A well-known puzzle in secure communication goes as follows:

You must send a confidential document via a courier.
The catch? The courier is untrusted—if they ever see the box unlocked or containing extra locks, they will confiscate everything.

  • You and your colleague each have a padlock, but only you know the key to yours, and only they know theirs.
  • You cannot share keys or meet in person.

How can you securely send the locked box and its contents?

Step-by-Step Solution: The Double-Lock Method

This problem is about using asymmetric locking to maintain confidentiality.

Step 1: You Lock the Box

  • Place the document in a box.
  • Lock it with your own padlock.
  • Send the box (locked) to your colleague.

The courier sees a locked box (no loose locks or keys)—safe.

Step 2: Colleague Adds Their Lock

  • Your colleague cannot open your lock, so they add their own padlock beside yours.
  • The box now has two padlocks (yours and theirs).
  • They send it back to you.

Again, the courier sees a locked box—still secure.

Step 3: You Remove Your Lock

  • You receive the doubly-locked box.
  • You remove your own padlock, leaving only your colleague’s lock.
  • Send the box to your colleague once more.

Still locked, and no key was shared.

Step 4: Colleague Unlocks and Retrieves

  • Now your colleague can unlock their own padlock.
  • They safely access the document inside.

Why It Works

  • The box is never unlocked in transit.
  • No keys are exchanged.
  • Both locks are used sequentially to ensure only the intended recipient can access the content.

Final Answer

Use the double-lock technique:

  1. Lock with your padlock and send.
  2. Colleague adds their padlock and returns.
  3. You remove your lock and send again.
  4. They unlock and retrieve the document.

Reference