For variable-length messages, there are three different ways to generate the tags.
$Tag = MAC_k(\Sigma_i m_i)$.
$t_i = MAC_k(m_i)$ and $Tag = (t_1, ..., t_l)$.
$t_i = MAC_k(i||m_i)$ where $i$ is a sequence number and $Tag = (t_1, ..., t_l)$.
Find an attack for each case.
Problem comes from Communication Systems Security, by Chen and Gong, page 179
I have been looking for various attacks for the above question. Can anyone help me with the answer?