First block as initialization vector in the CBC-MAC A fixed-length CBC-MAC uses zero block as IV (initialization vector) to prevent from changing the first block.

i.e. tag $t$ = $F(m_x \oplus ... F(m_3 \oplus F(m_2 \oplus F(m_1)))...)$

My question is, if the first block is used as IV, then is it secure (unforgeable), and why?
(not for encryption/confidentiality, and just for authentication)

i.e. tag $t$ = $F(m_x \oplus ... F(m_4 \oplus F(m_3 \oplus F(m_1 \oplus m_2)))...)$

Thank you.
My question is, if the first block is used as IV, then is it secure (unforgeable), and why?


Suppose with this scheme, you have the MAC for the message $[m_1, m_2, m_3]$.  Could someone without the key compute the MAC for the message $[m_2, m_1, m_3]$?