Send a secret mail

Question

You want to send a secret message to your friend Alex in the mail.
But Sam, who you don't trust, has access to all your mail.
So you put your message in a box with a lock.
But you can't send a key, because Sam could use or copy it!
How can you get your message through securely?

Note: Alex doesn't have a key to your lock.

score 3 · Answer 1 · answered Jul 25 '20 at 12:31

3

This is a very famous problem. Not sure if it’s been posted here before however.

The solution involves

Sending it back and forth three times.

This way

You send the message to Alex with your lock. Alex attaches his own lock and returns it. You remove your lock, so the box only has Alex’s lock on it, send it back to him, and he can unlock it and see your message. This way the message always has a lock on it.

answered Jul 25 '20 at 12:31

Beastly Gerbil

58,036
8
166
314

1

it was (posted here before): https://puzzling.stackexchange.com/questions/29937/challenge-for-lovers – melfnt Jul 25 '20 at 12:39
@melfnt expected so thanks for finding it! – Beastly Gerbil Jul 25 '20 at 12:41

score 2 · Accepted Answer · answered Jul 25 '20 at 12:36

2

Here is how

You send the locked message (without the key) to Alex.
Alex puts an extra lock on the message (Alex has the key to that extra lock).
Alex sends the double locked message (without any key) back to you.
You remove your lock, and send the message back to Alex.
Alex removes the lock that he put on.
Voila!

answered Jul 25 '20 at 12:36

risky mysteries

13,072
3
24
96

Except that Sam intercepts the box, sends it back with his own lock, and then intercepts the box with Alex's lock removed. – Florian F Jul 26 '20 at 13:14
@FlorianF Maybe change it to rot13? – risky mysteries Jul 26 '20 at 13:16
As soon as this site provides a way to encode and decode it. – Florian F Jul 26 '20 at 13:20
@FlorianF Good point. – risky mysteries Jul 26 '20 at 13:20
@FlorianF Do you have your own encoder? (just curious) – risky mysteries Jul 26 '20 at 13:21
I use rot13.com when I have to but it is a hassle to me and those who want to read it. – Florian F Jul 26 '20 at 13:22
@FlorianF Maybe this will make things more convenient. https://stackoverflow.com/a/8981312/13552470 – risky mysteries Jul 26 '20 at 13:23
Let us continue this discussion in chat. – Florian F Jul 26 '20 at 13:24

Send a secret mail

2 Answers2