted to minimise the slack so that no one can slip out the content.

d) once this system is established you and your friend can send and receive things without delay but safely.

Well if your friend locks it you can't open it.

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Well if your friend locks it you can't open it.

You did not read it carefully. Friend inserts the lock in one ring and locks and not on both the rings. And I mentioned there that the box is still empty and open. Please try to understand how chain locking is done. Also please read my previous explanation again.

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You want to send a valuable object to a friend.

You have a box to contain the object.

The box has a locking ring which is more than large enough to have a lock attached.

You and your friend have several locks with keys.

But your friend does not have the key to any lock that you have, and vice versa.

How do you do it?

Note that you cannot send a key in an unlocked box, since it might be copied.

Here's how I would do it, requiring only one transfer:

Send the box with many locks attached. In each lock, put a key that is the wrong key. Tell your friend which key matches which lock.

If you use, for example, 8 locks, there are 8! (8 factorial, or 40,320) combinations a thief would have to attempt in order to get into the box.

I'm surprised I'm the first to think of this.

Edited by j.green
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Ah, but the OP specified that ...

Note that you cannot send a key in an unlocked box, since it might be copied.

Ahh, but it doesn't say that you can't send a lock in an unlocked box. If you have your friend pit an unlocked lock inside the box and send the box (which is still unlocked) back to you then you will have a lock that your friend can unlock. Then you can add the valuable object to the box and lock the box with your friend's lock. She will then be able to unlock it and get the object without breaking any of the riddle's rules.

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You want to send a valuable object to a friend.

You have a box to contain the object.

The box has a locking ring which is more than large enough to have a lock attached.

You and your friend have several locks with keys.

But your friend does not have the key to any lock that you have, and vice versa.

How do you do it?

Note that you cannot send a key in an unlocked box, since it might be copied.

send the friend the unlocked box, have him send it back unlocked with a lock in it. Now send him the valuable object with the lock the friend sent locking the box. Or send the box with one of your locks and the key in a separate envelope. Or use a combination lock, and send him the code separately.

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Here's how I would do it, requiring only one transfer:

Send the box with many locks attached. In each lock, put a key that is the wrong key. Tell your friend which key matches which lock.

If you use, for example, 8 locks, there are 8! (8 factorial, or 40,320) combinations a thief would have to attempt in order to get into the box.

I'm surprised I'm the first to think of this.

not quite right... it's not 8! but 8 + 7 + 6 ... which as you probably know can be checked easily.

Also to anyone sending unlocked locks, it's not safe as your lock will be replaced somewhere along the way and then opened by whoever replaced it.

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Lock the box with your lock and send it to your friend.

Let him lock the lock again with his lock.( as it was mentioned that the locking ring is more than large enough to have a lock attached.)

and send it back to you.

he opens his own lock with his key and Voila.. gets the valuable object..

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1st, you put your lock on the box and send it to your friend. Then, since he can't unlock it, he simply puts another lock on and sends it back. Then, you can't open his lock, but you CAN open your original lock, leaving your friends lock on, which he can easily open and read the message! There is no point where an unlocked box is sent, and no point where a key is sent. It is totally secure!!!

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your friend could send you a lock and you lock the box with it and then he can open it because it was his lock.

ah but how would you lock it? you don't have the key!

Edited by joekemon
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Send the empty box putting your lock on the locking ring without attaching the ring to the hook where the ring should be hooked for locking the box. After receiving the box your friend put another lock by attaching his lock to your lock in the ring and to the hook, and returns the locked box to you. The two locks thus become like a chain. Now you can open the box, by opening your own lock, keep the valuable object in it, again close your lock in the same manner, and send the locked box to your friend. Now your friend can open his own lock to open the box, and receive the valuable object sent by you.

I think this is the right solution.

Edited by bhramarraj
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To make it simple, I assigned A & B as the friends in the problem

Edited by beryong27
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With regards to my answer, i assume that the box can be sent to one another many times. I also assigned A & B as the two friends in the challenge. The box originated from A

Step 1. A should unlock 2 padlocks, the 1st padlock stays with him and the key of 1st padlock must be put inside the box, the second padlock shall be put as the main lock of the box leaving the key to A's possession. A send the box to B

Step 2. Upon receiving of the box, B should put another padlock on the main lock of the box (now 2 padlocks are in place on the main lock of the box) leaving the key to B's possession. B send the box to A

Step 3. Upon receiving A should unlock his padlock leaving the padlock of B on the main lock of the box. A send the box to B

Step 4. Upon receiving B unlock his padlock and gets the key of the first padlock A unlocks, now B gets the key of the open padlock in A's possession (1st padlock in Step 1). B send the open and empty box to A

Step 5. Upon receiving A puts the valuable item in the box and lock it with the 1st padlock that the key B now in his possession. A send the box to B

Step 6. Upon receiving (for the last time) B opens the box and gets his valuable thing (i Hope thats really valuable ... lol)

Edited by plainglazed
fixed spoiler tags
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Well if your friend locks it you can't open it.

I think sudheeran is correct.

Nobody needs to open lock of other person. Each can open his own lock and since locks are put in the chain in the locking rings, therefore the box can be opened by opening any one lock.

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With regards to my answer, i assume that the box can be sent to one another many times. I also assigned A & B as the two friends in the challenge. The box originated from A

Step 1. A should unlock 2 padlocks, the 1st padlock stays with him and the key of 1st padlock must be put inside the box, the second padlock shall be put as the main lock of the box leaving the key to A's possession. A send the box to B

Step 2. Upon receiving of the box, B should put another padlock on the main lock of the box (now 2 padlocks are in place on the main lock of the box) leaving the key to B's possession. B send the box to A

Step 3. Upon receiving A should unlock his padlock leaving the padlock of B on the main lock of the box. A send the box to B

Step 4. Upon receiving B unlock his padlock and gets the key of the first padlock A unlocks, now B gets the key of the open padlock in A's possession (1st padlock in Step 1). B send the open and empty box to A

Step 5. Upon receiving A puts the valuable item in the box and lock it with the 1st padlock that the key B now in his possession. A send the box to B

Step 6. Upon receiving (for the last time) B opens the box and gets his valuable thing (i Hope thats really valuable ... lol)

Really brilliant thinking, though too long.

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Just guessing here, but why not put a lock on one ring and lock it. Then, put another lock through the second ring and lock it through the first lock. It would only work if you remember to put one of the keys in the box with the valuable item BEFORE you lock it shut. Just in case this isn't the correct answer, get the insurance also......

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