The question is quite similar to this one : Forge CBC-MAC given the MAC of two messages and of their concatenation
But I still cannot fully understand it, and here's my question:
given two messages with known MAC:
So from the given messages, I am able to forge a new message like this:
CBC-MAC(a || b || (d XOR x) || e ) = y (which is just like the post I read).
But how about if I want to forge the message like this:
CBC-MAC(a || b || c || d || e ) = z
The "c" maybe is something calculated from those two messages and its MAC, but I am not sure about it.
Is it possible to achieve this?