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