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Consider a Plaintext P1 and Ciphertext P2, which is known to the attacker. If the attacker knows that the IV used in encrypting P1 in OFB mode is the same used to encrypt another plaintext P2, how can he find out the content of P2 given the ciphertext C2?
Will the attacker have greater limitation if it is encrypted in CFB mode instead?
OFB converts a block cipher to a stream cipher (As does CTR mode). The plain text or any derivitive from it never enters the block cipher. The key stream is unrelated to what is being encrypted. The final ciphertext is just a xor of the plain text with the key stream and the key stream depends only on secret key and IV. reuse both and you get the same key stream. It is the same as reusing a one time pad. It's bad even without knowing a cipher text plain text pair. If you do, it becomes trivial to decrypt any message reusing the same key+IV.
