Notary Big in Smallville must send a secret document by mail to notary Small in Big Springs. He has a sturdy metal tube of which the lid can be locked with padlocks. Notary Big has two padlocks with matching keys, but notary Small does not have these keys. Notary Small has two padlocks with matching keys as well, but notary Big does not have those keys. The notaries do not want to send the keys as they find it too risky.
How can notary Big send his document securely to notary Small using the metal tube and padlocks?
Notary Big puts the document in the metal tube, locks the lid with one of his padlocks and sends it to notary Small. Notary Small adds one of his padlocks to the lid and sends the tube back to notary Big. Notary Big removes his padlock from the lid and sends the tube back again to notary Small. Notary Small can then open the tube by removing his own padlock again.
