I found this question about CBC-MAC forging and want to make sure I understand it:
Let a and be be two strings of block length one. Suppose the sender sends (a, CBC-MAC(a)), (b, CBC-MAC(b)), (a||b, CBC-MAC(a||b)). Find new messages / MAC pair which the attack can now forge from the messages / MACs given above.
Since given: CBC-MAC(x) = t, CBC-MAC(y) = t', then CBC-MAC(x || y $\oplus$ t) = t'
Then I can forge the permutation combination of the given pairs with that formula? ie. If CBC-MAC(a) = x, CBC-MAC(b) = y, then CBC-MAC(a || b $\oplus$ x) = y