Return an iterator that encrypts `data` using the Cipher Feedback (CFB)
    mode of operation.
    
    CFB mode can operate on `data` of any length.
    
    Each iteration, except the last, always returns a block-sized :obj:`bytes`
    object (i.e. 8 bytes). The last iteration may return a :obj:`bytes` object
    with a length less than the block-size, if `data` is not a multiple of the
    block-size in length.
    
    `init_vector` is the initialization vector and should be a
    :obj:`bytes`-like object with exactly 8 bytes.
    If it is not, a :exc:`ValueError` exception is raised.
    
    `data` should be a :obj:`bytes`-like object (of any length).

def encrypt_cfb(self, data, init_vector):
    """
    Return an iterator that encrypts `data` using the Cipher Feedback (CFB)
    mode of operation.
    
    CFB mode can operate on `data` of any length.
    
    Each iteration, except the last, always returns a block-sized :obj:`bytes`
    object (i.e. 8 bytes). The last iteration may return a :obj:`bytes` object
    with a length less than the block-size, if `data` is not a multiple of the
    block-size in length.
    
    `init_vector` is the initialization vector and should be a
    :obj:`bytes`-like object with exactly 8 bytes.
    If it is not, a :exc:`ValueError` exception is raised.
    
    `data` should be a :obj:`bytes`-like object (of any length).
    """
    S1, S2, S3, S4 = self.S
    P = self.P
    
    u4_1_pack = self._u4_1_pack
    u1_4_unpack = self._u1_4_unpack
    encrypt = self._encrypt
    
    u4_2_pack = self._u4_2_pack
    
    data_len = len(data)
    extra_bytes = data_len % 8
    last_block_stop_i = data_len - extra_bytes
    
    try:
      prev_cipher_L, prev_cipher_R = self._u4_2_unpack(init_vector)
    except struct_error:
      raise ValueError("initialization vector is not 8 bytes in length")
    
    for plain_L, plain_R in self._u4_2_iter_unpack(
      data[0:last_block_stop_i]
    ):
      prev_cipher_L, prev_cipher_R = encrypt(
        prev_cipher_L, prev_cipher_R,
        P, S1, S2, S3, S4,
        u4_1_pack, u1_4_unpack
      )      
      prev_cipher_L ^= plain_L
      prev_cipher_R ^= plain_R
      yield u4_2_pack(prev_cipher_L, prev_cipher_R)
      
    if extra_bytes:
      yield bytes(
        b ^ n for b, n in zip(
          data[last_block_stop_i:],
          u4_2_pack(
            *encrypt(
              prev_cipher_L, prev_cipher_R,
              P, S1, S2, S3, S4,
              u4_1_pack, u1_4_unpack
            )
          )
        )
      )