Known Plaintext Attack on Block Cipher in ECB and CBC Modes of Operations These 2 similar questions  are from Paar's Understanding Cryptography. I could not see the answer so if someone can help I will appreciate: 

Question1:  We consider known plaintext attacks on block ciphers by means of an exhaustive key search where the key is $k$ bits long.The block length counts $n$ bits with $n&gt;k$ .

(i)  How many plaintexts and ciphertexts are needed to succesfully break a block cipher running in ECB mode? How many steps are done in the worst case?

(ii) Assume that the initialization vector IV for running the considered block cipher in CBC mode is known. How many plaintexts and ciphertexts are now needed to break the cipher  by performing an exhaustive key search? How many steps need now maximally be done? Briefly describe the attack.

(iii) How many plaintexts and ciphertexts are necessary if you do not know the IV?

(iv) Is breaking a block cipher in CBC mode by means of an exhaustive search considerably more difficult than breaking an ECB mode block cipher?

Question 2: Keeping an IV secret in OFB mode does not make an exhaustive key search more complex. Describe how we can perform a brute force attack with unknown IV?What are the requirements regarding plaintext and ciphertext?
Let me proceed by giving the answers, some of which are probably considered wrong by the book:

(i) One. If you have two ciphertext blocks that are identical you leak information about the message itself, so the cipher would be broken. A message may well consist of multiple blocks with repeated plaintext after all. You'd need $m$ messages of a single block (max) before they start to repeat in the worst case, assuming non-random padding. Here $m$ is the number of messages in the message space, with $m \leq 2^n$ - so $2^n$ is the maximum  of the maximum.

(ii) One. You can verify the correctness of the key with high probability using the a single known block as $n &gt; k$. Even if this is not the case the message may occupy multiple blocks. You'll never be completely certain though. The maximum time for an exhaustive key search is of course $2^k$ (this one is probably correct by the book).

(iii) Still one, because you can simply consider any blocks after the first block.

(iv) Yes, generally it is considered more difficult. ECB is insecure and exhaustive key search is often not practically possible for high enough $k$.

2 You just consider the next blocks as you know the first (known) ciphertext block. The plaintext and thus ciphertext needs to consist of more than one block for this, but only if the IV is random instead of just "unknown"; the more bits you know of each message the better.

As indicated, horrible questions. I would presume I would have quite a few wrong, if just out of sheer unwillingness to understand.