I am trying to prove that the following MAC is insecure, but I don't know how to exploit the properties of the pseudorandom function $F$ involved:
Gen generates a uniform $k \in \{0, 1\}^n$.
To authenticate a message $m_1 || m_2$ with $|m_1| = |m_2| = n$, compute the tag $F_k(m_1)||F_k(F_k(m_2))$.
Any help?