0 1 2 3 4 5 6 7 <--------- n bytes ------------->
     +-+-+-+-+-+-+-+-+---------------------------------
     |0 0 0|h|k|  n  |      Partial IV (if any) ...    
     +-+-+-+-+-+-+-+-+---------------------------------
     |               |                                |
     |<--  CoAP   -->|<------ CoAP OSCORE_piv ------> |
        OSCORE_flags 

      <- 1 byte -> <------ s bytes ----->
     +------------+----------------------+-----------------------+
     | s (if any) | kid context (if any) | kid (if any)      ... |
     +------------+----------------------+-----------------------+
     |                                   |                       |
     | <------ CoAP OSCORE_kidctx ------>|<-- CoAP OSCORE_kid -->|