And the birthday attack is only possible when the attacker has {HEAD ~ control} over {TAIL ~ the message} that is signed 
over
And the birthday attack is only possible when the attacker has { HEAD ~ control } over { TAIL ~ the message } that is signed
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH