code: def add_round_key(state, rkey):
      
      state = state.reshape(4, 32)
      rkey = rkey.reshape(4, 32)
      return fcat(
          state[0] ^ rkey[0],
          state[1] ^ rkey[1],
          state[2] ^ rkey[2],
          state[3] ^ rkey[3],
      )
docs: Transformation in the Cipher and Inverse Cipher in which a Round Key is
      added to the State using an XOR operation. The length of a Round Key equals
      the size of the State (i.e., for Nb = 4, the Round Key length equals 128
      bits/16 bytes).