applied cryptography - protocols, algorithms, and source code in c

Old keys are valuable even if they are never used

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Unformatted text preview: key. One hopes Alice knows when her key was compromised. If a KDC is managing the keys, Alice should notify it that her key has been compromised. If there is no KDC, then she should notify all correspondents who might receive messages from her. Someone should publicize the fact that any message received after her key was lost is suspect, and that no one should send messages to Alice with the associated public key. The application should be using some sort of timestamp, and then users can determine which messages are legitimate and which are suspect. If Alice doesn’t know exactly when her key was compromised, things are more difficult. Alice may want to back out of a contract because the person who stole the key signed it instead of her. If the system allows this, then anyone can back out of a contract by claiming that his key was compromised before it was signed. It has to be a matter for an adjudicator to decide. This is a serious problem and brings to light the dangers of Alice tying all of her identity to a single key. It would be better for Alice to have different keys for different applications—just as she has different physical keys in her pocket for different locks. Other solutions to this problem involve biometrics, limits on what can be done with a key, time delays, and countersigning. These procedures and tips are hardly optimal, but are the best we can do. The moral of the story is to protect keys, and protect private keys above all else. 8.10 Lifetime of Keys No encryption key should be used for an indefinite period. It should expire automatically like passports and licenses. There are several reasons for this: — The longer a key is used, the greater the chance that it will be compromised. People write keys down; people lose them. Accidents happen. If you use the same key for a year, there’s a far greater chance of compromise than if you use it for a day. — The longer a key is used, the greater the loss if the key is compromised. If a key is used only to...
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