Garage Door Protocol

Norm Hardy wrote:

> The "Garage Door Protocol" provides simple protection for commands in a
> one way channel. http://cap-lore.com/Garage/

If you're allowed to resync every once in a while, you can use S/Key, http://en.wikipedia.org/wiki/S/KEY.  (Did you notice the pun in the URL?)  The transmitter starts with a secret and a counter c as in all the other cases.  The receiver starts with the secret hashed c times.  The transmitter hashes the secret c-1 times, transmits the result, and decrements the counter.  The receiver hashes the received value.  If it matches the stored value, the door opens, and the receiver stores the received value.  You handle lost or garbled transmissions by having the receiver hash multiple times bounded by the last known value of the counter.  Of course, you have to resync with a new secret when c=1.

S/Key is often combined with encryption on the channel, but that's not needed here.  A successful MITM attack requires blocking the radio link between transmitter and receiver, and even if successful, only allows the attacker to open the door on your request.  An attacker gains nothing by learning the value stored on the receiver.

________________________
Alan Karp
Principal Scientist
Enterprise Services, Office of the CTO
Hewlett-Packard Company
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http://www.hpl.hp.com/personal/Alan_Karp