What is more important, in terms of security, the size of the MAC or the size of the key that it uses? I'm asking this because I have to answer the following homework question:
What is more secure? A MAC with a 12-byte key and 6-byte MAC length, or a MAC with a 8-byte key and 12-byte MAC length?
For the first one, it's easier to find a pre-image of the result. But for the second one, it's easier to find the key.