Your choices change the outcome
Courier: test a few retained bits, then deliver or discard the rest. Testing reveals both values publicly, so every tested bit is removed from the key. Spy: secretly choose how many signals to intercept. More listening can reveal more bits, but usually causes more detectable errors.
A shift has four packets. The Courier and Spy compare their total points at the end. All messages and bits are fictional; this game does not create a real encryption key.
| What happens | Game points |
|---|
| Deliver a correct bit Eve does not know | Courier +1 |
| Deliver a bit Eve knows or Bob reads incorrectly | Courier −2, counted once |
| Eve knows a delivered bit | Spy +2 |
| Discard after finding a mismatch | Courier +4; Spy 0 |
| Discard without finding a mismatch | Both 0 |
What is actually quantum here?
Alice prepares each bit in basis Z or X. Bob independently chooses a basis. Matching bases preserve the prepared bit in the ideal model; different bases give a random measurement result. They publicly compare bases and keep only matching positions. Eve does not know their bases when she intercepts.
If Eve measures in a different basis, her re-prepared signal can change Bob’s later result. The full random-basis intercept-and-resend attack produces a 25% average error rate among retained bits in the ideal model. A small sample can miss it. In Noisy Night, an additional 6% independent detector readout error can also cause mismatches.
Evidence has limits
No mismatches does not prove that no one listened. A mismatch shows disturbance; it need not identify its cause. The after-round replay reveals the simulator’s hidden information to explain the score. Real players cannot see that information during a real protocol.
The scoring system is a game rule, not a security calculation. Real quantum key distribution also needs an authenticated classical channel, reconciliation, privacy amplification, and a finite-key security analysis.
Play together on one phone
Choose Pass & Play. The Spy privately sets a plan, then passes the covered phone to the Courier team. Two players can take one role each; with three players, one Courier teammate chooses tests and the other makes the delivery decision. Swap roles after a shift. This is a classroom honor system.
Physics: Bennett and Brassard’s BB84 protocol. Educational precedents include DeVore and Singh’s tutorial and CrypTool’s BB84 activity. Game design and code prepared for SpookyQubits.