are there any good attack models of HMAC ? like how it can be attacked ? if there are any possible attacks of HMAC i would be happy to know about it.
HMAC as message authentication code should be resistant to existential forgery under chosen message attack, this is known as EUF-CMA.
HMAC as a pseudo-random function should be resistant to distinguishing attack under, again, chosen message attack, which is known as IND-CMA.
I'm quite certain about the MAC part, but I'm not absolutely confident about PRF part. I don't know good references on this, feel free to add to here.