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

Eve can effectively become alice it is vital that

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Unformatted text preview: ing the key into two halves, storing one half in the terminal and the other half in the ROM key. The U.S. government’s STU-III secure telephone works this way. Losing the ROM key does not compromise the cryptographic key—change that key and everything is back to normal. The same is true with the loss of the terminal. This way, compromising either the ROM key or the system does not compromise the cryptographic key—an adversary must have both parts. Hard-to-remember keys can be stored in encrypted form, using something similar to a key-encryption key. For example, an RSA private key could be encrypted with a DES key and stored on disk. To recover the RSA key, the user has to type in the DES key to a decryption program. If the keys are generated deterministically (with a cryptographically secure pseudo-random-sequence generator), it might be easier to regenerate the keys from an easy-to-remember password every time they are required. Ideally, a key should never appear unencrypted outside the encryption device. This isn’t always possible, but it is a worthy goal. 8.8 Backup Keys Alice is the chief financial officer at Secrets, Ltd.—“We don’t tell you our motto.” Like any good corporate officer, she follows the company’s security guidelines and encrypts all her data. Unfortunately, she ignores the company’s street-crossing guidelines and gets hit by a truck. What does the company’s president, Bob, do? Unless Alice left a copy of her key, he’s in deep trouble. The whole point of encryption is to make files unrecoverable without the key. Unless Alice was a moron and used lousy encryption software, her files are gone forever. Bob can avoid this in several ways. The simplest is sometimes called key escrow (see Section 4.14): He requires all employees to write their keys on paper and give them to the company’s security officer, who will lock them in a safe somewhere (or encrypt them all with a master key). Now, when Alice is bowled over on the Interstate, Bob can ask his security office...
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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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