Unformatted text preview: mple of this design. However, this approach can sometimes aid in differential cryptanalysis . Actually, a better approach is making sure that the maximum differential is as small as possible. Kwangjo Kim proposed five criteria for the construction of S-boxes , similar to the design criteria for the DES S-boxes. Choosing good S-boxes is not an easy task; there are many competing ideas on how to do it. Four general approaches can be identified. 1. Choose randomly. It is clear that small random S-boxes are insecure, but large random S-boxes may be good enough. Random S-boxes with eight or more inputs are quite strong [1186,1187]. Twelve-bit S-boxes are better. Even more strength is added if the S-boxes are both random and key-dependent. IDEA uses both large and key-dependent S-boxes. 2. Choose and test. Some ciphers generate random S-boxes and then test them for the requisite properties. See [9,729] for examples of this approach. 3. Man-made. This technique uses little mathematics: S-boxes are generated using more intuitive techniques. Bart Preneel stated that “...theoretically interesting criteria are not sufficient [for choosing Boolean functions for S-boxes]...” and that “...ad hoc design criteria are required” . 4. Math-made. Generate S-boxes according to mathematical principles so that they have proven security against differential and linear cryptanalysis, and good diffusive properties. See  for an excellent example of this approach. There has been some call for a combination of the “math-made” and “man-made” approaches , but the real debate seems to be between randomly chosen S-boxes and S-boxes with certain properties. Certainly the latter approach has the advantage of being optimal against known attacks—linear and differential cryptanalysis—but it offers unknown protection against unknown attacks. The designers of DES knew about differential cryptanalysis, and its S-boxes were optimized against it. They did not seem to know about linear cryptanalysis, and the DES S-boxes are very...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips