pycryptodome/lib/Crypto/Cipher/_mode_ofb.py

# -*- coding: utf-8 -*-
#
#  Cipher/mode_ofb.py : OFB mode
#
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"""
Output Feedback (CFB) mode.
"""

from Crypto.Util import Counter

class ModeOFB(object):
    """*Output FeedBack (OFB)*.

    This mode is very similar to CBC, but it
    transforms the underlying block cipher into a stream cipher.

    The keystream is the iterated block encryption of the
    previous ciphertext block.

    An Initialization Vector (*IV*) is required.

    See `NIST SP800-38A`_ , Section 6.4 .

    .. _`NIST SP800-38A` : http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
    """

    def __init__(self, factory, **kwargs):
        """Create a new block cipher, configured in OFB mode.

        :Parameters:
          factory : module
            A cryptographic algorithm module from `Crypto.Cipher`.

        :Keywords:
          key : byte string
            The secret key to use in the symmetric cipher.
          IV : byte string
            The initialization vector to use for encryption or decryption.
            It is as long as the cipher block.

            **The IV must be a nonce, to to be reused for any other
            message**. It shall be a nonce or a random value.

            Reusing the *IV* for encryptions performed with the same key
            compromises confidentiality.
        """

        #: The block size of the underlying cipher, in bytes.
        self.block_size = factory.block_size

        #: The Initialization Vector originally used to create the object.
        #: The value does not change.
        self.IV = kwargs.pop("IV", None)

        try:
            key = kwargs.pop("key")
            if self.IV is None:
                self.IV = kwargs.pop("iv")
        except KeyError, e:
            raise TypeError("Missing parameter: " + str(e))

        self._cipher = factory.new(key,
                                   factory.MODE_OFB,
                                   self.IV,
                                   **kwargs)

    def encrypt(self, plaintext):
        """Encrypt data with the key and the parameters set at 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 : byte string
            The piece of data to encrypt.
            It can be of any length.
        :Return:
            the encrypted data, as a byte string.
            It is as long as *plaintext*.
        """

        return self._cipher.encrypt(plaintext)

    def decrypt(self, ciphertext):
        """Decrypt data with the key and the parameters set at initialization.

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

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

        That is, the statement:

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

        is equivalent to:

             >>> c.decrypt(a+b)

        This function does not remove any padding from the plaintext.

        :Parameters:
          ciphertext : byte string
            The piece of data to decrypt.
            It can be of any length.

        :Return: the decrypted data (byte string).
        """

        return self._cipher.decrypt(ciphertext)