+--------------------------------------------+
| PKIMessage                                 |
| +----------------------------------------+ |
| | header                                 | |
| +----------------------------------------+ |
| +----------------------------------------+ |
| | body                                   | |
| +----------------------------------------+ |
| +----------------------------------------+ |
| | protection (OPTIONAL)                  | |
| +----------------------------------------+ |
| +----------------------------------------+ |
| | extraCerts (OPTIONAL)                  | |
| +----------------------------------------+ |
+--------------------------------------------+