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 $n>k$ .
(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?