The strongest attack known against {HEAD ~ HMAC} is the birthday attack, based on the frequency of {TAIL ~ collisions} for the hash function 
The strongest attack known against { HEAD ~ HMAC } is the birthday attack , based on the frequency of { TAIL ~ collisions } for the hash function
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH