Perform cyclical exclusive or operations on ``data``.

    The ``key`` can be a an integer *(0 <= key < 256)* or a byte sequence. If
    the key is smaller than the provided ``data``, the ``key`` will be
    repeated.

    Args:
        key(int or bytes): The key to xor ``data`` with.
        data(bytes): The data to perform the xor operation on.

    Returns:
        bytes: The result of the exclusive or operation.

    Examples:
        >>> from pwny import *
        >>> xor(5, b'ABCD')
        b'DGFA'
        >>> xor(5, b'DGFA')
        b'ABCD'
        >>> xor(b'pwny', b'ABCDEFGHIJKLMNOPQRSTUVWXYZ')
        b'15-=51)19=%5=9!)!%=-%!9!)-'
        >>> xor(b'pwny', b'15-=51)19=%5=9!)!%=-%!9!)-')
        b'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

def xor(key, data):
    """
    Perform cyclical exclusive or operations on ``data``.

    The ``key`` can be a an integer *(0 <= key < 256)* or a byte sequence. If
    the key is smaller than the provided ``data``, the ``key`` will be
    repeated.

    Args:
        key(int or bytes): The key to xor ``data`` with.
        data(bytes): The data to perform the xor operation on.

    Returns:
        bytes: The result of the exclusive or operation.

    Examples:
        >>> from pwny import *
        >>> xor(5, b'ABCD')
        b'DGFA'
        >>> xor(5, b'DGFA')
        b'ABCD'
        >>> xor(b'pwny', b'ABCDEFGHIJKLMNOPQRSTUVWXYZ')
        b'15-=51)19=%5=9!)!%=-%!9!)-'
        >>> xor(b'pwny', b'15-=51)19=%5=9!)!%=-%!9!)-')
        b'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    """

    if type(key) is int:
        key = six.int2byte(key)
    key_len = len(key)

    return b''.join(
        six.int2byte(c ^ six.indexbytes(key, i % key_len))
        for i, c in enumerate(six.iterbytes(data))
    )