The second computer system receives {HEAD ~ an encrypted message} from the first computer system and processes the encrypted message to generate {TAIL ~ a response message} 
The second computer system receives { HEAD ~ an encrypted message } from the first computer system and processes the encrypted message to generate { TAIL ~ a response message }
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL