intuition.
rewrite <- BVxor_id_r at 1.
rewrite <- (BVxor_same_id x).
rewrite <- BVxor_assoc.
rewrite H.
rewrite  BVxor_assoc.
rewrite (BVxor_same_id x).
rewrite BVxor_id_r.
trivial.
Qed.

(* Specifications: *)
Theorem A_NMAC_RKA_equiv :
Pr[A_NMAC] == Pr[RKA_G1 _ _ _ (Rnd b) (Rnd c) (BVxor b) HMAC_RKA_A].