Unformatted text preview: ll those weird correlations and strange results. Every pseudorandomsequence generator is going to produce them if you use them in a certain way. And that’s what a cryptanalyst will use to attack the system. Cryptographically Secure PseudoRandom Sequences
Cryptographic applications demand much more of a pseudorandomsequence generator than do most other applications. Cryptographic randomness doesn’t mean just statistical randomness, although that’s part of it. For a sequence to be cryptographically secure pseudorandom, it must also have this property: 2. It is unpredictable. It must be computationally infeasible to predict what the next random bit will be, given complete knowledge of the algorithm or hardware generating the sequence and all of the previous bits in the stream. Cryptographically secure pseudorandom sequences should not be compressible...unless you know the key. The key is generally the seed used to set the initial state of the generator. Like any cryptographic algorithm, cryptographically secure pseudorandomsequence generators are subject to attack. Just as it is possible to break an encryption algorithm, it is possible to break a cryptographically secure pseudorandomsequence generator. Making generators resistant to attack is what cryptography is all about. Real Random Sequences
Now we’re drifting into the domain of philosophers. Is there such a thing as randomness? What is a random sequence? How do you know if a sequence is random? Is “101110100” more random than “101010101”? Quantum mechanics tells us that there is honesttogoodness randomness in the real world. But can we preserve that randomness in the deterministic world of computer chips and finitestate machines? Philosophy aside, from our point of view a sequence generator is real random if it has this additional third property: 3. It cannot be reliably reproduced. If you run the sequence generator twice with the exact same input (at least as exact as humanly possible), you will get two completely unrelated random sequences. The output of a...
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 Fall '10
 ALIULGER
 Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips

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