initialization.

A cipher object is stateful: once you have encrypted a message
you cannot encrypt (or decrypt) another message using the same
object.

The data to encrypt can be broken up in two or
more pieces and `encrypt` can be called multiple times.

That is, the statement:

    >>> c.encrypt(a) + c.encrypt(b)

is equivalent to:

     >>> c.encrypt(a+b)

This function does not add any padding to the plaintext.

:Parameters:
  plaintext : bytes/bytearray/memoryview
    The piece of data to encrypt.
    It can be of any length.
:Keywords:
  output : bytearray/memoryview
    The location where the ciphertext must be written to.
    If ``None``, the ciphertext is returned.
:Return:
  If ``output`` is ``None``, the ciphertext is returned as ``bytes``.
  Otherwise, ``None``.
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