code: def subword(w):
      
      w = w.reshape(4, 8)
      return SBOX[w[0]] + SBOX[w[1]] + SBOX[w[2]] + SBOX[w[3]]
docs: Function used in the Key Expansion routine that takes a four-byte input word
      and applies an S-box to each of the four bytes to produce an output word.