Design a protocol for a garage-door opener.  A garage-door opener consists
of 

1. The motor and receiver installed inside the garage.
2. The handheld transmitter that resides in the car.

The user presses a button on the transmitter, causing it to send an
"OPEN" command to the receiver.  Your goal is to prevent an attacker
from opening the garage.

There are a few requirements on the design:

- The transmitter and receiver are set up at the factory, so you can 
  install shared keys or whatever you want at that time.
- The receiver inside the garage cannot transmit at all.  Thus, once
  the devices are shipped out of the factory, the only communication
  between the devices is from the hand-held transmitter to the receiver.
- You should avoid the use of public-key crypto, because it requires too
  much computational power.
- You should avoid keeping long-term state on the transmitter and receiver,
  as that requires extra hardware.

Present your protocol as a diagram (like we've drawn in class).  Your
protocol should fit on 1/2 page of paper.

Note: To my knowledge, there is no "perfect" solution to this problem.
Don't worry if your solution uses a little long-term state, or if you
include sme public-key crypto, or if you can see some attacks against
your protocol.  Just do the best you can as cheaply as you can.