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