Unformatted text preview: n’t win, but they will certainly make it cheaper for a company to license than fight. RC4 is in dozens of commercial cryptography products, including Lotus Notes, Apple Computer’s AOCE, and Oracle Secure SQL. It is part of the Cellular Digital Packet Data specification [37]. 17.2 SEAL
SEAL is a softwareefficient stream cipher designed at IBM by Phil Rogaway and Don Coppersmith [1340]. The algorithm was optimized for 32bit processors: To run well it needs eight 32bit registers and a cache of a few kilobytes. Using a relatively slow operation, SEAL preprocesses the key operation into a set of tables. These tables are then used to speed up encryption and decryption. Pseudorandom Function Family
One novel feature of SEAL is that is isn’t really a traditional stream cipher: it is a pseudorandom function family. Given a 160bit key k, and a 32bit n, SEAL stretches n into an Lbit string k(n). L can take any value less than 64 kilobytes. SEAL is supposed to enjoy the property that if k is selected at random, then k(n) should be computationally indistinguishable from a random Lbit function of n. The practical effect of SEAL being a pseudorandom function family is that it is useful in applications where traditional stream ciphers are not. With most stream ciphers you generate a sequence of bits in one direction: Knowing the key and a position i, the only way to determine the ith bit generated is to generate all the bits up until the ith one. But a pseudorandom function family is different: You get easy access at any desired position in the key stream. This is very useful. Imagine you need to secure a hard drive. You want to encrypt each and every 512byte sector. With a pseudorandom function family like SEAL, you can encrypt the contents of sector n by XORing it with k(n). It is as though the entire disk is XORed with a long pseudorandom string, where any piece of that long string can be computed without any trouble. A pseudorandom function family also simplifies the synchronization problem encountered with standard stream ciphers. Suppose you send...
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 Fall '10
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 Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips

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