Unformatted text preview: designed by International Business Machines (IBM), as a federal Data Encryption Standard (DES). Marty Hellman and I criticized the proposal on the ground that its key was too small, but manufacturers were gearing up to support the proposed standard and our criticism was seen by many as an attempt to disrupt the standardsmaking process to the advantage of our own work. Public key cryptography in its turn was attacked, in sales literature [1125] and technical papers [849,1159] alike, more as though it were a competing product than a recent research discovery. This, however, did not deter the NSA from claiming its share of the credit. Its director, in the words of the Encyclopedia Britannica [1461], pointed out that “twokey cryptography had been discovered at the agency a decade earlier,” although no evidence for this claim was ever offered publicly. In the real world, publickey algorithms are not a substitute for symmetric algorithms. They are not used to encrypt messages; they are used to encrypt keys. There are two reasons for this: 1. Publickey algorithms are slow. Symmetric algorithms are generally at least 1000 times faster than publickey algorithms. Yes, computers are getting faster and faster, and in 15 years computers will be able to do publickey cryptography at speeds comparable to symmetric cryptography today. But bandwidth requirements are also increasing, and there will always be the need to encrypt data faster than publickey cryptography can manage. 2. Publickey cryptosystems are vulnerable to chosenplaintext attacks. If C = E(P), when P is one plaintext out of a set of n possible plaintexts, then a cryptanalyst only has to encrypt all n possible plaintexts and compare the results with C (remember, the encryption key is public). He won’t be able to recover the decryption key this way, but he will be able to determine P. Previous Table of Contents Next Products  Contact Us  About Us  Privacy  Ad Info  Home Use of this site is subject to certain Terms & Conditions, Copyright © 19962000 EarthWeb I...
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 Fall '10
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 Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips

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