Comments by "Lawrence D’Oliveiro" (@lawrencedoliveiro9104) on "Feistel Cipher - Computerphile" video.

  1. 2:56 The key lies in observing that each half of the original data block does not go through the F-block to get to the next stage. The only use of the output of the F-blocks is to xor with data blocks. And xor is reversible: xoring with the same bitstream twice gives you back the original bitstream. That’s why the same sequence of bit-mashing works for both encryption and decryption. It really is that simple.
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  2. OK, here’s a fun thought: how would you generalize this idea to a base other than 2? For example, base-26, using the letters of the (uppercase, unaccented) English alphabet? The obvious answer is to do arithmetic modulo 26, assigning numerical values like A = 0 ... Z = 25. The symmetry of xor would no longer apply, so you would need separate encryption and decryption blocks: the encryption blocks would use addition in place of xor, while the decryption ones would subtract.
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