Why are much shorter output lenghts of, e.g 80 bits, sufficient for MACs ? Assume a message x that is sent in clear toghether with its MAC over the channel: (x,MACk(x)). What has Oscar to do attack this system?
I think i can awnser the first question: Its because of the Birthday Paradox But for the second, i don´t know how to start. Its not given how the MAC is calculated. I could do a secret pre- or suffix attack ?
Thanks for awnsers