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.

The question: How can notary Big send his document securely to notary Small using the metal tube and padlocks?

The answer: Click here!