Applied Cryptography

In CBC IVs are selected at random. Your friend, after buying and reading a used crypto book, tries to implement a cipher and decides that random IVs are a bit excessive. They propose that instead IVs for different messages be chosen iteratively rather than randomly (i.e. the IV for message m_2 is simply one larger than the IV for message m_1). Build an Chosen Plaintext Attack (CPA) against your friend's system demonstrating that this method of using CBC is not secure