I have a hypothetical encryption scheme where somebody uses the one-time pad in CBC mode. That is, the block cipher is $E(k, m) :=k⊕m$, and that block cipher is used in CBC mode.
Now, I am assuming that a message $m=m_1||m_2||m_3||m_4$ is used for encryption.
How to compute $m_3⊕m_4$ from the resulting ciphertext (without using the key)?
Also: using this, how can I prove that it is IND_CPA insecure?