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I understand what substitution-permutation networks look like. I understand what Feistel networks and Feistel-like networks look like. A Feistel network or a Feistel-like network features substitution and permutations, so why is it considered distinct from an SP-network?

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  • $\begingroup$ Your question is not clear. A Feistel network could contain an SPN in its $f$ function but that is distinctly different from SPN itself $\endgroup$
    – sashank
    Commented Apr 7, 2015 at 16:53

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In the substitution step of a typical Substitution-Permutation Network (e.g. in AES SubBytes), the whole state is broken in parts and each part substituted. That's not the case in (the core of) a Feistel cipher, where at each step/round some sizable part of the state is bound to remain unchanged (in order that each step be reversible).

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