The remote service obtains a service authentication code that has been generated, using {HEAD ~ a code generation} algorithm, based on {TAIL ~ a first secret key} 
The remote service obtains a service authentication code that has been generated , using { HEAD ~ a code generation } algorithm , based on { TAIL ~ a first secret key }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL