Initiator                         Responder
-----------                       -----------
HDR(IKE_INTERMEDIATE,MID=2),
SK(SK_ei2,SK_ai2) {...}  -->

         <Calculate IntAuth_i2 = prf(SK_pi2, ...)>

                             <--  HDR(IKE_INTERMEDIATE,MID=2),
                                  SK(SK_er2,SK_ar2) {...}

         <Calculate IntAuth_r2 = prf(SK_pr2, ...)>