Initiator                         Responder
-----------                       -----------
HDR(IKE_INTERMEDIATE,MID=1),
SK(SK_ei1,SK_ai1) {...}  -->

         <Calculate IntAuth_i1 = prf(SK_pi1, ...)>

                             <--  HDR(IKE_INTERMEDIATE,MID=1),
                                  SK(SK_er1,SK_ar1) {...}

         <Calculate IntAuth_r1 = prf(SK_pr1, ...)>