The diagram above shows the standard HMAC algorithm. However consider the algorithm of HMAC without xoring the key with outer or inner pad. In other words let the structure of HMAC remain the same without xoring the key with outer and inner pad. Let the key go in as input alone at both points. Is it possible to commit existential forgery with this modified architecture ?
I understand that the standard HMAC architecture prevents an attacker from computing MACs for extended messages (i.e. message blocks appended to a message for which MAC is known). But in my view this can also be prevented without xoring with inner and outer pad. Please suggest.
