VADER: Where is that shuttle going?
PIETT: Shuttle Tydirium, what is your cargo and destination?
PILOT VOICE: Parts and technical crew for the forest moon.
VADER: Do they have a code clearance?
PIETT: It’s an older code, sir, but it checks out. I was about to clear them.
In modern cryptography, a system is designed so that it is difficult to break. The only part that must be kept secret are the keys used to encrypt the message. Changing Keys is necessary from time to time because an attacker who learns the key can easily break any message that is encrypted.
If the rebels steal their keys, re-entry of periodic information ensures that the problem is limited in time.
In a military situation where your opponent will go far to steal the keys, there is a logistics problem of how to distribute new keys. You can not send on a channel that is set by the old keys. You must use a separate system (and how to ensure that these keys are safe?) Or by mail. But when their units are scattered throughout the planet or galaxy, one can not have a daily courier service. You should also plan your mail service to fail, either because the mail is intercepted, or the rendezvous point is not available. So you have to send a set of keys to be used in the next N cycles.
In World War II, the Allies took advantage of it, attacking the Nazis weather ships. (The plan may come from Ian Fleming, who went on to write novels of James Bond.) For the capture key, the Allies were able to read the Nazi traffic.
In any case, Piett was about to authorize the landing of the shuttle. The history of cryptography is full of examples that happened not too long ago, but the pattern is the same. The desire to believe that all is well, the pressure of the routine, and the belief that the operator that the little abnormal is pretty close to normal combine to justify bending the rules a bit.
Piett is about to accept a key release date, a decision that is militarily and psychologically cryptographically probably take sensible. The process design means that such anomalies are expected. This is why the expectation is heroic effort worth stealing keys. (Such efforts are the reasons behind the work of Jack Shaftoe in Stephenson “the Cryptonomicon.”) Even with systems designed in accordance with the principle of Kerkhoff, key management is a difficult challenge.
Also, be sure to check out the gallery Piett, whose image today, asked me borrow. On Friday, we can make a detour to Tatooine to answer a question reader, or I can start in the classic Saltzer and Schroeder. I’m always looking for a good web version that I can link. Finally, thanks to DM noticed some flaws in the first draft.