Question on MAC Forgery In MAC forgery, is it assumed that the attacker has the secret key or not? 
I find this in Wikipedia but still not clear.


  Existential forgery is the creation (by an adversary) of at least one message/signature pair, ( m , σ ), where σ was not produced by the legitimate signer. The adversary need not have any control over m, m need not have any particular meaning; the message content is irrelevant — as long as the pair, ( m , σ ), is valid, the adversary has succeeded in constructing an existential forgery.
The key part of the entry is "...where σ was not produced by the legitimate signer.". The legitimate signer is the key holder. We assume the adversary does not possess the key, but would like to have a way to produce MAC tags anyways.