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

People write keys down people lose them accidents

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Unformatted text preview: r for her key. Bob should make sure to have the combination to the safe himself as well; otherwise, if the security officer is run over by another truck, Bob will be out of luck again. The problem with this key management system is that Bob has to trust his security officer not to misuse everyone’s keys. Even more significantly, all the employees have to trust the security officer not to misuse their keys. A far better solution is to use a secret-sharing protocol (see Section 3.7). When Alice generates a key, she also divides up that key into some number of pieces. She then sends each piece—encrypted, of course—to a different company officer. None of those pieces alone is the key, but someone can gather all the pieces together and reconstruct the key. Now Alice is protected against any one malicious person, and Bob is protected against losing all of Alice’s data after her run-in with the truck. Or, she could just store the different pieces, encrypted with each of the officer’s different public keys, on her own hard disk. That way, no one gets involved with key management until it becomes necessary. Another backup scheme [188] uses smart cards (see Section 24.13) for the temporary escrow of keys. Alice can put the key to secure her hard drive onto the smart card and give it to Bob while she is away. Bob can use the card to get into Alice’s hard drive, but because the key is stored in the card Bob cannot learn it. And the system is bilaterally auditable: Bob can verify that the key will open Alice’s drive, and when Alice returns she can verify if Bob has used the key and how many times. Such a scheme makes no sense for data transmission. On a secure telephone, the key should exist for the length of the call and no longer. For data storage, as just described, key escrow can be a good idea. I’ve lost about one key every five years, and my memory is better than most. If 200 million people were using cryptography, that same rate would equal 40 million lost keys per year. I keep copies of my...
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This note was uploaded on 10/18/2010 for the course MATH CS 301 taught by Professor Aliulger during the Fall '10 term at Koç University.

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