The AddRoundKey transformation

This transformation is performed by simply adding the state and the key modulo 2 inside the coprocessor:

   state$$=$$   Key$$\oplus$$   state$$.$$

No temporal register is therefore needed. The Key register used will be Key1 or Key2 in the case of 256-bit keys, depending on the round number (Key1 for ``even" rounds and Key2 for ``odd" rounds).