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