Can anyone explain why CBC-MAC is not secure for variable length message?
I read many books, but there are still some confusion. Like the highlighted parts in following pictures
First question, why need to set T"=C0?
Second question which is biggest question i don't understand, why T"' = T". How to prove this relationship?
Third question, why is new message = P0.P1.(P0 xor T')?
Can anyone help me. Thanks!