
 Security Forums Dot Com :: View topic  Principles of operation of symmetric cryptosystems 
  Predominantly, as previously mentioned, they are based on the surrounding theory of Feistel networks and SubstitutionPermutation Networks, iterated and/or product; along with them come many variations, such as unbalanced Feistel networks or modified SubstitutionPermutation Networks, for example. 
  In a Feistel network, we generally have a "round function" that is iterated for n rounds; in a SubstitutionPermutation Network, we generally have layers of routines that perform substitutions and permutations, in a round transformation, obviously, iterated for n rounds. 
  As for formulas that demonstrate the mathematical constructions of these design strategies, refer to the NIST's FIPS publications for DES [FIPS 463], a Feistel network, and AES [FIPS 197], a SubstitutionPermutation Network. 
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