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Unformatted text preview: ce a month or once a year. You have to balance the inherent danger in keeping a key around for a while with the inherent danger in distributing a new one. Encryption keys used to encrypt data files for storage cannot be changed often. The files may sit encrypted on disk for months or years before someone needs them again. Decrypting them and re-encrypting them with a new key every day doesn’t enhance security in any way; it just gives a cryptanalyst more to work with. One solution might be to encrypt each file with a unique file key, and then encrypt all the file keys with a key-encryption key. The key-encryption key should then be either memorized or stored in a secure location, perhaps in a safe somewhere. Of course, losing this key would mean losing all the individual file keys. Private keys for public-key cryptography applications have varying lifetimes, depending on the application. Private keys used for digital signatures and proofs of identity may have to last years (even a lifetime). Private keys used for coin-flipping protocols can be discarded immediately after the protocol is completed. Even if a key’s security is expected to last a lifetime, it may be prudent to change the key every couple of years. The private keys in many networks are good only for two years; after that the user must get a new private key. The old key would still have to remain secret, in case the user needed to verify a signature from that period. But the new key would be used to sign new documents, reducing the number of signed documents a cryptanalyst would have for an attack. 8.11 Destroying Keys
Given that keys must be replaced regularly, old keys must be destroyed. Old keys are valuable, even if they are never used again. With them, an adversary can read old messages encrypted with those keys . Keys must be destroyed securely (see Section 10.9). If the key is written on paper, the paper should be shredded or burned. Be careful to use a high-quality shredder; many lousy shredders are on the market. Algorithms in this boo...
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- Fall '10