Also, Schneier is trying to teach people how to implement cryptosystems, not how to break them or how to design them (which you allude to by saying "applying kn

Also, Schneier is trying to teach people how to implement cryptosystems, not how to break them or how to design them (which you allude to by saying "applying known systems"). One difference between classical and modern cryptography is that classical cryptography usually works at the letter or character level, while modern cryptography usually treats plaintexts and ciphertexts as uninterpreted sequences of bits and uses bit-level operations. Sometimes these two perspectives have equivalences or similarities, but mostly not if you're looking at stuff like block cipher S-boxes or block cipher modes of operation or Merkle-Damgård hash functions or whatever.