How do you solve the diamond-and-padlock puzzle? (Herbert wants to send Angelika a valuable diamond by mail, but the courier is untrustworthy.)

Question

Accepted Answer

Angelika sends Herbert one of her padlocks (unlocked) in a normal package. Herbert locks the box with it and sends it back.
The puzzle:

Herbert wants to send a valuable diamond to his friend Angelika
The courier is untrustworthy and will steal anything from an unlocked box
Herbert could lock the box with a padlock, but Angelika doesn't have a key for any of his locks

Solution in two steps:

Step
Action
Box State

1
Angelika sends Herbert one of her padlocks (unlocked) in a normal package
Just a lock, nothing valuable to steal

2
Herbert puts the diamond in the box, locks it with Angelika's padlock → sends to Angelika
Locked with Angelika's lock — only she has the key

Why it matters:

This is a physical analogy for public-key cryptography
The unlocked padlock represents a public key — anyone can use it to lock (encrypt) something
Only Angelika has the matching key (private key) to unlock it
The courier (attacker) sees the lock being sent but can't do anything useful with it — there's nothing valuable to steal at that point

Tip: This puzzle beautifully illustrates why public-key cryptography was revolutionary — you can share your "lock" (public key) openly, and only you can open what others lock with it. This is exactly how HTTPS/TLS works when your browser connects to a website.
Go deeper:

Public-key cryptography (Wikipedia) — the lock-it-with-the-recipient's-padlock idea formalized as public/private keys.

Step	Action	Box State
1	Angelika sends Herbert one of her padlocks (unlocked) in a normal package	Just a lock, nothing valuable to steal
2	Herbert puts the diamond in the box, locks it with Angelika's padlock → sends to Angelika	Locked with Angelika's lock — only she has the key