Unformatted text preview: e x. Taking a watch apart is a good example of a trapdoor oneway function. It is easy to disassemble a watch into hundreds of minuscule pieces. It is very difficult to put those tiny pieces back together into a working watch. However, with the secret information—the assembly instructions of the watch—it is much easier to put the watch back together. 2.4 OneWay Hash Functions
A oneway hash function has many names: compression function, contraction function, message digest, fingerprint, cryptographic checksum, message integrity check (MIC), and manipulation detection code (MDC). Whatever you call it, it is central to modern cryptography. Oneway hash functions are another building block for many protocols. Hash functions have been used in computer science for a long time. A hash function is a function, mathematical or otherwise, that takes a variablelength input string (called a preimage) and converts it to a fixedlength (generally smaller) output string (called a hash value). A simple hash function would be a function that takes preimage and returns a byte consisting of the XOR of all the input bytes. The point here is to fingerprint the preimage: to produce a value that indicates whether a candidate preimage is likely to be the same as the real preimage. Because hash functions are typically manytoone, we cannot use them to determine with certainty that the two strings are equal, but we can use them to get a reasonable assurance of accuracy. A oneway hash function is a hash function that works in one direction: It is easy to compute a hash value from preimage, but it is hard to generate a preimage that hashes to a particular value. The hash function previously mentioned is not oneway: Given a particular byte value, it is trivial to generate a string of bytes whose XOR is that value. You can’t do that with a oneway hash function. A good oneway hash function is also collisionfree: It is hard to generate two preimages with the same hash value. The hash function is public; there’s no secrecy to the process. The secur...
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 Fall '10
 ALIULGER
 Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips

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