Message Delivery — Double-Lock Puzzle
Alice must send a secret document to Bob through an untrusted courier. Alice and Bob each own a padlock and keep their own key.
How can they deliver the message securely?
The box must never travel while unlocked.
🔒 Secure protocol
👩💻
Alice
🔑 A
←
→
📄
A
👨💻
Bob
🔑 B
1
2
3
4
1. Alice locks the document
Alice → Bob
Alice puts the document in the box, attaches padlock A,
and sends it to Bob. The courier cannot open the box.
Current box state
DocumentInside box
Alice's lockOn 🔒
Bob's lockOff
Can courier open it?No ✓
Why does this work?
① Alice → Bob
Lock A protects the document.
Lock A protects the document.
② Bob → Alice
Bob adds lock B, so both locks are attached.
Bob adds lock B, so both locks are attached.
③ Alice → Bob
Alice removes A. Lock B remains during the final trip.
Alice removes A. Lock B remains during the final trip.
Key idea: the box is never transported while unlocked, and neither person ever needs to give the other person a key.
Try the protocol yourself
Start with Alice holding the document. Choose the actions in the correct order.
Alice has the document. The box is currently unlocked.