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Unformatted text preview: s not limited to encryption algorithms: If you let someone else specify any algorithm which is used on your message before encryption, then you had better be sure that your encryption will withstand a chosenplaintext attack. (Note that the most common algorithm used for compressing and digitizing speech to modem speeds, used before any encryption, is CELP—designed by the NSA.) This can be better phrased: Using a chosenplaintext attack, a cascade of ciphers is at least as hard to break as any of its component ciphers [858]. A previous result showed that the cascade is at least as difficult to break as the strongest algorithm, but that result is based on some unstated assumptions [528]. Only if the algorithms commute, as they do in the case of cascaded stream ciphers (or block ciphers in OFB mode), is the cascade at least as strong as the strongest algorithm. If Alice and Bob do not trust each other’s algorithms, they can use a cascade. If these are stream algorithms, the order doesn’t matter. If they are block algorithms, Alice can first use Algorithm A and then use Algorithm B. Bob, who trusts Algorithm B more, can use Algorithm B followed by Algorithm A. They might even add a good stream cipher between the two algorithms; it can’t hurt and could very well increase security. Remember that the keys for each algorithm in the cascade must be independent. If Algorithm A has a 64bit key and Algorithm B has a 128bit key, then the resultant cascade must have a 192bit key. If you don’t use independent keys, then the pessimists are much more likely to be right. 15.8 Combining Multiple Block Algorithms
Here’s another way to combine multiple block algorithms, one that is guaranteed to be at least as secure as both algorithms. With two algorithms (and two independent keys): (1) Generate a randombit string, R, the same size as the message M. (2) Encrypt R with the first algorithm. (3) Encrypt M • R with the second algorithm. (4) The ciphertext message is the results of steps (2) and (3). Assuming the randombit str...
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 Fall '10
 ALIULGER
 Cryptography

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