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