Sending     |     Receiving
MK      CK      RK      CK      MK
--      --      --      --      --
   ECDH(A0,B0)  + 
               /|
              / |
             /  +  ECDH(A1,B1) 
     CK-A1-B0    \
         |        \
 MK-0  --+         \
         |          CK-A1-B1
 MK-1  --+              |
         |              +--  MK-0  
 MK-2  --+              |
                        +--  MK-1